Tunnel Kiln For Brick Firing Process? All Answers

Furnace that generates high temperatures

Indian brick kiln

An empty, intermittent furnace. This specific example is a “car oven”; The base is on wheels and has been rolled out of the oven – this makes loading and unloading the oven easier.

An oven is a thermally insulated chamber, a type of furnace that produces temperatures sufficient to complete a process such as curing, drying, or chemical changes. Kilns have been used for millennia to transform clay objects into pottery, tiles and bricks. Various industries use rotary kilns for pyroprocessing – to calcine ores, calcine limestone to lime for cement, and many other materials.

Pronunciation and etymology[edit]

Kiln derives from Old English cylene (/ˈkylene/), borrowed from Old Welsh ‘cylyn'[1][2], which in turn was borrowed from Latin culīna, kitchen, stove, fireplace.

According to the Oxford English Dictionary, Kiln was written in Old English as cyline, cylene, cyln(e). In Middle English as kulne, kyllne, kilne, ofen, kylle, kyll, kil, kill, keele, kiele. By the 14th century the final “n” had fallen silent in most locales, leading to the spelling “kill” instead of the etymological kiln.[3][4]

In James A. Bowen’s book English Words as Spoken and Written for Upper Grades (1915) he refers to the ‘ln’ in ‘kiln’ as a digraph, a combination of two letters producing a sound, and treats ‘kill’ and ‘ Kiln” as homophones: “The digraph ln, n silent, occurs in the kiln. Falling into the kiln can kill you.”[5][6]

The modern pronunciation of this word, in which the “n” is pronounced, is common. This is due to a phenomenon known as spelling pronunciation, where the pronunciation of a word is derived from its spelling and differs from its actual pronunciation.

Using kilns[edit]

Pit-fired pottery was made thousands of years before the earliest known kiln, which dates back to around 6000 BC. and found at the Yarim Tepe site in modern Iraq.[7] Neolithic kilns could generate temperatures in excess of 900 °C (1652 °F).[8] Uses include:

Ceramic kilns [ edit ]

Kilns are an essential part of the production of all ceramics. Ceramics require high temperatures, so chemical and physical reactions occur that permanently change the unfired body. In pottery, clay materials are shaped, dried, and then fired in a kiln. The final properties are determined by the composition and preparation of the clay body and the temperature at which it is fired. After a first firing, glazes can be used and the ware is fired a second time to fuse the glaze to the body. A third firing at a lower temperature may be necessary to set the overglaze decoration. Modern kilns often have sophisticated electronic control systems, although pyrometric devices are often used as well.

Clay consists of fine-grained particles that are relatively weak and porous. Clay is combined with other minerals to create a workable clay body. The firing process includes sintering. This heats the clay until the particles partially melt and flow together, creating a strong, uniform mass consisting of a glassy phase interspersed with pores and crystalline material. Firing reduces the pores, causing the material to shrink slightly. This crystalline material consists mainly of silicon and aluminum oxides.

Broadly speaking, there are two types of furnaces: batch and continuous, both are insulated boxes with controlled internal temperature and atmosphere.

A continuous kiln, sometimes called a tunnel kiln, is long with only the middle section directly heated. From the cool entrance, the ware is moved slowly through the kiln and its temperature is steadily increased as it approaches the central, hottest part of the kiln. As it continues through the oven, the temperature is reduced until the goods leave the oven at almost room temperature. A continuous furnace is energy efficient because the heat given off during cooling is reused to preheat the incoming goods. In some designs, the dishes stay in one place while the heating zone moves across them. Furnaces of this type include:

Hoffmann oven

Bull’s Trench Oven

Habla (zigzag) oven

Roller kilns: A specific type of kiln common in tableware and tile manufacture is the roller hearth kiln, in which goods placed on bats are carried through the kiln on rollers.

With the intermittent kiln, the goods are placed in the kiln, the kiln is closed, and the internal temperature is increased according to a schedule. After firing is complete, both the kiln and the ware are cooled. The dishes are removed, the oven is cleaned and the next cycle begins. Kilns of this type include:[10]

pinch oven

Skove oven

Scottish oven

downdraft furnace

Shuttle kilns: This is a shuttle kiln with a door at one or both ends. The burners are positioned at the top and bottom of each side, creating a turbulent circular flow of air. This type of kiln usually has a multi-car design and is used for the batch processing of white goods, technical ceramics and refractory. Depending on the size of the ware, bogie hearth kilns may be equipped with bogie moving devices to move fired and unfired ware in and out of the kiln. Bogie hearth kilns can be either updraft or downdraft kilns. A shuttle kiln derives its name from the fact that kiln cars can enter a shuttle kiln from either end of the kiln, while a tunnel kiln only flows in one direction.

The furnace technology is very old. Kilns evolved from a simple earth ditch filled with pots and fuel pit firing to modern methods. An improvement was to build a combustion chamber around pots with baffles and a stoke hole. That saved heat. A chimney improved the airflow, or draft, of the stove, thus burning the fuel more completely.

Chinese kiln technology has always been a key factor in the development of Chinese ceramics and was the most advanced in the world until recent centuries. The Chinese developed before 2000 BC. kilns that could burn at around 1,000 °C. These were updraft kilns, often built underground. Two main types of kilns were developed around AD 200 and have remained in use into modern times. These are the dragon kiln of hilly southern China, which is usually wood-fired, long and thin and running up a slope, and the horseshoe-shaped Mantou kiln of the northern China plains, smaller and more compact. Both could reliably generate the temperatures of up to 1300 °C or more required for porcelain. In the late Ming period, the egg-shaped kiln or zhenyao was developed in Jingdezhen and was mainly used there. This was something of a compromise between the other types, offering locations in the combustion chamber with a range of firing conditions.

Both ancient Roman pottery and medieval Chinese pottery could be fired in industrial quantities, with tens of thousands of pieces in a single firing.[12] Early examples of simpler kilns found in Britain include those used to make roof tiles during Roman occupation. These kilns were built on the side of a slope so that a fire could be lit at the bottom and the heat would rise up into the kiln.

Traditional kilns include:

Dragon furnace in southern China: thin and long, climbing a slope. This type spread to the rest of East Asia and gave rise to the Japanese anagama kiln, which arrived via Korea in the 5th century. This kiln usually consists of a long combustion chamber pierced on one side with smaller ware-stacking openings, with a firebox at one end and a flue at the other. The burning time can vary from one day to several weeks. Traditional anagama kilns are also built on a slope to allow for a better draft. The Japanese Noborigama kiln is an evolution of the Anagama design as a multi-chamber kiln, stacking wood first from the front firebox and then through the side heating holes only, with the benefit of the air heating to 600 °F (1,112 °C). becomes F) from the front firebox, allowing for more efficient firing.

Cambodia During the restoration of a traditional Cambodian kiln at the Khmer Ceramics & Fine Arts Center in Siem Reap

Khmer kiln: quite similar to Anagama kiln; However, traditional Khmer kilns had a flat roof. Chinese, Korean or Japanese kilns have an arched roof. These types of ovens vary in size and can measure tens of meters. The burning time also varies and can last several days.

: very similar to the Anagama furnace; However, traditional Khmer kilns had a flat roof. Chinese, Korean or Japanese kilns have an arched roof. These types of ovens vary in size and can measure tens of meters. The burning time also varies and can last several days. Bottle kiln: a type of intermittent kiln, usually fired with coal, formerly used for firing pottery; Such a kiln was surrounded by a tall brick hut or cone of typical bottle shape. The crockery was locked in sealed chamotte bags; As the heat and smoke from the fires passed through the kiln, they were fired at temperatures of up to 1,400 °C (2,552 °F).

: a type of intermittent kiln, usually fired with coal, formerly used for firing pottery; Such a kiln was surrounded by a tall brick hut or cone of typical bottle shape. The crockery was locked in sealed chamotte bags; As the heat and smoke from the fires passed through the kiln, they were fired at temperatures of up to 1,400 °C (2,552 °F). Biscuit Oven: The first firing took place in the biscuit oven.

: The first firing took place in the biscuit oven. Glazing Oven: The biscuits were glazed and given a second glaze firing in the larger glazing ovens.

: The biscuit tableware was glazed and subjected to a second smooth firing in the larger kilns. Mantou kiln from northern China, smaller and more compact than the dragon kiln

from northern China, smaller and more compact than the dragon kiln muffle kiln: this was used to fire overglaze decorations at a temperature below 800 °C (1,472 °F). In these cool kilns, the smoke from the fires flowed through chimneys outside the kiln.

: This was used for firing overglaze decorations at a temperature below 800 °C (1,472 °F). In these cool kilns, the smoke from the fires flowed through chimneys outside the kiln. Catenary Arch Kilns: Typically used for firing pottery using salt, because of their shape (a catenary arch) these tend to retain their shape over repeated heating and cooling cycles, while other types require extensive metalwork.

: Typically used for firing pottery using salt, due to their shape (a catenary arc) these tend to retain their shape over repeated heating and cooling cycles, while other types require extensive metalwork. Sèvres kiln: Invented in Sèvres, France, it efficiently generated high temperatures of 1,240 °C (2,264 °F) to produce waterproof ceramic bodies and readily available glazes. It features a down-draft design that produces high temperatures in less time, even when burning wood.

: Invented in Sèvres, France, it efficiently generated high temperatures of 1,240 °C (2,264 °F) to produce waterproof ceramic bodies and readily available glazes. It features a down-draft design that produces high temperatures in less time, even when burning wood. Bourry box oven, similar to the previous one

Modern ovens[ edit ]

With the industrial age, kilns were designed to use electricity and more refined fuels, including natural gas and propane. Many large industrial pottery kilns use natural gas because it is generally clean, efficient, and easy to control. Modern kilns can be fitted with computerized controls that allow for fine adjustments during firing. A user can choose to control the rate of temperature rise or rise, hold or penetrate the temperature at any given point, or control the rate of cooling. Both electric and gas kilns are common for smaller scale production in industry and handicrafts, handicrafts and sculpture.

The temperature of some furnaces is controlled by pyrometric cones – devices that begin to melt at certain temperatures.

Modern ovens include:

Retort furnace: a type of furnace that can reach temperatures around 1,500 °C (2,732 °F) for long periods of time. Typically these kilns are used for industrial purposes and have moving carriages that form the floor and door of the kiln.

: A type of furnace that can reach temperatures around 1,500 °C (2,732 °F) for long periods of time. Typically these kilns are used for industrial purposes and have moving carriages that form the floor and door of the kiln. Electric ovens: Electric ovens were developed in the 20th century, primarily for smaller scale use such as in schools, universities, and hobby centers. The atmosphere in most electric furnace designs is rich in oxygen because there is no open flame to consume oxygen molecules. However, reducing conditions can be created by appropriate gas supply or by special use of saggers.

: Electrically powered stoves were developed in the 20th century, mainly for smaller applications such as schools, universities and hobby centers. The atmosphere in most electric furnace designs is rich in oxygen because there is no open flame to consume oxygen molecules. However, reducing conditions can be created by appropriate gas supply or by special use of saggers. Feller stove: brought contemporary design to wood firing by reusing unburned gas from the chimney to heat the intake air before it enters the firebox. This results in an even shorter firing cycle and less wood consumption. This design requires external ventilation to prevent the radiator from melting in the chimney, which is typically metal. The result is a very efficient wood-burning stove that burns one cubic meter of ceramic with one cubic meter of wood. [citation required]

: brought contemporary design to wood firing by reusing unburned gas from the chimney to heat the intake air before it enters the firebox. This results in an even shorter firing cycle and less wood consumption. This design requires external ventilation to prevent the radiator from melting in the chimney, which is typically metal. The result is a very efficient wood-burning stove that burns one cubic meter of ceramic with one cubic meter of wood. Microwave Assisted Firing: This technique combines microwave energy with more conventional energy sources such as radiant gas or electric heating to process ceramic materials to the high temperatures required. Microwave-assisted firing offers significant economic benefits.

: This technique combines microwave energy with more conventional energy sources such as radiant gas or electric heating to process ceramic materials to the high temperatures required. Microwave-assisted firing offers significant economic benefits. Microwave Oven: These small ovens are designed to be placed in a standard microwave oven. The furnace body is made of a porous ceramic material lined with a coating that absorbs microwave energy. The microwave oven is placed in a microwave oven and heated to the desired temperature. The heating process is much less controlled than most modern electric ovens as there is no built in temperature control. The user must closely monitor the process to achieve the desired results by adjusting the time and power levels programmed on the microwave oven. Through a small hole in the lid of the oven, the internal temperature can be estimated optically, as hot materials glow. Microwave ovens are designed to reach internal temperatures in excess of 1400°C, hot enough to work some types of glass, metal and ceramics while keeping the outside of the oven cool enough to be handled with hot pads or tongs. After firing, the kiln should be removed from the microwave oven and placed on a heat-resistant surface while it is allowed to cool. Microwave ovens are limited in size, typically no more than 8 inches in diameter. [13]

: These small kilns are designed to be placed in a standard microwave oven. The furnace body is made of a porous ceramic material lined with a coating that absorbs microwave energy. The microwave oven is placed in a microwave oven and heated to the desired temperature. The heating process is much less controlled than most modern electric ovens as there is no built in temperature control. The user must closely monitor the process to achieve the desired results by adjusting the time and power levels programmed on the microwave oven. Through a small hole in the lid of the oven, the internal temperature can be estimated optically, as hot materials glow. Microwave ovens are designed to reach internal temperatures in excess of 1400°C, hot enough to work some types of glass, metal and ceramics while keeping the outside of the oven cool enough to be handled with hot pads or tongs. After firing, the kiln should be removed from the microwave oven and placed on a heat-resistant surface while it is allowed to cool. Microwave ovens are limited in size, typically no more than 8 inches in diameter. Top Hat Kiln: an intermittent kiln sometimes used for firing pottery. The ware is placed on a fireproof hearth or base over which a box-shaped cover is lowered.

Wood drying oven [ edit ]

Fresh wood coming directly from the felled tree has far too high a moisture content to be commercially viable and will rot, warp and split. Both hardwoods and softwoods need drying until the moisture content is between 18% and 8%. This can be a long process if not speeded up by using a kiln. Today there is a wide variety of furnace technologies: conventional, dehumidification, solar, vacuum and high frequency.

Conventional lumber drying kilns are built either of the pack (side-loader) or track (tram) type. Most hardwood kilns are side-loading kilns, which use forklifts to load packs of lumber into the kiln. Most softwood kilns are rail-type, where the wood (US: “lumber”) is loaded onto kiln/rail cars to load the kiln. Modern conventional high temperature, high air velocity kilns can typically dry 25mm (1 inch) thick green wood to 18% moisture content in 10 hours. However, a 2.5 cm thick green red oak takes about 28 days to dry to a moisture content of 8%.

Heat is typically supplied via steam flowing through fin/tube heat exchangers controlled by pneumatic on/off valves. Moisture is removed through a system of vents, the specific placement of which is usually specific to a particular manufacturer. Generally, cool, dry air is introduced at one end of the furnace while warm, moist air is exhausted at the other. Traditional hardwood kilns also require the supply of moisture either via steam spray or cold water mist systems to prevent the relative humidity inside the kiln from falling too low during the drying cycle. Fan directions are typically reversed periodically to ensure even drying of larger oven loads.

Most softwood stoves operate at temperatures below 115°C (239°F). Hardwood kiln drying schedules typically keep the dry bulb temperature below 80 °C (176 °F). Difficult-to-dry species should not exceed 60°C (140°F).

Dehumidification ovens are similar to other ovens in basic design and the drying times are usually comparable. The heat comes mainly from an integrated dehumidification unit, which also removes moisture. Auxiliary heat is often provided early in the schedule to supplement the dehumidifier.

Solar ovens are traditional ovens typically built by hobbyists to keep initial investment costs low. Heat is provided by solar radiation, while internal airflow is typically passive.

Vacuum and high frequency ovens reduce air pressure to speed up the drying process. There are a variety of these vacuum technologies, differing primarily in the method by which heat is introduced into the wood batch. Hot water platen vacuum furnaces use aluminum heating plates in which the water circulates as a heat source and typically operate at significantly reduced absolute pressure. Batch and SSV (superheated steam) use atmospheric pressure to introduce heat into the furnace charge. The entire furnace charge is brought to full atmospheric pressure, the air in the chamber is then heated and finally a vacuum is pulled while the charge cools. SSVs operate at partial atmospheres, typically around 1/3 of full atmospheric pressure, in a mix of vacuum and conventional kiln technology (SSV kilns are significantly more popular in Europe, where the locally harvested wood is easier to dry than North American woods.) RF/V (radio frequency + vacuum) furnaces use microwave radiation to heat the furnace load and typically have the highest operating costs because the heat of vaporization is provided by electricity rather than local fossil fuels or scrap wood sources.

The economics of various wood drying technologies are based on total energy, capital, insurance/risk, environmental impact, labor, maintenance, and product deterioration costs. This cost, which can represent a significant portion of the facility cost, includes the different effects of having drying equipment in a given facility. Every piece of equipment, from the green cutter to the feeding system at the planing mill, is part of the “drying system”. The true cost of the drying system can only be determined by comparing the total system costs and risks with and without drying.

Kiln-dried firewood was developed in the 1980s and later widely used in Europe due to the economic and practical advantages of selling wood with a lower moisture content (although an optimal moisture content of below 20% is much easier to achieve).[15] [16][17][18]

The total (harmful) air emissions from wood stoves, including their heat source, can be significant. Typically, the higher the temperature at which the furnace operates, the greater the amount of emissions produced (per pound of water removed). This applies in particular to the drying of thin veneers and the high-temperature drying of softwood.

Gallery [ edit ]

Brick kilns, Mekong Delta. The cargo ship in the foreground carries the rice chaff, which is used as fuel for the furnace.

A wood-fired pottery kiln in Hoi An, Vietnam.

Rice chaff is placed in a brick kiln in the Mekong Delta

A catenary arc kiln used for firing electron tube quality alumina ceramics at high temperatures

A two-story porcelain kiln with ovens á alandier in Sèvres, France, circa 1880

CAD representation of a beehive oven

CAD representation of a tunnel kiln

A kiln courtyard with several kilns

See also[edit]

Notes [edit]

References[edit]

: a heap of green bricks, curved to contain the fuel for their combustion

: a heap of green bricks, curved to contain the fuel for their combustion

2 : a stack of green bricks, arched to contain the fuel underneath to burn them

a kiln in which bricks are baked or fired

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How to make a kiln

Build your own pottery kiln

Furnace assembly instructions

An old oil barrel or a metal trash can with a lid.

Ceramic fiber blanket (e.g. Kaowool)

Refractory Cement

A torch from the hardware store (like those used for soldering).

An oven shelf that fits inside.

A propane tank.

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One way to fire pottery at home is to build your own kiln. A small homemade kiln can be cheap and easy to build – a great solution for at-home pottery classes. While it may seem daunting, crafting a furnace is very possible. I looked at the options and found the easiest way to put one together. Wouldn’t this be a great homeschooling project? And ideal if you have children interested in firing their own clay creations. Let’s take a look at what you need and how to (safely!) assemble it at the Pottery Store. If you’re using an oil drum, you’ll need to cut a lid into the top. Both the drum and trash can require a hole near the bottom large enough to fit the end of the burner. Then cut and glue the fiber to the inside of the container. Don’t forget to line the bottom and lid. The best video I’ve found that explains this process is this one by Simon from Sleachpots. Simon says that you should get about 20 ignitions from the gas cylinder, depending on how long it lasts and how many pots are burning. Although the kiln was used for raku in the video (not something you should attempt in homeschooling – it’s too toxic and dangerous to do yourself), I think it would work just as well for firing small clay projects at home. Fire it up until it starts to glow red inside, then turn off the burner and let it cool down as slowly as possible. Remember it gets very hot so place it on a fire retardant surface. Too much for you? Check out the other ceramic firing options:

These high school ceramics students are looking for clay. Not only did they make pottery from the clay, they built a kiln and fired the pots in the kiln with wood. Eric Good Kaufmann, her teacher, is an accomplished potter and teaches art at Bethany Christian High School in Goshen, Indiana, USA. Mr. Kaufmann is a Class of ’97 graduate of Goshen College.

PROCESS for REFURBISHING good clay that is becoming too dry for use. Processing of home-dug clay follows below and home-dug clay is shown in the photos on the left of this page.

In order to prepare hard clay, it must first be completely dry. Dry clay does not need to be crushed.

PRECAUTIONS:

Make sure there are no gypsum shavings in it – gypsum causes spalling when biscuit is fired. Leather hard clay or wet clay does not erase well because it is not porous like dry clay. Instruct the students to handle the dry clay without creating dust. Dust in the air is not healthy to breathe. Other contaminants such as paper, sponges, etc. can cause mold to form in the clay in addition to being a nuisance in the clay.

see page Health hazards Put the completely dry clumps in clear water in something like garbage cans. Use enough water so that the clay is completely submerged. Just leave it on in clear water. Never stir. Stirring will clog the porosity and prevent good deglazing (soaking into a mush). In a few days or less, even huge chunks of dry clay will crumble to a pulp. Go to step 6 below and dry sufficiently to use as in steps 7, 8 and 9.

HOME-BUILT SOUND VARIANT

If you dig clay yourself, it often has impurities that need to be removed. Most children love to help and there are few better learning experiences. If you are a teacher, invite students to bring samples to test. If it works well, ask them to bring more.

Allow the clay to dry completely. Delete it as described in 3 above. When everything is soft and mushy, stir it until it’s a slip. I use a blender on an electric drill or mixer. Add water if needed to liquefy. Pour the slip through a regular window screen, which you can get at any hardware store. Sifting removes stones, roots, and other debris that causes problems. The main culprit is limestone. Limestone, like gypsum, causes pots to crack after firing. When the clay has settled and turned into a mush, remove excess water from the top. Drain or siphon water. Spread the slurry a few inches thick on clean, dry, porous surfaces. I use dry plaster, clean concrete, canvas, denim, etc. Smooth the top to avoid getting small dry bits on the surface. If you want it to dry faster, use a fan and/or place everything on a grid to let air underneath. When it’s almost dry enough I make coils as thick as my arm and lay them around like big bows (12″ high) and they’re ready to wedge and use in 24 hours or less. This clay can be stored forever in an airtight plastic.

In ancient China, potters stored wet clay in caves for the next generation to enhance the clay’s plasticity. If it is to be stored long-term, double wrap it. Double packaging in plastic bags from the supermarket works. Students can contribute hundreds of these.

Clay Digging Notes

WHERE IS SOUND? – Check stream banks, construction sites, road cuttings and any place that gets slippery and sticky after a rain as it starts to dry. When dry it is almost rock hard. Many of us can find clay beneath the mother earth in our backyards.

PLASTICITY – Some clays are too sandy and others are too sticky. When I dig, I look for clay that I can roll between my hands into a pencil-thick roll of soft clay and wrap it around my finger without breaking. If the spool breaks, it may be too gritty or its clay particles are too large. Sticky clay sticks to my hands too much. It often has severe drying shrinkage and tends to crack during drying. Potters often mix several clays to get the right properties. See photos on the left.

Commercial clays can be added to balance the mix. Commercial Ball Clay adds plasticity (making it less prone to cracking when flexed). On the other hand, coarsely ground chamotte, china clay (kaolin), fine sand and/or chamotte reduce plasticity (make it less sticky and shrink less). Do a web search for “ceramic chemicals and clay” to find sources of commercially available clays in your area. See photos on the left.

IMPURITIES – Most clay contains impurities, often in the form of iron oxide, sand, roots and other debris. Disturbing impurities can be removed with a thin slip. The sand settles first on the bottom. Allow the sand to settle briefly. Then decant the clay water (the good slide from top to bottom to the sand) and discard the sand in the bottom. Allow the clay (slip) to settle and process as outlined in the 9 steps above.

Iron contamination is very common and not easy to remove. Iron gives it the red-brown color when fired and makes the clay melt more easily. It may not work for earthenware, but most common clays work well for earthenware. Most of it is shot at cone 05 with no problems.

GOOD USE OF IMPURE CLAY – Potters who make high refractory stoneware sometimes add small amounts of impure local clay to their clay bodies to add character and blemishes. I regularly add some ordinary brick clay to add character to my pottery. Paint and iron stains look more natural and give a warmer feel. Stoneware potters also use local clay as a source of glaze material. These “slip glazes” have been used to line pitchers and traditional tableware for hundreds of years.

AESTHETICS OF HOME-DUG CLAY: Most native home-dug clay fires look like ordinary clay flower pots. Some potters polish it (rub the nearly dry pieces with a polished stone or the back of a spoon). Some Native American potters make beautiful polished black pottery from home-excavated clay. Black is achieved by smothering the fire with ash at the end to prevent air from reaching the hot ceramic and the carbon from remaining fuel blackens the ceramic. Typically, tribal pottery is unglazed and fired without kilns. Sometimes the potters use colored and white clay (slip) for decoration. Search (Google) for terra sigillata for more info on how to get a highly polished finish without glaze.

TIP: Clay that is thick or not dry enough often explodes when moisture turns to vapor when heated quickly. If this happens, make it thinner, dry better, heat more slowly at first, and/or add something like sand to the clay to open up the clay body more and let the steam out.

FIRING OUTDOORS WITHOUT A FURNACE

Responsible adult supervision is required

Never leave an outdoor fire unattended

Never fire when there is a possibility of wildfire

Have emergency fire extinguishers ready

Leave the site cleaner than you found it

Obey all laws and codes

WHY WILL CLAY BURN?

Clay becomes ceramic at temperatures around 1,000 degrees F (beginning of red hot – about 540 C). Traditionally, tribal earthenware is fired at about 1,400 degrees F (760 C). Heat removes the molecular water in the clay. The heat converts clay molecules into molecules that don’t dissolve or quench in water. In modern societies, pottery and bricks are fired in kilns at temperatures between 1,800 F and 2,400 F. Most of the common clays found in our backyards, like the clay shown here at left, will begin to deform and melt when fired higher than about 1,900 F. Modern toilets are fired from clay that contains fewer pollutants. It is fired to 2,300 to 2,400 F. making it very strong and impermeable.

FIRING WITHOUT A STOVE

Kilns were invented to store heat in order to reach a higher temperature with less fuel. In tribal settings, it is traditional to use an outdoor campfire lit with enough wood lit beneath the pottery to exceed the red-hot heat as it burns. Ceramic temperatures reach 1,000 F and hotter.

WHERE and how to do it SAFELY. Consider fire safety and local fire safety regulations. Many cities and towns are very strict about open fires. In any case, do not do this if there is a possibility that the fire will spread from your fire. Have an adequate supply of water nearby. Have a shovel and dirt handy so you can quickly put out an accidental fire. Don’t leave it unattended. Teach children and students careful and strict safety habits. Temperatures are much hotter than a cooking fire.

THE “UNKILN” CONSTRUCTION STEPS

1. Burning in the “Unofen” begins with a pile of dry kindling. Some potters place this in a shallow pit or in a ring of steel, brick, or stone. Be sure to clean the area to prevent the fire from spreading.

2. A stack of pottery is carefully stacked on the kindling. Stack it in a way you think will survive when the wood burns and your pots fall into the ashes. Optionally, you can try supporting the pottery pile with some carefully placed support stones, bricks, or some old pieces of fired pottery; but leave enough room for kindling to stoke the fire.

3. OPTIONAL: Some potters place broken pieces of pot on top of the potter’s pile. You can also cheat with some tin scraps, flattened tin cans, etc. Leave a generous exhaust port at the top and several combustion air vents at the bottom around the perimeter.

4. Cover it with a thick layer of natural material such as tall green marsh grass or animal dung to retain heat. A little moisture in the dung and grass prevents it from burning too early. This insulating layer will hold the heat long enough to fire the clay, but it will also burn towards the end of firing.

5. OPTIONAL: In some cases, this insulation layer is coated with a coating that forms a thin shell. This bowl can be made of a clay/sand/straw or grass mix.

6. There is a generous vent hole at the top of the mound and several vents are provided at the bottom to allow the wood to breathe and burn with enough momentum to turn the clay red hot. The size depends on how big your burn is. The openings around the base provide a place to light the wood and allow sufficient combustion air to enter. The top opening must be large enough to allow rapid airflow at the bottom and small enough to contain heat.

BURN

1. Light the kindling with some wads of paper at the openings. OPTIONAL (if you’re worried about breakage): Once you’re sure the wood is burning, you can partially cover the top vent with some tin or pottery shards to limit the burn and heat the contents more slowly initially. Open these early enough to allow most of the fuel to burn quickly and very hot. Most of the fuel is needed to reach a high enough temperature for the clay to fire.

2. OPTIONAL: When the fuel is completely burned, immediately cover it with a layer of dry soil (if you have wood ash, this works too). This will stall the air so the pots come out smokier and darker. Some potters can obtain completely black pottery this way.

3. When it has cooled to around 500F or cooler, feel free to use sticks to gently examine and roll out your hot treasures.

TROUBLESHOOTING

Mistakes happen, but enjoy the process. Think about it and try again. Many mistakes lead to new ideas and possibilities.

breakage problems?

Experiment and learn. The steam pressure will destroy most pots. If pots are not made in a uniform thickness, they sometimes crack because the drying shrinkage is different. If pots break it may mean they are too thick or the clay needs some opener. Sand or grog in clay is an opener. It allows the moisture to evaporate (easier to escape) in the early stages of heating. Sand must not contain lime. After firing, clay chunks will spall as the chunks of limescale contaminants expand by absorbing atmospheric moisture.

Modern computer controlled electric ovens use a longer heating cycle at 200 degrees F. This is just below the point where moisture turns to vapor. Rapidly fired clay must be COMPLETELY dry before it reaches steam molding temperature. This prevents clay explosions that often occur when clay is heated too quickly.

When firing without a kiln, it may be helpful to pre-dry the clay pieces in a kitchen oven set at 190 degrees F. Using a kitchen oven, the pots are dried by “baking” them for several hours below the boiling temperature of water. I set the oven to 190 F. This will NOT fire the pots, but it will dry them out so they can be fired in an outdoor campfire or pit fire with less breakage from steam explosions.

CAUTION: A kitchen oven cannot be set hot enough to fire pots. Firing pots in an indoor kiln is never recommended. It can cause a house fire. The temperatures required to fire clay are too hot (1,000 F degrees and hotter). This temperature would make any oven red hot and exceed the safety designed into any oven. That’s a lot hotter than a self-cleaning oven burning off the residue in a dirty oven. Clay will not turn into pottery unless fired to 1,000 F (red hot) or hotter.

What if the burnt pots dissolve in water?

That means the fire wasn’t hot enough. Tribal-fired pottery is often fired to around 1,400 F. Clay turns into ceramic at around 1,000 F. The water that evaporates as the clay dries is simply physical water. However, at around 1,000 F, the chemical water is removed. This leads to a molecular change – the clay becomes a stone-like substance that no longer softens in water.

What if you don’t like the color and texture?

Pit fire and campfire pots have natural variations. These are not defects. Experiment. Pay attention to everything. Try polishing. Try coatings. Never use toxic substances on the inside of pottery that could be used by anyone to eat or drink now or in the distant future. If you like boring and reliable uniformity, use an electric oven.

What if water seeps through the pot?

This is not a defect. Pit fired pots without glaze are all porous and some water will seep through, but the structure of the clay is fine if fired hot enough. If I want to use a porous pot for a vase, I heat it in an oven and then seal the inside with melted wax by pouring melted paraffin wax in and out of the pot. Porous ceramics are used for self-cooling water jugs that keep the contents cold through evaporation on the outside. Porous ceramics are also used to filter water. Colloidal silver is added to water filters to eliminate bacteria. If you do, buy it from a reputable company (some websites have sold unsafe counterfeit materials as colloidal silver). Water filters are typically fired in kilns to reach the right temperature for proper functioning. This link contains more information about porous ceramic filtration for drinking water.

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Kilns are insulated chambers that use fuel or electricity to reach high temperatures. When something is heated in a kiln, it is said to be “fired”. There are different types of kilns to fire different materials. For example, there are kilns specifically designed for ceramic, glass, metal, brick, metal clay, and enamel.

This article is about the different kilns used for pottery and ceramics. There are many different types of kilns used in pottery. And the design of pottery kilns has evolved over the years.

First I will take a look at the different kilns commonly used by potters today. Then I’ll take a look at different types of kilns that have been used historically. This story is interesting in itself. But it’s also relevant because many potters still use kilns that are older, more traditional, or even primitive in design.

Contemporary stove types

Broadly speaking, kilns used by modern potters can be divided into two categories. The first is the way the furnace is heated. Some ovens are electrically heated. Other furnaces are heated by burning fuel. These are known as kilns. The fuel used can range from natural gas, propane, wood and sawdust. These are some of the combustible materials used to heat a kiln.

The other distinction made with furnace types is whether a furnace is continuous or intermittent. Most smaller kilns used by individual and hobby potters are intermittent or periodic kilns. These ovens are loaded with dishes, turned on, heated, allowed to cool, and then unloaded.

In contrast, continuous furnaces remain switched on. The ware is loaded onto a moving platform and moves through one side of the kiln and out the other. The oven stays hot and the goods cool down when they leave the oven. These kilns are more typical of commercial or industrial settings where very large quantities of ceramics are produced.

Electric kilns are the most commonly used among individual potters and hobby potters. However, gas kilns are also a common way to fire ceramics. And gas ovens have been around longer than electric ovens. So let’s take a look at gas stoves first.

1) Gas ovens

Gas stoves use either natural gas or propane. The furnace consists of a chamber insulated with fireclay bricks. Heat is supplied to the chamber through burner ports at the bottom of the furnace. Burner ports are basically openings through which lighted gas flows. Gas ovens have either an updraft or downdraft design.

chimney furnaces

Typically, updraft furnaces have burner ports on either side of the base of the furnace. The gas flames travel up the side of the oven and heat the chamber. Some gas ovens have what is called a pouch wall that sits between the flame and the ceramic.

The sack wall usually consists of loosely stacked firebricks that deflect the flame. This will prevent the flames from touching and marking the dishes. The stones are placed with spaces between them to allow heat to enter the chamber through the spaces.

Regardless of whether the kiln has bag walls or not, the heat rises from the bottom to the top of the kiln. The chamber is heated en route. Updraft kilns have a chimney on the top of the kiln. The heat exits the top of the kiln along with smoke and gases produced during firing.

During firing, a draft is drawn through the bottom of the furnace to the chimney. Hence the name Aufwind. The amount of draft can be regulated with a flap on top of the chimney. When the flap is closed, the flow of the train is reduced.

Closing the damper reduces the amount of oxygen drawn into the furnace. It also increases the amount of atmospheric pressure that builds up in the furnace.

The amount of oxygen in the kiln affects the appearance of the clay and glaze after firing. And the pressure building up in the oven forces the heat to spread to all areas of the oven chamber.

So as you adjust the damper, the appearance of the ceramic will change when unloaded. These processes are an important feature of the gas oven, so it is worth looking at them in more detail now….

oxidation and reduction

The amount of oxygen in the kiln determines whether the kiln burns in oxidation or reduction. These terms can seem technical and daunting, but they don’t have to be. Here is a simple overview of oxidation and reduction:

During firing, kilns rely on the combustion of material to generate heat. Combustion requires oxygen to take place. When the furnace chamber contains a lot of oxygen, the flame feeds this supply.

However, if the oxygen supply in the furnace is limited, the flame will look elsewhere for oxygen. Both clay and glazes contain oxygen at the chemical level in the form of metal oxides. For example iron oxide or copper oxide.

When the flame cannot find oxygen in the kiln atmosphere, it pulls it out of the clay and glaze. This chemically alters the clay and glaze by reducing the number of oxygen atoms it contains. This process is therefore referred to as “reduction firing” and the furnace is said to have a “reduction atmosphere”.

This chemical change affects the color and texture of the ceramic. Glazes and clays fired “in reduction” often have a more organic, rustic appearance.

On the other hand, if the atmosphere is rich in oxygen, the flame uses that oxygen to burn. As a result, oxygen atoms are not removed from the clay and the glaze in the same way. When ceramics are fired in an oxygen-rich atmosphere, they are described as being fired in “oxidation”.

When the flap of a gas oven is opened, the draft flowing through the oven draws in more oxygen. This creates an oxidizing atmosphere.

Conversely, when the dampers are closed, the oxygen flow is reduced and the furnace goes into reduction. So, gas furnaces have the flexibility to work in either oxidation or reduction.

downdraft furnaces

Another type of gas furnace is the downdraft furnace. As with the updraft kiln, the heat enters the downdraft kiln at the bottom through gas burner openings. In contrast to the updraft oven, however, the downdraft oven does not have a chimney at the top.

Since the heat cannot escape at the top, it has to circulate back down. The hot draft travels down into the oven chamber and is effectively used to heat the ware twice. For this reason, downdraft furnaces are considered to be more efficient than updraft furnaces.

The flue of a downdraft furnace is located at the bottom of the furnace. After the draft is pushed down, it exits the kiln into a chimney via the low chimney. The chimney carries the heat, fumes and gases out of the furnace.

Like updraft kilns, downdraft gas kilns often have what is known as a “skip arc” design. Jump arch furnaces have an arched ceiling resting on straight walls and a straight base. Iron angle is then used to support the walls of the furnace. Then the entire construction is encased in an iron mantle that supports and protects the chamber.

The shape of an oscillating arc oven somewhat resembles the cross-section of a loaf of bread. The sweeping arch is achieved through the use of conical bricks. And the degree of curvature depends on how tapered the bricks are.

Most gas ovens are front-loading, meaning the door to the oven is on the front. These doors can be hinged. But they can also be free-standing. When freestanding, they are wheeled into position and clamped onto the kiln as they fire.

2) electric ovens

Electric furnaces are a recent development in furnace construction. Despite being the new kid on the block, electric kilns have become very popular. They are particularly popular with hobby potters and small pottery and ceramic outfitters.

The reason for their popularity is that they are convenient and relatively inexpensive to install and operate. Some electric ovens must be connected directly to a power supply. Other electric ovens can be connected to a household power supply.

Like gas furnaces, electric furnaces have an inner chamber insulated with refractory bricks. A refractory material is a material that can withstand high temperatures and has insulating properties.

However, the heat in an electric furnace is generated by metal coils. These metal coils or elements are positioned in grooves cut into the brick wall. An electric current flows through the elements and the metal coils heat up.

The elements are positioned around the side of the oven as they soften as they heat. At 1800F (982C) the elements actually go limp and therefore cannot be used on the top of the kiln (source).

The types of furnaces used to fire glass are designed for lower temperatures below 926°C (1700°F). Therefore, glass kilns have elements on the ceiling.

The heat then radiates around the oven cavity. Many modern electric ovens are controlled by digital oven controls. These allow you to pre-program the kiln to run through a carefully controlled firing schedule. Once you become familiar with the controls, you can program the oven and let it “do its thing”.

Although you can program an electric kiln, all kilns must be monitored when they are burning. It is not safe to leave a kiln unattended while it is burning.

oven sitter

Other, older electric ovens rely on an “oven minder” control. A stove sitter is a partially mechanical device that uses a pyrometric cone. These are specially made ceramic rods that bend when heated. Different cones bend at different temperatures.

As the oven heats up, the cone will begin to bend. The kiln is designed to shut off the kiln if the cone deflects sufficiently. Once the oven has reached the desired temperature, the cone will have flexed enough to trigger the off switch.

oxidation burn

Because electric furnaces do not burn fuel, they do not require an oxygen supply during firing. Oxygen is not burned, so no draft is required. Although oxygen is not drawn into the furnace, it is not used for combustion. Therefore, electric furnaces work in an oxygen-rich “oxidation atmosphere”.

The chemical processes that the clay and glaze go through in an oxidation or reduction firing are very different. So the difference in color and texture of pottery fired in these two atmospheres is striking. For example, if there is copper in a glaze, it will turn red in the reduction. However, the same glaze turns green in an oxidation atmosphere.

The results of firing in an electric kiln are much more predictable and repeatable. And colored glazes fired in oxidation tend to be lighter.

ventilation

Electric ovens are neither updraft nor downdraft. However, they must be vented, and some ventilation systems rely on creating a draft. During the firing process, moisture, gases and vapors are released from the heated ceramic. These must be vented from the oven.

An electric oven does not have a vent as such. However, some have a hole with a bung on the lid for venting purposes. Others don’t have a hole in the lid, but they do have peepholes on the side. The peepholes or spy holes are sometimes used to see the potter firing into the kiln. But they are also sometimes used to allow some ventilation during firing.

A simple ventilation system is a cross-draft arrangement. The stove is near an open window. And a fan is used to blow heat and fumes away from the stove.

Other more efficient venting systems include positioning a vent on or near the stove to vent the fumes. Sometimes these systems pull moisture and gases out of the bottom of the oven. This is a downdraft venting system. Other vents are positioned above the stove and pull the fumes and water away from the top of the stove.

Electric Furnace Designs

Some electric ovens have a hinged door on the front of the oven. These are referred to as front loading ovens. Other ovens have a hinged lid that opens up like a freezer. These are called top loading ovens. Electric ovens can vary widely in size and capacity. They range from a small countertop oven to a large freezer-sized oven.

Another type of oven is the “top hat oven”. In a hood kiln, the goods are loaded onto a base or plinth. The heating chamber is then lowered onto the base. Once the ceramic has fired and cooled, the chamber is lifted off and the base can be unloaded. The chamber is usually raised with a hand crank that uses a system of rollers that suspend the lid.

Cylinder kilns can be used for scaling fires and glazing fires. However, they are also a popular choice for raku firing. So let’s look at another type of kiln called a raku kiln.

3) Raku kilns

Raku is a particular approach to firing that involves removing the pottery from the kiln when it is red hot. The pottery is then placed in a closed container along with some combustible material such as paper. The combustible material will burn if it comes into contact with the hot dishes.

Since the fire takes place in a closed container, a reducing atmosphere is formed. This creates certain characteristic raku effects on the ceramic glaze. The container can be as simple as an inverted steel bucket. As long as it can withstand the heat of the pottery and flames.

When making raku, the ceramic is removed when the kiln is hot. Therefore, electric kilns are not typically used for the raku firing process. The sudden drop in temperature when the oven is opened can shorten the life of the electrical elements.

Most often, raku is made in gas kilns. A common choice for raku is propane gas. Raku kilns come in all shapes and sizes. They can be commercially manufactured front-loading or top-loading gas ovens. Or they can just be homemade raku kilns.

The top hat design is one of the most popular kiln styles used for raku. Because when the hood is folded up, the hot dishes can be easily removed and removed with tongs.

4) car ovens

Some types of ovens have a static base. With these kilns, the door is placed on the base and locked. As described above, these moveable doors can be front-loading, top-loading, or top-hat designs.

An alternative design is that the chamber of the kiln is static and the base of the kiln moves. These ovens are called car ovens because the bottom of the oven moves. And the goods drive through the furnace on a car.

Some car kilns are giant industrial kilns known as tunnel kilns. These are continuous kilns that remain on and the ware is fired as it moves through the tunnel.

However, not all auto kilns are continuous, nor are massive industrial kilns. Some kilns used by smaller potteries or schools also have a movable base.

The oven door is attached to the wagon floor. The goods are loaded onto the base, then the base slides into the oven chamber. Then the door is clamped and the oven is switched on. These furnaces are batch or periodic as they are allowed to cool before being discharged.

Another variant of wagon kiln construction is the bogie hearth kiln. Bogie hearth furnaces have a door on both sides. The cart base slides in and out of the kiln. It can be loaded at one end and run through the kiln. It can then drive out the other door and be unloaded. Hence the name shuttle kiln.

Traditional oven types

Each of the kilns described above is quite modern in its design and use. However, kilns have been used to produce useful pottery for thousands of years. And many of the features of modern kilns have been inherited from more traditional kilns. So let’s take a look at some of these original ovens….

5) climbing ovens

Climbing kilns are large hanging kilns that were first built in China around 500 AD (source). The stoves that originated in China were called Dragon stoves and are the basis from which the Japanese Anagama stoves developed.

Climbing furnaces were built along the slope of a hill and take advantage of the fact that heat rises. Some climbing kilns were simply one long tunnel not divided into chambers. These single-chamber kilns had shelves on either side of the kiln on which wares were placed.

Other climbing kiln designs had multiple chambers. Heat would flow through one chamber and then enter the next chamber. Some climbing kilns had a firebox beneath each chamber. Others had openings in the side of the kiln and fuel could be added that way. Most of these kilns were wood-fired, but some were charcoal-fired.

NEXT-EXIT at ja.wikipedia, CC BY-SA 3.0, via Wikimedia Commons

6) wood stoves

Although most large climbing kilns were wood-fired, there are many different types of wood-burning kilns. They range from giant anagama kilns to small igloo-shaped brick kilns built by individual potters in their backyards.

Despite this range, there is a common construction method used today by potters who want to fire with wood. Typically, wood-burning stoves today are built in the form of a catenary arch. The arc furnace described above has a vaulted roof supported by straight walls. In contrast, the shape of a chain arc furnace is a smooth curve from floor to ceiling.

Catenary arc furnace under construction. Wikimedia Commons.

They are usually constructed by building the arch around a supporting arched structure. The bricks are then cemented and insulated from the outside. A common type of insulation is adobe, a primitive building material made from earth, straw, grass, and water.

Once sealed and insulated, the arch is self-supporting. At this point the original supporting arched structure can be removed.

A wall is then built at each end of the arch to enclose the inner chamber. A chimney is built at one end of the arch. And at the other end an opening is left to feed wood into the kiln during firing.

In simple designs, the fire is positioned at only one end of the stove. However, sometimes a firebox is built to separate the fire. The fire at one end and the chimney at the other create an updraft.

Burning the wood stove

Using a wood stove is a lot of work. The fire has to be stoked continuously to keep the temperature in the kiln high enough. It usually takes two or three people working together to keep the fire going. Often the kiln burns for about 72 hours.

Pottery that is fired in a wood-fired kiln often comes into the kiln unglazed. Because the hot ash from the burning wood settles on the pots and creates a natural glaze. Also, the effect flames touching the pots directly can leave a beautiful, unpredictable finish.

7) Soda Stoves

Like wood-fired ovens, soda ovens are made of brick and are usually large enough to walk inside. They are typically made with a spring arch or catenary arch design with a chimney at one end. Soda stoves are often powered by natural gas, although they are sometimes wood-fired.

During the firing of pottery, a solution of sodium is sprayed into the kiln. It is often sprayed into the oven when the oven has reached around 2336F (1280C). This is the temperature of cone 9, so soda firing is suitable for firing stoneware.

The solution is made by dissolving sodium bicarbonate in water. This vaporizes, coats the pottery and reacts with the clay to form a glaze.

The icing often has a nubbed orange peel texture. And is characterized by changes in color and tone where the sodium solution hits the pots. This is called the lightning effect.

Soda ovens often don’t have a door. Instead, the kiln is bricked up at the entrance each time it is fired. The sodium solution is sprayed through a hole in the door where a brick was removed. Once the kiln has cooled, which usually takes a few days, the “door” is removed brick by brick.

8) Beehive ovens

One of the earliest kiln types is the Beehive kiln. These are large brick structures used for the industrial production of pottery.

Beehive kilns are squat round kilns with straight walls and a domed roof. They actually look a bit like the top half of a beehive. Beehive kilns can be downdraft or updraft kilns. Almost all beehive ovens are no longer in operation.

However, there are a small number of exceptions to this. An exception is the downdraft beehive kiln at Errington Reay Pottery in Northumberland, UK. This kiln produces salt-glazed pottery and is coal-fired. (Source)

Large beehive furnaces were usually downward facing and had a chimney. These kilns were heated by placing fuel in a firebox on the side of the kiln. The heat then entered the oven and was conducted up through a bag wall. When heat reaches the closed ceiling, it is forced to circulate back down.

The heat and fumes then exit the kiln through a chimney in the floor. It travels underground through the chimney until it reaches a chimney separate from the furnace.

Jason Woodhead, CC BY 2.0, via Wikimedia Commons

Other simple beehive stoves have a simple updraft design. In these ovens, the heat rises through the oven and exits through a short flue on the top of the oven. These kilns are similar in design to another early kiln design called the bottle kiln.

Because of their similarity, the terms beehive and bottle oven are sometimes used interchangeably. There are some differences though, so now let’s look at bottle ovens….

9) Bottle Ovens

Bottle ovens were developed later than beehive ovens and have a different shape. As the name suggests, bottle ovens are tall bottle-shaped structures. Like beehive ovens, they are traditionally used for industrial-scale production. They fired large batches of crockery at once, and one factory aimed to fire the kiln once a week.

However, bottle furnaces are intermittent, meaning they fire, cool, and discharge. With most industries today relying on tunnel/auto conveyor ovens, bottle ovens are increasingly a historical curiosity.

Another reason bottle ovens fell out of favor is that they were usually coal-fired. And coal has contributed significantly to air pollution.

FDleyda, CC BY-SA 3.0, via Wikimedia Commons

A distinction is made between bottle ovens and bottle ovens. Bottle kilns were used for simmering and glazing fireware. While bottle kilns were either muffle kilns or calciners.

Muffle furnaces were bottle furnaces that were fired at a lower temperature. They were used to enamel pottery or to fire decorative designs onto already glazed pottery. Calcining kilns were used to burn ingredients that were ground like grog and added to a clay body.

Bottle ovens have a vent at the top, so it would be reasonable to assume they had an updraft design. While some are updraft, others had an inner chamber designed with a downdraft flow in mind.

Primitive Types of Kilns

In this section I will consider two primitive types of kilns; Pit firing and sawdust kilns. Pit fires and sawdust kilns, both sometimes made of metal barrels, which is hardly a primitive material.

However, the original pit firing technique dates back to 800 AD and modern pit firing and sawdust kilns are based on these original principles. Pit firing is still a popular low-tech method of firing clay, as are sawdust kilns.

10) pit firing

One of the earliest, primitive types of kilns is the shaft firing. In pit burning, goods are placed in a pit in the ground along with combustible material. Some of the fuels traditionally used are wood, straw and manure.

The pit is lined with combustible material and the dishes are laid on top. It is then covered with more material and once it is completely covered the fire is lit. The fire is allowed to burn until the combustible material is consumed by the flames. It is then allowed to cool and the ware is removed, cleaned and often polished.

A kiln in the usual sense of the word is not used in a pit fire. However, it uses the same principles of kiln firing.

Kilns are basically insulated heating chambers. While pit fires are dug rather than built, the walls of the pit form a chamber. And this chamber is insulated by the floor, not unlike an insulated kiln.

In addition, like other fuel kilns, pit firing consumes fuel. However, unlike most kilns, a pit fire places the fuel on top of the kiln. And the insulating walls of the pit are under fire. As a result, a lot of heat is lost from the pit as it radiates upwards. Because of this, pit firing is not the most efficient way to fire pottery.

However, heat in a pit fire can be contained. Once the fire is well established, the pit can be partially covered with sand or manure. This retains heat and helps create a reducing atmosphere.

One way to adapt this approach is to use a barrel as an alternative to a pit in the ground. This is known as a barrel oven.

11) Sawdust Furnace

The principle of a sawdust furnace is similar to that of a pit fire. As in a pit fire, the ceramics to be burned are buried in the combustible fuel, the sawdust.

As with each of the types of kilns described above, sawdust kilns differ in their construction. However, many sawdust kilns are built of brick. Their design can be very simple, consisting of one layer of building bricks for the base. With brick walls to build a four-sided simple box-shaped stove.

A layer of sawdust is placed on the bottom of the kiln and the pieces are positioned on this layer. These are then covered with more sawdust packed between the pottery. Further layers of ceramic can then be laid on top of the first layer buried in the sawdust.

A simple shelf can be used to separate the layers of pottery. This shelf can be made from a metal mesh that is held in place by sandwiching it between the wall bricks.

Once the brick box is filled with sawdust and pottery, the sawdust is lit. Once the fire is established, the top of the kiln can be covered with a refractory lid. The atmosphere in the furnace can be regulated by a gap between the lid and the walls. There must also be some gaps in the wall of the oven to allow some oxygen.

The stove is allowed to burn until all the sawdust has burned away. As with pit firing, an alternative sawdust kiln design is to use a metal barrel.

Final Thoughts

There are many types of kilns that have been used over the years and are currently available to potters. These ovens are so diverse that using the word “oven” to refer to them all is almost a stretch. For example, consider the difference between a simple sawdust kiln and a continuous kiln for mass production. While very different, they still share the ultimate goal of burning clay. With so many kiln types available, you’re bound to find a firing method that suits you.

Properly built and operated brick kilns are without a doubt one of the most effective methods of charcoal production. They have proven over decades of use to be low in capital cost, moderate in labor and capable of surprisingly good yields of high quality charcoal suitable for all industrial and domestic uses.

There are many designs of brick kilns used around the world and most are capable of giving good results.

The brick kiln must meet a number of important requirements in order to be successful. It must be simple to construct, relatively insensitive to thermal stresses during heating and cooling, and strong enough to withstand the mechanical stresses of loading and unloading. It must be rain and weather resistant for six to ten years.

The oven must allow controlled air entry at all times and be able to be hermetically sealed to prevent air entry during the cooling phase. It must be of reasonably light construction to allow for relatively easy cooling and still provide good thermal insulation for the wood being carbonized, otherwise the formation of cold spots due to wind action on the furnace walls will prevent and lead to proper burning of the charcoal excessive production of partially carbonized pieces of wood (“brands”) and low yields. The brick kiln’s ability to retain the heat of carbonization is an important factor in its high wood-to-charcoal conversion efficiency.

Photo. 13. Reinforced Concrete Missouri Kilns. Note steel doors, charcoal stock. Missouri, USA Photo A. Baker

The designs of traditional brick kilns have been refined over many centuries, but other types of brick kilns are also used, which have been the subject of systematic experimentation in recent years to improve them. These are the Brazilian beehive kiln, the Argentine half-orange kiln, the European Schwartz kiln, and the USA’s Missouri kiln. The first, second and fourth examples burn part of the charged wood within the furnace to char the remainder. The Schwartz kiln uses the hot flue gases from an external fire grate passed through the kiln to provide heat to dry and heat the wood to start carbonation. The Schwartz furnace requires significant amounts of steel for bandages on the furnace chamber and steel grilles and doors for the furnace. Since in practice its yield (when the firewood is counted) is not superior to the others, it cannot be recommended for widespread use in the developing world. The fourth type of stove proven in practice is the Missouri stove, which was developed in the United States and is still used. It is usually made of reinforced concrete or concrete blocks and has steel chimneys and doors. Its yield is similar to that of the Argentine and Brazilian kilns. It is equipped with large steel doors that allow mechanical devices to be used for loading and unloading. It has two disadvantages for use in developing countries: it requires a lot of steel and cement for its construction, both expensive and usually imported goods, and it is not as easy to cool as the other furnaces. It is therefore more suitable for use in moderately cooler climates where the materials and skills for steel and reinforced concrete construction exist and low air temperatures allow for easy cooling. It is attractive where working front loaders etc are readily available.

The advantages of Argentinean and Brazilian ovens are:

– They can be built in medium and large sizes. – They are constructed entirely of soft-fired, locally produced adobe/sand bricks and adobe mortar. They require no steel except for a few flat bars over doors and as reinforcement at the base of the dome in the case of the Brazilian blast furnace. – They are sturdy and not easily damaged. They are not easily damaged by overheating; they can stand unprotected in the sun and rain without corrosion or damage and have a service life of 5 to 8 years. – The bricks set in mud can be recycled and reused when moving the kilns. – Firing control is relatively easy, especially with the Argentine kiln. – The kilns are easy to cool with clay mud and easily hermetically sealed while cooling. A recent development in rapid cooling is water injection in dollars. – The operating systems for furnace groups (battery) are well researched and standardized so that labor and resource efficiency is maximized. – The charcoal produced is suitable for all uses including household, metallurgical and activated charcoal manufacture.

The main disadvantage of these two types of kilns is that they are not suitable for the recovery or recycling incineration of tar or gas as a by-product. This increases air pollution and slightly lowers potential thermal efficiency. However, in fairness it must be added that there are no commercially proven brick kilns capable of easily recovering tar without the need for steel components which greatly increase the cost and complexity of the kiln.

Photo. 14. Build a half-orange brick oven. Note the wood radius bar, the way the bricks are laid, and the double layer of brick over part of the wall to reinforce the shell of the stove. Argentina. Photo. J.Bim.

A free area of ​​​​4 000 – 5 000 m² is required for a battery of 12-14 stoves. The wood obtained from this clearing, with the exception of logs that could be used for sawing or poles, is used as firewood. The site on which a furnace is to be built must be slightly compacted and then backfilled to bring it back to the level of the entire site to allow for easy drainage of water from the furnace.

The design of this oven is shown in Figure 5. The oven is built entirely of bricks. Charcoal and mud are used as mortars, usually without iron or steel support at any point. The shape is hemispherical with a diameter of about 6 m (range 5-7 m). The size of the bricks is 0.24m x 0.12m x 0.06m. To build a kiln requires a total of 5,500 to 6,000 bricks, accounting for breakage during construction.

The oven has two doors that are diametrically opposed. The line of doors must be perpendicular to the direction of the prevailing winds. The height of each door is 160-170m, the width at the bottom is 1.10m and at the top 0.70m. One door is used to load firewood into the stove while the other is used to unload the charcoal. The kiln doors are closed with the bricks built up after charging is complete and both are opened when the carbonation process is complete. This is a simple process that is repeated each time the kiln is loaded. It’s all about laying bricks over bricks and covering them in mud. Approximately 100 bricks per door are required and can be used until the bricks begin to break from handling. The top of the oven has a hole (referred to as the “eye”) approximately 0.22m to 0.25m in diameter. Around the base at floor level are ten equally spaced holes (0.06m height x 0 ,12 cross section). These holes are air inlets and the eye is the smoke outlet. The foundation consists of a double row of three-layer bricks set in clay mortar.

Photo 15. A half orange oven just built. Note the reinforced door to prevent damage to the oven when loading and unloading. Notice how bricks are cross-joined in a double skin wall around the door compared to a single skin wall shown in the upper right corner. Argentina. Photo. J.Bim.

Fig. 5. Argentine half-orange or beehive brick kiln. The oven is hemispherical in shape with two opposing doors to facilitate loading and unloading and to provide ventilation. Shell is most often a single sheet of brick with a double layer surrounding each door. Additional brick pillars on each side of the door are common. About 6,000 ordinary hand-made bricks are needed, set in clay mortar and finely mixed with charcoal.

Photo. 16. Closure dome of half an orange oven under construction. Note the radius rod and the orientation of the bricks in the dome. Argentina. Photo. J.Bim.

Photo. 17. Partially Completed Half Orange Oven and Completed Water Storage Tank. Note the cross-bonding of the masonry in the two-tier part of the wall and the way bricks are laid in the single-tier part. Argentina. Photo J. Bim.

The firewood to be used is cut to a length of about 1.00 m to 1.30 m with a minimum diameter of 0.05 m and a maximum diameter equal to the width of the door. Firewood coming from the forest via transport (trailers or animals) must be placed as close as possible to the loading door. An air drying time of at least four to five weeks is recommended. This depends on local weather conditions. Manual or mechanical means may be used to debark the wood. Many barks simply fall off during the drying period. The stove can be loaded with about 30 tons of air-dried wood with a moisture content of 25% and an average specific weight of about 850 kg/m³.

Loading through the door near the firewood pile is the most convenient. This operation requires two men and the time it takes to complete should not exceed six hours. Stringers over which the firewood will be placed must be prepared with small pieces of wood with a diameter of no more than 0.08-0.10 m. This is to avoid direct contact of the firewood with the ground. The logs with a larger diameter must be placed in the middle, where higher temperatures are reached for a longer period of time. The firewood in the stove is stacked vertically to a height of 1.20 m (wood length). Logs are placed in a horizontal position above the vertical logs, allowing the stove to reach its full capacity, as shown in Fig. 5 and photo. 18 of a partially loaded furnace. It is particularly important to ensure that the holes in the bottom of the furnace (air inlets) are not blocked. Some dry wood is placed on top of the charge under the eye to help light the stove. When the loading is complete, both main doors must be sealed with mud-covered bricks.

Photo. 18. Typical charge of half an orange oven, Argentina.

All holes on the bottom and on the furnace eye must be open. A few pieces of burning charcoal, dry leaves and small twigs are thrown through the eye to ensure the firewood burns well. After a few minutes, a visible stream of dense white smoke begins to come through the eye. This phase represents the first distillation and the wood loses its water content in this phase. The white smoke lasts for a few days (depending on water content) and then begins to turn blue, indicating effective carbonation is underway. This process is controlled by opening and closing the air inlets at the bottom of the oven. No flame should appear through the eye. When the carbonation process is complete, the smoke becomes almost as transparent as hot air. At this point, the holes at the base must be plugged with mud or covered with earth and sand. This phase is called “flushing”. After this phase, the upper eye hole is closed, which starts the cooling phase. Cooling is accelerated by throwing mud (diluted with water) onto the stove. Aside from cooling, this helps to cover any indentations or cracks in the walls, preventing air from getting in. The slurry of mud and water needs to be applied about three times a day.

Before the charcoal is discharged, sufficient water must be available when the stove has cooled down sufficiently in order to avoid reignition when the stove door is opened. A barrel with about 200 liters is enough for an oven. The furnace is unloaded by two or three men. The charcoal is conveniently removed from the kiln with a special fork known as a stone fork. It has 12-14 teeth and a tooth spacing of 0.02 m. This allows most of the fines (less than 20 mm) to fall through and remain in the kiln. The charcoal is placed on a four foot piece of canvas and carried out of the kiln by two men.

A typical schedule looks like this:

Charging 6 hours Burning 6-7 days Flushing 1-2 days Cooling 3.4 days Discharging 3.4 days

A total of 13-14 days should be enough to complete a cycle of producing 9-10 tons of charcoal with a 7m diameter kiln.

For a 6m diameter kiln the approximate yield per fire is 7.5t or 15t/month. The annual average of the largest charcoal producer in Argentina, Salta Forestal S.A., was 3.75 tons of firewood per ton of charcoal in 1978. Both wood and charcoal are always weighed. Lower yields can be obtained when using wood with lower density or higher moisture content.

During the first three to four burns, when the bricks and soil dry out, the kiln is considered “green” or “immature” and yields are slightly lower. The service life is at least five years and no special maintenance is required. Whenever small cracks appear on the walls, small pieces of brick and mud are used to close them.

The usual number of kilns per battery is 10-14 depending on the forest type, area affected and transport distance. A water supply is also required. A tank with a capacity of about 3,000 liters can be made from bricks and cement. A battery is operated by 3 men: a burner and two helpers.

The type of brick used for kilns is important. An ideal brick is fairly porous, has good resistance to thermal shock, and is a good insulator. The kiln walls must protect the wood to be carbonized from excessive heat loss, particularly from wind, and yet conduct heat during the cooling phase to allow rapid cooling.

For reasons of economy, bricks should be made and fired near where the oven batteries are built. A sandy clay with a clay content of about 65% is produced. To increase the porosity of bricks, about 20% sawdust can be added to the raw clay mix. Dry bricks are self-burned in large piles using wood fuel.

Dense, machine-made, high-strength bricks used in permanent urban buildings are not as suitable, being more prone to heat cracking. They also cost much more delivered than bricks made and fired on site.

Mud (clay) supplies are important. A good type of mud will have a fairly high sand and organics content and will not shrink or peel as it dries. It shouldn’t dry too hard either, as the clay will need to be scraped off the kiln periodically as the thickness builds up after several chilling cycles. This clay can be recycled.

Photo. 19 shows a typical brick factory. The bricks are made of sandy clay dug from an alluvial bank of a nearby stream. The compacted wet mix is ​​cut with a spade to form each brick and placed to dry as shown. The dry bricks are stacked in a large pile of 20,000 to 30,000 bricks. The stack is built with internal channels opening along the top of the stack and emanating from fire holes built along the base of the four sides. When the heap is complete, log fires are lit in the fire holes and poking is continued for 10-12 days or more to raise the temperature of the heap to around 900°C. The heap is then allowed to cool and disassembled. Well-fired bricks are separated from under-fired ones, which form the outside of the stack. The underburned bricks can be burned again in the next batch or used for inferior construction.

Typical brick factory. Salta, Argentina. Photo. H Stand.

Photo. 20. Brick making using hand methods and fired in a batch with chimneys using wood fuel. Embarcación, Argentina. Photo. H Stand.

The Brazilian charcoal industry, which forms the base of this country’s charcoal-iron industry, is one of the most successful charcoal industries based on the existing brick kiln technology in the world today. Any country that wants to build its charcoal production on a solid footing can learn a lot from the Brazilian experience. The description below closely follows H. Meyers’ report on the Brazilian charcoal-iron industry. (23)

The kilns that are widely used and successfully operated in Brazil and especially in the state of Minas Gerais are internally heated, fixed batch kilns. The major iron and steel companies operate several thousand of them. They are round, have a domed roof and are built of ordinary bricks. The circular wall is in full contact with the outside air. This type of kiln is called a “beehive brick kiln”. See Figure 6.

Fig. 6. Brazilian beehive oven

Furnace diameter 5 m Furnace nominal volume 48.94 m³ Furnace useful volume 45.31 m³ Number of air inlet openings 18 Number of chimneys 6 Number of outlet openings 6 Number of emergency exit openings 50 Number of bricks required 8 500

The Brazilian beehive oven has the following advantages:

– The gases flow through the wood batch. The heat contained in the gases is partly used in wood drying and carbonization. – Good yield, up to 62% volume = 1.6 st. wood/m³ charcoal with proper operation. – Low cost, approximately $700 (1978) including truck access roads. – Simple construction. Two men can build a furnace in 6 days. – Simple materials. 8,500 fired clay bricks with only one steel band for the dome. No concrete foundations. – Long life span. Up to 6 years at the same location. Can be dismantled and rebuilt elsewhere without significant loss of bricks. – Carbonation time of 9 days with a production of 5 t/cycle. – Uniform carbonation. – Uniform cooling as the walls are in full contact with the outside air. – Short operational plan; about nine days. This time could be shortened by forced cooling with fine water jets. – Uniform control of the internal combustion through 18 air intake openings for the entry of the necessary combustion air. – Simple and inexpensive maintenance, few repairs, no wall cracks, no electricity, very little water, approx. 100 liters per oven and batch.

Photo. 21. Battery of brick kilns at different stages of the firing cycle. Minas Gerais, Brazil. Photo. J.Bim

A 7-furnace battery requires a site with the following dimensions:

Length … 70 m

Width … 25 m.

This area is necessary for the seven kilns, storing and curing the charcoal for two days, access roads for the trucks bringing in wood, storage areas for a certain amount of wood, access roads for the trucks removing the charcoal, and a truck turning area. In order to prepare the ground for the ovens and the charcoal loading platform, earthmoving with crawler tractors is always required. The terrain must be slightly inclined to allow rainwater to drain away. Two or more oven batteries are often combined in one line. This is the case when the surrounding forests are extensive and large amounts of firewood are available at close range. All batteries should consist of seven ovens and the total area will be many times the area given for one battery. Building large numbers of batteries allows for good centralized operation and monitoring, resulting in good charcoal quality and yield.

The total number of kilns in a center must be limited to 35 or 42 because of the fumes from the chimneys which, while not harmful to health, are irritating to the eyes and lungs. Charcoal production centers should therefore be located at least two kilometers from villages. The prevailing wind direction should also be taken into account.

When laying out a battery, the center line is first marked on the ground. The centers of the ovens are eight meters apart. The center of each kiln is marked with a two meter long tube that is driven vertically into the ground. The inner circumference of the furnace is measured at a diameter of 5.00 m and the outer circumference at a diameter of 5.40 m.

The two one meter wide doors, the foundations for the door pillars, the six chimneys and the furnace foundations are marked and the foundation trench is excavated. The kiln foundations must extend four layers of bricks below ground level and one layer of bricks above ground level. All lanes must be laid carefully and evenly.

Fig. 7. Section through a beehive furnace showing construction techniques.

A 2.50 m long wooden pole is fastened horizontally to the central tube, which serves as a guide for building the wall. When building the walls, openings for the doors should be left, but pillars should be built for them. Mortar consists of ten parts clay and one part charcoal, previously sifted. When laying the first row of bricks, the necessary openings for the air vents should be left, three between each pair of chimneys, a total of 18, symmetrically distributed. The sizes of the air connections are: width: 0.10 m; Height: 0.08 m. The chimneys are built at the same time as the wall. The internal dimensions of the chimneys are: 0.12 m x 0.10 m. When building the oven wall, care must be taken to ensure that the various stone layers are level. A wooden guide should be used. After laying five layers of bricks, two emergency vents should be left vertically above the two air intake vents located next to the chimneys. After a second shift of five aisles, a central emergency opening should be left. After the next five layers, two emergency openings should be located vertically above the first. The emergency openings are 0.07 m x 0.07 m. Each pair of stacks should have five emergency ports. When the wall has reached a height of 1.60m, the steel angle lintels should be placed on the jambs and construction of the perimeter wall continued. This completes the two door openings, 1.00 m wide and 1.60 m high. After loading the stove with firewood and closing the doors with bricks, the burner should leave an air intake hole in each door wall at the same level as the others.

The total height of the vertical wall is 1.80 m and the last row of bricks must be leveled well. On top of this another row of bricks is to be laid with mortar and laid against this last layer four loosely bolted segments of steel strip.

The first row of bricks for the dome should be adjusted by cutting the topmost bricks of the wall on its edge. The central tube should be removed and replaced with a stake or short stake driven into the ground flush with the last layer of bricks in the foundation. To this slatted frame should be fixed the beam compass with a guide length of about 3.10 m for the construction of the dome, which will be built with a minimum of mortar half a brick thick. The strength of the dome is ensured by the pressure of the stones against each other.

Emergency openings 0.07 m x 0.07 m should be left on the fifth row of bricks, another ten on the tenth row and six on the fifteenth. The ignition hole, triangular and 0.10m x 0.10m x 0.10m, should be left at the top of the dome. After the dome was completed, the steel strip was to be tensioned, the walls plastered with fine clay mortar, and the dome brushed with clay slurry to close all cracks and openings.

Hillside type beehive brick kiln

Figure Fig. 8.1

Figure Fig. 8.2

Furnace diameter 4.0 m Furnace nominal vol. 24.8 m³ Effective kiln volume 21.6 m³ Number of air inlet openings 1 Number of chimneys 3 Number of exit openings 4 Number of emergency exit openings 4 Number of bricks 2 000

A variant of the beehive brick kiln is shown in Figure 8. It is a circular oven, 4 meters in diameter, built into a slope or mound forming its side and rear walls. This is called a slope-type oven. It uses significantly fewer stones. Many thousands of these kilns are in operation in Minas Gerais and other states in Brazil. They are very popular with the small independent charcoal producers. Their operation is slightly simpler than the Beehive Brick Kiln as they only have one air port to control compared to 18 for the Beehive Kiln. The chemical and physical composition and yield of charcoal produced in hillside kilns is very close to that of charcoal produced in beehive kilns. No significant differences between the qualities of the two types of charcoal are reported.

Photo. 22 Brazilian hillside oven installed on a hillside, sealed and ready to light. Brazil. Photo. J.Bim.

A circular area with a diameter of six meters is to be cleared on the furnace site and a circle with a diameter of four meters is to be marked. The 1.40 m high furnace chamber was to be excavated and the floor leveled well. The position of the door with a width of 0.60 m and three or four chimneys (0.35 m x 0.35 m cross-section) with a certain distance between them should be marked. (Fig. 8).

The openings for the door and the three chimneys were to be dug into the bank to a height of 1.40m and the chimneys raised to a height of 1.50m with bricks on top.

A picket was to be driven into the center of the floor, with the beam compass attached to build the domed dome. The guide length corresponds exactly to the inner height of the dome, i.e. 2.45 m.

If the height of the embankment or hill is not sufficient, the wall height of 1.40 m must be achieved by adding a few layers of half-stone. The mortar used is the same as for the dome. A brick arch was to be erected over the doorway. The necessary bevel cut for the dome overlay should be made around the dome base.

The dome is made of half brick, with a little mortar, from ten parts loamy earth and one part sifted charcoal. Four symmetrically located emergency holes, 0.07m x 0.07m, should be left in the tenth row of bricks, and after ten more rows, four more holes 0.08m x 0.08m should be left. An ignition opening of 0.10m x 0.10m should be left at the top of the dome.

The arch over the doorway is made of bricks laid down their sides and connected to the dome brick structure. An opening is made in the bench near the oven door to facilitate handling of the firewood. After loading the stove with wood and closing the door, a porthole should be left for air intake at the level of the sphere. Dig the necessary water drainage ditches around the stove.

From time to time the excess clay formed by successive brushing of clay slurry should be shaved off. This improves the cooling of the charcoal.

The structure of the stove can be damaged by impacts; B. by trucks, carts, logs, etc., and this should be avoided. The door pillars should be protected by two corner bars. Bricks that have fallen or come loose from the walls should be replaced and rammed down.

The outlet and escape hatches are to be closed with wedge-shaped stones without mortar and coated on the outside with clay slurry. The chimney drafts must be carefully cleaned with a long, flexible wooden stick.

The chimneys must end above the steel band of the dome to reduce corrosion from exhaust gases. In any case, corrosion of the belt takes place after a few years. The steel band of the dome should be regularly tensioned and any corroded parts replaced.

The oven floor should always be kept level. If necessary, some moist clay soil should be introduced and tamped down. If the bottom of the kiln pedestal cracks, it must be closed to avoid damage to the kiln foundations, for example from water ingress. The drainage ditches are to be kept clear at all times.

Stray animals should be kept away from the charcoal factory by fences with barbed wire or other material.

The Missouri stove was developed over many years in the hardwood forest zones of the state of Missouri, USA. Das Design war um 1960 mehr oder weniger standardisiert, und empfohlene Abmessungen und Konstruktionsdetails wurden von Pitts Jarvis in “The Wood Charcoal Industry in the State of Missouri“ (English Series Bulletin No. 48, Engineering Experiment Station, Columbia, Missouri). Der Ofen in Missouri ist an ein strenges Winterklima, hohe Arbeitskosten, mechanisiertes Be- und Entladen angepasst und verwendet Konstruktionsmethoden und Materialien, die in den Entwicklungsländern möglicherweise kostspielig und schwer zu beschaffen sind.

Der Missouri-Ofen muss strengen eisigen Winterbedingungen standhalten, die für leicht gebaute Ziegelöfen sehr zerstörerisch sein können. Zu den größten Gefahren zählen das Aufwirbeln von Baugrund und das Gefrieren von Wasser in Mauerwerksrissen. Eine starke Verstärkung von Fundamenten, Wänden und Dach sowie eine monolithische Stahlbetonkonstruktion sind unerlässlich, um eine schnelle Verschlechterung des Ofens zu vermeiden. Niedrige Lufttemperaturen erfordern auch eine gut isolierte Struktur, um Wärmeverluste und eine übermäßige Produktion von unverbrannten Marken in der Ladung zu reduzieren.

Der Missouri-Ofen ist so konzipiert, dass er einen einfachen Zugang für mechanische Geräte wie kleine Traktoren, Lastwagen und Frontlader ermöglicht, damit der Ofen einfach und schnell be- und entladen werden kann. Dies ist eines der attraktivsten Merkmale, wenn die Arbeitskosten hoch sind.

Typische Brennöfen in Missouri haben ein Volumen von etwa 4 m³ bis 350 m³. Das optimale mittlere Volumen scheint etwa 180 m zu betragen. Diese Größe ergibt relativ niedrige Kosten pro m³ Volumen und macht den Ofen dennoch nicht so groß, dass er schwierig zu kontrollieren ist. Ein solcher Ofen ist etwa dreimal so groß wie ein halber Orangen- oder Bienenstockofen. (Siehe 7.1 und 7.2).

Der Ofen hat einen rechteckigen Grundriss (Abb. 9), ist etwa 7 m breit und 11 m lang. Die Wände sind etwa 2,5 m hoch, um den Zugang zu mechanischen Geräten zu ermöglichen. Die maximale Höhe des gewölbten Daches beträgt ca. 4 m über dem Boden. Die Wände sind etwa 250 mm dick. Foto 13 zeigt eine typische Gruppe von Brennöfen in Missouri.

Die Materialmengen, die zum Bau eines Missouri-Ofens benötigt werden, sind beeindruckend und müssen von potenziellen Bauherren sorgfältig kalkuliert werden. Tabelle 5 vergleicht das Material, das für einen Brennofen in Missouri mit einer Kapazität von 180 m³ (50 Stränge) benötigt wird, der mit vier Brennöfen brasilianischer Bauart mit einer Gesamtkapazität von etwa 200 m³ zusammengesetzt ist. Der größte Nachteil des Missouri-Ofens in Entwicklungsländern ist sein hoher Gehalt an Zement und Stahl, die beide normalerweise importiert und oft teuer und schwer zu beschaffen sind. Die Fähigkeit, mit ihnen in einem typischen Bereich der Holzkohleherstellung zu arbeiten, kann begrenzt sein. Der Vorteil, dass der Ofen leicht mechanisch be- und entladen werden kann, muss von Fall zu Fall sorgfältig beurteilt werden.

Die Kosten für Stahl können in Entwicklungsländern kritisch sein, daher ist es wichtig, die relative Effizienz zu vergleichen, mit der er von verschiedenen Ofentypen verwendet wird.

Ein nützlicher Index ist die Berechnung der Produktion von Holzkohle pro kg. von Stahl, der im Bauwesen für verschiedene Arten verwendet wird. Offensichtlich sind Erdhügel und -gruben am besten, da sie überhaupt keinen Stahl verwenden und der Index unendlich ist. Wir können den Missouri-, den Brasilianer-, Bienenstock- und den tragbaren Stahlofen sinnvoll vergleichen. Unter Verwendung von Werten für Holzkohle, die über die Lebensdauer des Ofens bei effizienter Nutzung produziert wird, die diesem Handbuch entnommen wurden, lauten die Indizes wie folgt: Brasilianischer Bienenstock: 8.200 kg Holzkohle pro kg Baustahl; Missouri-Öfen 550 kg Holzkohle pro kg Baustahl und tragbarer Stahlofen: 330 kg Holzkohle pro kg Baustahl. Der Bienenstockofen nutzt somit Stahl 15-mal effizienter als der Missouri. Der Missouri verwendet Stahl etwa 1,5-mal so effektiv wie der tragbare Ganzstahlofen.

Der Missouri-Ofen ist eine technische Struktur, die sorgfältig gemäß den Konstruktionsspezifikationen konstruiert werden muss. Ein Einsturz der Struktur wird eintreten, wenn unzureichender Bewehrungsstahl verwendet oder nicht richtig positioniert wird. Wenn kein Blähschieferzuschlagstoff verwendet wird und/oder der Ofen überhitzt wird, bricht das gewölbte Dach aufgrund des Nachgebens des Bewehrungsstahls im Dach und in den Wänden zusammen. Collapse of such a costly kiln would be an economic disaster for most producers and, if skilled construction labour and the specified materials are not available, construction should not be undertaken.

Table 5 – Use of materials for Missouri kiln and four equivalent beehive kilns

Material One 180 m³ Missouri kiln Four beehive kilns total volume 200 m³ Concrete using expanded shale aggregate 46 m³ nil Common bricks nil 34 000 Steel tons total 4.4 m.t. 0.58 m.t. Reinforcement 1.56 m.t. 0.58 Door frames 0.74 m.t.

Air ducts 0.34 m.t.

Doors 1.11 m.t.

Miscellaneous 0.20 m.t.

Stoneware flue pipes 37 m of 150 mm diameter nil

The walls, floor and roof of the kiln are of poured reinforced concrete construction with doorway frames of steel cast in place. The recommended aggregate is lightweight expanded shale to improve thermal insulation and resistance to heat spelling. Expanded shale aggregate is usually not available in most charcoal making areas of the developing world. The double doors of the kiln are a critical and costly component. They must be easy to open, yet capable of being sealed tight to prevent air leaks during cooking. Rolled steel plate 10 cm thick is recommended to reduce warping which prevents proper sealing. The seal is made by bolting the doors to the steel door frame embedded in the walls and to each other where they meet. 18 mm (¾ inch) bolts are recommended. Each door leaf weighs more than a quarter of a ton so they must be well hung on heavy duty hinges to stand repeated opening and closing.

The kiln is fitted with eight chimneys made from 15 cm (6 inch) stoneware drainage pipes. Steel or cast iron pipe can also be used. Each chimney is about 4.5 m high and is supported by brackets off the walls of the kiln.

Kilns are usually grouped in batteries of three to six or more. This makes for economical use of equipment and labour. To reduce pollution sometimes in the U.S.A. batteries of kilns are connected to a central flue and after-burner system feeding to a common chimney stack. The normal flues are dismantled and the smoke outlets are connected by horizontal steel flues to the after burner. The after burner, which is fuelled by oil, is needed to heat the incoming flue gases and ensure that they are totally reduced to carbon dioxide and water by mixing with the oil burner flame before being discharged via a fan into the atmosphere. Obviously, such a system can only be economic where the price of charcoal is high, making it economic to burn some fuel oil in order to produce charcoal.

The conversion yield of Missouri type kilns is similar to brick kilns of the Brazilian or Argentine type when all types are operated under optimum conditions. The high construction cost of the Missouri kiln is traded off against the labour savings brought about by using mechanical loading and unloading equipment. Theoretically, since a large Missouri kiln has the volume of about 4 Brazilian type kilns, there should be additional savings in burning labour costs. In practice, the Missouri kiln does not achieve its full potential, especially in warm climates, because of its slow cooling. There is a close relation between kiln surface area, volume, ambient temperature and cooling rate. Very large kilns and pounds are slow to cool and, if construction cost is high, they represent an inefficient use of capital because of the reduction in output which slow cooling implies.

The burning of the kiln is controlled in a similar way to the portable metal kiln. The gas circulation system is rather similar. Yields are usually better because the better thermal insulation and greater ratio of volume to surface area means that the endothermal heat of carbonization is better utilised and the kiln is not so subject to the cooling effects of winds and rain as the uninsulated metal kiln.

Missouri kilns are usually equipped with thermocouples to read the temperature at several points within the kiln. This is important with such large kilns as it enables cold and hot spots to be readily detected and corrective action taken by the operator by closing or opening air vents along the base of the kiln. The cooling process can also be checked and the kiln opened only when the temperature of the charcoal is low enough. This avoids fires which, in such large kilns, are not easy to control even with mechanical handling.

A crew of two men are needed for loading and unloading, equipped with a front-end loader and truck. One operator per shift is sufficient to control the burning and, providing thermocouples are used, one man per shift can supervise a number of kilns.

The kiln cycle is usually about 25 to 30 days depending on cooling rates. The capacity of two 180 m kilns is about equal in wood consumption to a standard battery of seven Brazilian kilns. But because the cycle time is different, the utilization of labour is not as efficient as it could be unless there were more kilns to a battery. Utilisation of mechanical equipment is not optimised unless the number of kilns is sufficient to keep it working more or less continuously.

The cycle time of a Missouri kiln in a warm climate is at least one month, made up as follows:

Loading 3 days 2 men plus machines Burning 7 days 2 men on 12 hour shifts or 3 men on 8 hour shifts Cooling 21 days (min.) 1 man part-time supervision Unloading 2 days 2 men plus machines.

The total time is 33 days. If machines are not available the cycle time can stretch to two months or more.

The Missouri type kiln was and still is an important method of charcoal making in the U.S.A. In 1958 when the industry was at its peak about 45 000 tons of charcoal were produced in Missouri by this method. (This production is miniscule compared to production by other methods in the developing world but is enough to attract interest in the method.)

Missouri kiln – volume 150 M³

Vent hole covers

Chimneys

Steel plate doors

The Missouri kiln’s greatest advantage compared with brick kilns is the possibility, in fact the necessity, of mechanical loading and unloading.

Its disadvantages are its high cost due to high use of steel and concrete and its immobility. Unlike brick kilns it cannot be demolished and rebuilt. Hence a ten-year wood supply must be available within economic haul distance of any group of kilns. The amount of wood for a group of some 180 m³ kilns would be 120 000 m³ approximately. About 4 000 ha of forest capable of yielding 30 m³ per hectare would have to be set aside for ten years to supply this amount of wood. Such an area would give a mean haulage distance of about 2.5 km, which is reasonable.

Missouri type kilns or their equivalent would be more acceptable if construction were lighter and hence cheaper and if they could be cooled more rapidly, perhaps using water injection. The Brazilian industry is experimenting in this direction. Faster cooling and simpler construction has been achieved in the U.S.A. by building the kilns from insulated steel sheathed panels. Unfortunately, construction cost steel remains high – a barrier in the developing world where such products must be imported.

As far as transferring the technology is concerned, the experience of the FAO/UNDP project in Ghana is useful. A Missouri kiln was built and, technically, operation of this kiln is a success. The problems are the high cost of construction and the mechanical loading/unloading equipment.

Battery of half orange kilns more than 8 years old and still in use. Los Tigres, Argentina. Photo H. Booth.

The beehive brick kilns are grouped in batteries of 7, 14, etc., (always a multiple of 7). The slope type kilns are grouped in batteries of 14, 28, etc., (always a multiple of 14).

Fig. 10. Operating cycle for beehive brick kilns

Charcoal discharging and firewood charging 8 hours Carbonization 96 hours Cooling 88 hours Total cycle: 192 hours or 8 days

Each battery is attended by only two men, one charcoal operator or burner and one helper.

A charcoal production centre comprises one or more batteries of kilns each complete with the infrastructure necessary for continuous operation. For example, stockyard for firewood, for charcoal storage, for charcoal loading facilities, access roads, water supply, etc.

The operational cycle of each of the seven kilns starts on successive days. If kiln No. 1 is discharged and recharged (eight hours) on a Monday, then kiln No. 2 is discharged and recharged on a Tuesday, etc. Kiln No. 1 will then be ready for discharging and recharging on the following Tuesday and kiln No. 2 on the following Wednesday, etc.

Sunday is a rest day on which no kiln discharging or charging is done. Kilns due to be discharged on Sunday are dealt with the following Monday. Thus the cycle recommences on a Monday for each kiln in turn after six weeks.

Table 6 : Characteristics of beehive and slope type kilns

Type of kiln Volume of Firewood in steres Ratio Real/Nominal Charcoal volume in m³ per batch Average yield Firewood/Charcoal Real Nominal Beehive brick kiln 5 m diam. 48 37.34 70% 17.8 2.1 : 1 (vol.) Slope type kiln 4 m diam. 24.8 17.40 60% 8.9 2.2 : 1 (vol.)

The above average yields are those obtained until about 1975. Since 1976, through continuous research and experiment, the improvement of operational conditions, the training of charcoal burners and better supervision, the yields of company operated kilns have continuously improved to: 1.9 : 1 (53%); 1.8 : 1 (55%) and 1.7 : 1 (59%). Yields of 1.6 : 1 (60%) are being obtained recently in routine operation (1977).

Production of a 7 beehive kiln battery in 30 days 30/8 x 7 = 26.25 batches – say 26 batches. Each batch = 17.8 m³ of charcoal. Monthly production 17.8 x 26 = 462.8 m³ charcoal and yearly production 17.8 x 26 x 12 = 5 553.6 m³ charcoal.

(i) Charging First fit two logs crosswise on the inside of the discharging door. Then block up the discharging door with bricks laid without mortar. The outside of the door should be brushed with a clay slurry but only after charging has been completed. Charging can now commence. The logs are placed vertically, the thinner pieces against the wall, the thicker ones towards the centre of the kiln where the temperature will be higher. Put the chisel-shaped bases of the logs on the kiln floor to make circulation of the gases easier. The wood piled under the dome ceiling must be placed horizontally on top of the vertically piled floor wood. Fill up well into the dome. The wood must be piled as close together as possible to obtain a maximum amount of material inside the kiln. Use a loose structure and some kindling close to the ignition opening to make ignition easier. If there is some deteriorated wood, this must be placed near the discharge opening as the charcoal produced from it will have a tendency to ignite easily and, if that happens, it can be rapidly removed when discharging the kiln. Close the charging door in the same manner as was done for the discharge door. When the kiln is ready for ignition, all portholes and openings must be kept open. (ii) Ignition of kiln Introduce through the central opening in the dome a shovelful of glowing (incandescent) charcoal. In the rainy season it may be necessary to help with some kerosene or used lubrication oil. Use the central opening only for ignition, as the carbonization process must proceed from top to bottom. At the start of the ignition period smoke will issue from the ignition opening first white and minutes later dark coloured. This is a signal that the fire has caught. The opening must then be plugged with a brick brushed with clay slurry. (iii) Carbonization Immediately after ignition, smoke issues from the outlet portholes, initially white coloured, which means that the carbonization area is increasing. The emergency outlet portholes and the ports located in the dome are now plugged. The stacks (chimneys) start smoking. The kiln operates from now on exclusively with the controlled air supplied through the air inlet portholes and on the draft of the stacks expelling the carbonization gases. The carbonization process proceeds from top to bottom and also horizontally. The chimneys must be watched to ensure they work uniformly. This is achieved by controlling the draught of air entering the air inlet portholes by varying the position of a brick loosely inclined against the porthole entry. The charcoal burner controls the carbonization by observation of the colour of the smoke issuing from the stacks. Carbonization proceeds as long as the colour is white. Later it turns bluish white and then transparent blue. When this colour becomes steady, the air inlet portholes must be closed. At the end of the carbonization the smoke becomes colourless and transparent. When a zone of approximately 20 cm height of colourless smoke appears on top of the stacks, the chimneys are closed. The stacks do not present simultaneously the same smoke colour even after every precaution has been taken. It is therefore necessary to regulate, one after the other, the air inlet portholes and to close the corresponding stacks. These will continue to issue smoke some time after the air portholes have been plugged. The stacks should not be closed too soon to avoid the presence of uncarbonised pieces of firewood. Once the stacks have been closed, the carbonization process is terminated. After closing all openings, they must be carefully brushed with clay slurry to prevent any air entering. (iv) Cooling the kiln The kiln is brushed all over with several layers of clay slurry to close all openings, leaks and cracks. The number of brushings varies between three and six. The better this operation is done, the faster will be the cooling of the kiln. When leaks are not fully closed, air will continue to penetrate into the kiln, preventing the extinction of the fire, causing loss of charcoal through its combustion and an increased ash content. (v) Unloading the kiln and curing the charcoal The kiln is opened and the charcoal is discharged when the kiln is sufficiently cool. The burner knows the correct temperature – 60 – 70°C, by feeling the door wall with the back of his hand. A kiln must never be opened until it is sufficiently cool to avoid spontaneous fire. Such a fire may be extinguished with water but, in most cases, it will be necessary to close immediately the kiln. The result is always a loss of charcoal. Sufficient water, at least one barrel – 200 litres – must be readily available before the kiln is opened. The space in front of the kiln where the charcoal will be stored, must be clean. Fresh charcoal must never be placed on top of older charcoal. The kiln is opened rapidly. The burner will observe by the smell of the issuing gases, whether there is fire in any place and, in that case, will extinguish it with water spray. The bricks from the door opening are put on one side not to impede the discharge operations, which are done manually with a special large fork and a basket. It is good practice to separate all uncharted pieces of wood bricks, ashes, charcoal fines and clay remnants. Incompletely carbonised pieces of wood are separated and reloaded with the next batch. The discharged charcoal is heaped and stored in a way to allow thorough aeration. This is also called curing. Fresh charcoal absorbs oxygen. This chemical reaction is accompanied by a rise in temperature which can cause spontaneous ignition. Therefore, fresh charcoal is required to “cure” in the open air for two days before being transported to the intermediate storing houses or to the iron and steel plants. It is, of course, difficult to control whether this operation is always done with the necessary care. Principally at the end of the month, when charcoal operators are anxious to complete their monthly quotas of charcoal production, it frequently happens that the charcoal is insufficiently cured, causing fire hazards. During curing, the charcoal heaps should not exceed 1.50 m height or depth, to permit a thorough contact between the charcoal and the outside air. After unloading, the bottom of the kiln is cleaned. All air ports and stacks are opened and cleared from carbonization residues. The entire inside of the kiln becomes heavily coated with hard tar or pitch which condenses and builds up during successive charges and protects the tricks.

Photo. 24. Unloading charcoal from a brick kiln in Salta, Argentina. Fork is used to allow fires to be separated. Photo M. Trossero

This is the same as in beehive brick kilns. The operation is simpler because there is only one air inlet porthole to watch and regulate. These kilns are frequently located at places of difficult access without roads. The unloaded charcoal must be transported to the nearest road or to a reloading place by mules. The following points of comparison may be considered in making the correct choice between the two types of kilns:

$189.00 – $9,755.00

Insulating Firebricks (IFB), also known as Fire Bricks, are used in high temperature applications ranging from 2,000°F (1,093°C) to 3,200°F (1,760°C). IN-30 are rated for 3000℉.

If you require a specific grade, finish or other shapes, please contact our office directly for a custom quote.

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Insulating firebricks must be shipped freight prepaid to ensure safe delivery. Orders less than a full pallet will incur a $50 defective pallet fee. Please contact us directly for a custom shipping quote or place your order and a CeraMaterials representative will contact you to confirm shipping costs before your order is processed. If you would like to collect the material, please contact the office directly to inquire about available options. If you would like to collect the material, please contact the office directly to inquire about available options.

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Firing pottery can seem like a mysterious process to the new potter. There are many technical terms that can sound quite foreign. So I thought it would be helpful to write an overview of the stages of clay firing. Here is a guide to the 3 stages of clay firing.

There are 3 main stages in clay firing. The first stage is the drying process. Clay must be bone dry before firing. The second stage is the bisque firing. In bisque firing, soluble green clay is converted into ceramic material. The third stage is the glaze fire.

Each of these processes is important. So let’s take a closer look at each of these stages of clay firing.

The stages of firing clay

Proper drying of your pottery is essential for successful firing. Therefore, before turning on your oven, it is important to understand something about the drying process.

Stage 1 – Drying your pottery

Once you’ve made your pottery, you’ll need to let your clay dry thoroughly before firing it. When clay is completely dry, it is referred to as bone-dry clay. This refers to when clay is as dry as possible before it is fired.

It is important that clay is dry before firing, as wet clay can explode in the kiln. Because when moisture reaches the boiling point of water at 100°C, it turns into steam. When water turns into steam, it expands rapidly.

When making pottery, it’s a good idea to trap air bubbles in your clay. But sometimes air pockets form in your ceramic. And when water turns to steam and quickly expands into those air pockets, it can cause your pot to explode.

To avoid this, make sure your pots are as dry as possible before firing.

How to dry your pottery clay

It is often said that the best way to dry your pottery is slowly and naturally. This means letting your clay dry naturally without trying to rush the process.

However, it is more important that your ceramic dries out evenly. If one part of your pottery dries out faster than another, it will put a lot of strain on the pottery.

For example, a handle on a mug tends to dry out faster than the mug itself. This is because the handle is more exposed and has a larger surface area. In this case, there is a risk that the handle will break at the junction.

This is because the clay shrinks as it dries. If the handle shrinks faster than the cup, it can cause the handle to tear at the joint.

So the most important thing when drying your pots is to dry them evenly. A tip is to wrap extremities and details like handles with a piece of plastic bag. And loosely cover the whole piece with a bag to prevent drafts from drying out one area faster than another.

Once your clay is dry to the touch, you can place it in the kiln over low heat for a while. That’s called candlelight.

Candling your pottery in the kiln

Even if your pottery is bone dry to the touch, there may be moisture embedded in the deeper layers of clay. It is therefore a good idea to heat your pots in the kiln at a low temperature before firing.

Some potters put their pots in the kiln overnight at a low temperature. A low temperature at this stage is anything below the boiling point. When candling overnight, the oven is often set to a very low temperature, e.g. B. 50 ° C. This is also known as “water smoking”.

During the candling process, the lid of the oven is opened about 2.5 cm with an oven stone. This allows any moisture to evaporate from the oven. For the same reason, the peepholes, also called peepholes, are left open on the side of the stove.

As an alternative to candling overnight, many potters preheat in the kiln. Preheating your oven involves setting the oven at a low temperature for a few hours. For example, you could set your oven to heat up to 80-90°C for a few hours.

These temperatures are below boiling point, but hot enough to ensure your pottery is completely dry.

If the walls of your pottery are thick or uneven, it may be a good idea to preheat longer. For example, pottery teachers at school often preheat the kiln for about 4 hours. This is because student pottery is often a bit chunkier and takes longer to fully dry.

Now let’s take a look at the next stages of firing clay, starting with bisque firing…

Stage 2 – biscuit fired pottery

Once the pottery is bone dry, it is usually fired twice. The first firing in the kiln is called biscuit firing. This is sometimes referred to as a biscuit burn.

Bone-dry pottery that has not been fired is still raw clay. Bone Dry Clay is:

Fragile – it breaks very easily. If accidentally bumped lightly, it will break and details such as handles will break off easily.

Soluble – When you immerse bone dry clay in water it will dissolve and return to its workable state.

For these reasons, bone dry clay is not very practical or useful.

During a biscuit fire, pottery is transformed from clay into pottery. Ceramic materials are:

Hard and Brittle – Bisque is much stronger than bone dry clay. For example, you can pick up a mug made of biscuit brandy without having to worry about the handle breaking off.

Insoluble – this means that if you immerse bisque porcelain in water, it will absorb water but not dissolve. Wet biscuit stays intact, unlike unfired clay, which dissolves or “melts off” in water.

Changes that ceramics undergo in a biscuit fire

When a furnace burns, the temperature in the inner chamber rises. Typically, a biscuit firing will reach around 1832F (1000C). I’ll say more about biscuit firing temperatures later.

Exactly how the oven heats up depends on the type of oven you are using. Most ovens are electric and are heated by electrical elements that sit in grooves along the wall of the oven. But there are also other types of ovens, such as gas ovens or wood-burning ovens.

As the temperature in the kiln increases, the clay undergoes many significant chemical and physical changes. Here are some of the changes that occur at various points in the biscuit firing process:

Residual moisture in the clay, so-called mechanical water, evaporates.

Organic carbon and sulfur burn out of the clay.

Clay also contains “chemically bound water” which is found inside the clay particles. It is part of the chemical composition of the clay platelets or molecules. At about 660F (350C) chemical water begins to be expelled from the clay.

After the chemically bound water has been expelled, the clay has become ceramic. It is no longer soluble and cannot be converted back into editable clay.

Chemicals and mineral compounds are burned out of the clay.

The clay undergoes a process called sintering. This is when the surface of the clay particles begin to bond together. The particles move closer together and the sound becomes denser (source).

So, by the end of the bisque firing, a piece of pottery will have undergone some significant changes.

How is bisque ceramic?

Once clay has been fired biscuit-like, it has become ceramic. Bisque porcelain is hard but porous due to the changes it undergoes during firing. This means that it absorbs water but does not dissolve.

During bisque firing, the clay begins a maturing process. Clay is considered mature when it is as dense and hard as possible. Some tones can get much denser and harsher than others.

The three most common types of clay that potters use are earthenware, stoneware, and porcelain. Stoneware clay and porcelain can be harder and denser than earthenware.

Earthenware Clay is a “Low Fire Clay” and usually gets as hard as possible during scalding fire.

Stoneware and porcelain, on the other hand, harden and condense further during later firing. Stoneware and porcelain are known as “medium or high fire clays” which are aged at higher temperatures than a biscuit firing.

Earthenware is therefore more or less mature after a biscuit firing. While stoneware and porcelain are semi-mature and still have a long way to go before they reach maturity.

However, regardless of the type of clay you use, bisque pottery has some things in common. Bisque china is porous and hard, and makes a sharp “ding” sound when you gently move it with your finger

Biscuit Firing Temperatures

Typically, potters fire their clay to a target temperature in the range of 1823-1940F (995-1060C). Pottery is usually fired in this temperature range, regardless of what type of clay is used.

By slightly adjusting the bisque firing temperature, potters can change the characteristics of the bisque pottery a bit. But biscuit firing is almost always a “low-fire” process.

Once the clay has been fired it can be glazed. Ceramic glaze consists of ceramic materials suspended in water to form a liquid. They apply liquid glaze to biscuit porcelain and then go into a glaze firing.

Glaze firing is the final step in firing clay. So let’s take a look at the Glaze firing.

Stage 3 – Glaze firing ceramic

There are two main purposes in glazing pottery. The first is decorative. Glazing can give the potter a range of colours, textures and finishes. The second is functional. Glaze coats the pottery with a glass-like layer that makes the pottery waterproof and sometimes waterproof.

Ceramic glaze is applied to biscuit ware in liquid form. It can be applied by brush, pour, dip and airbrush. When glaze is applied to biscuit dishes, it dries out quickly. The reason is that the biscuit ceramic is very porous and the water quickly draws out of the glaze.

As the icing dries on the pot, a layer of the icing materials will adhere to the surface. Biscuit porcelain usually requires several layers of glaze to be applied. However, the number of coats varies depending on the application and the type of glaze used.

As soon as the glaze has dried on the pot, it is ready for the second firing. This is called glaze firing or sometimes smooth firing. During the glaze firing, the ingredients of the glaze change.

Some of the materials in glazes are known as glass formers. Once they reach a certain temperature, they melt and form liquid glass. This liquid glass coats the pot, and as the kiln cools, the glaze hardens again to form a glazed surface.

glaze firing temperatures

The temperature at which you glaze your pottery depends on the type of clay you used. If you used earthenware clay, the glaze firing is usually at about the same temperature as a biscuit firing.

In fact, when glazing earthenware, the glazing fire can often be at a slightly lower temperature. In a glaze firing, the clay itself matures further. Sometimes carbon and impurities in the clay are not completely burned out in the biscuit fire.

If the temperature of the glaze firing is higher than that of the biscuit firing, the burnout process continues. These impurities can form gases when escaping from the clay mass, which can lead to defects in the glaze. Flaws include bubbles in the glaze, small holes in the texture, and a cloudy finish.

If the glaze firing temperature is lower than the biscuit firing temperature, you can avoid these glaze defects. Therefore, when glazing stoneware, it is often recommended to heat biscuit type fires at around 1940F (1060C). And to glaze the fire just below that temperature, at around 1823F (995C).

Like clay, glaze matures. And glazes that are suitable for earthenware clay melt at lower temperatures than earthenware glazes.

If you made your pots from stoneware clay, you will need to choose a different type of glaze. For stoneware clay you will need a medium or high fire glaze. That means choosing a glaze that ages at a higher temperature.

How to choose the right glaze

You can buy powdered icing, which you then mix with water before using. Or you can buy ready-made liquid glaze in different pot sizes.

Small containers of ready-made icing are popular because they are handy and reliable. And they don’t take up much space like big buckets of frosting.

An important point when choosing a glaze is to make sure it is suitable for the clay you are using. This means that the glaze should be fired in the same temperature range.

Normally, the back of the glaze container will list the correct temperature range for firing. This also applies to clay. Generally, the clay manufacturer will indicate on the clay bag what firing temperature range applies to that clay body.

Check the clay body’s firing temperature range, then purchase a glaze that matches that range.

If you study firing temperatures, you’ll hear potters speak of “cones.” The cone system is shorthand for talking about firing temperatures and heat work.

Heat work is a combination of temperature and time used to fire a piece of pottery or glaze. If you want to learn more about what the cone system means, read this article.

Stoneware firing temperatures

There is a wide range of stoneware firing temperatures. Different stoneware clays are designed to be fired at different temperatures. Again, you need to check the firing range of the clay before firing. However, roughly speaking, firing temperatures for earthenware range from about 2158 F (1181 °C) to 2377 F (1300 °C).

During glaze firing, stoneware clay continues to mature through a process called vitrification. Like glaze, clay also contains glass-forming substances. When the clay is fired, these materials soften and melt.

The glassy liquid migrates into the interstices between the clay particles and fills in the interstices. When the liquid solidifies again, the ceramic is less porous because the pores have been filled with glass. This is the process of glazing. If stoneware clay is glazed, it is called matured.

The more glass-forming materials the clay contains, the denser and less porous the clay will be after firing. Stoneware clay contains more glass-forming ingredients than earthenware.

Therefore, once fired at a high enough temperature, earthenware is less porous than earthenware. Some stoneware contains more glass-forming material than others. Some stoneware is more glassy and less porous than others when fired.

A word on single fire

Not every pottery biscuit burns its greens. Some prefer to glaze unfired clay directly and proceed directly to glaze firing. This is sometimes referred to as a “single firing” or “raw firing”.

Firing once has the advantage of being more direct, less time consuming, and uses less energy to fire. However, since the raw clay has to give off moisture, carbon and gases, this can lead to glaze defects during firing. Because of this, single firing can be more difficult to master.

If you want to learn more about the single firing versus bisque and glaze firing, read this article.

Final Thoughts on the Stages of Firing Clay

There are many different types of kilns, regardless of which the stages of firing clay remain the same. Most commonly, the clay is thoroughly dried, then it is fired, followed by glaze firing. While raw glazing is a passion for some potters, I think it helps to master biscuit and glaze firing first. Clay firing isn’t rocket science, but it does involve chemistry and there’s a lot to learn. Enjoy the trip!

Soul Ceramics is one of the best suppliers I have worked with in quite some time. Serious. Communication through every step of my order and weekly updates on my order delivery times. The even heat is fantastic. I ordered one with the Tap controller, not sure how it compares to the other options as this is the first time I’ve had to use an oven for heat treatment. Before that I only used forgeable steels, mostly O1. I look forward to testing some stainless blades soon. I personally would prefer to make carbon blades, but many of my customers have asked for stainless steel. Overall I would recommend you to buy your knife heat treatment furnace here.

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Most of our top loading ovens do not use element clamps.

The elements sit in sunken, recessed grooves for better protection. Without staples, they are easier to change.