Black Iron Oxide Wash? The 73 Top Answers

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FAQ: Bluing & Cold Blackening

Black Oxide is a finish applied to iron and steel. There is also a “black oxide” process that is applied to the copper inner layers of printed circuit boards, but that’s another topic.

There are two general types of black oxide for iron and steel: hot black oxide (or hot blacking) and room temperature blacking (or cold blacking).

hot blackening:

Hot bluing can be done from generic blends of caustic soda, sodium nitrite/nitrate, wetting agents and stabilizers, or from proprietary blends.

The result of the process is an attractive, but very thin and slightly corrosion resistant, dark black iron oxide surface. This black finish is familiar to consumers of gears and sprockets, some brands of spark plugs, and socket wrenches and other tools. It is also used for firearm components such as gun barrels.

The parts are usually cleaned, burnished and then waxed or oiled (with intermediate rinsing).

While most metal finishing processes use toxic chemicals, the Black Oxide process is particularly dangerous and amateurs are definitely discouraged from attempting hot blackening! One of the things that makes black oxidation so dangerous is that the black oxide bath operates at around 290°F. Note that this is well above the boiling point of water and the difference is the biggest problem. Water evaporates quickly from the black oxide tank, but when replacement water (which turns to vapor at 212 degrees) is introduced into a 290-degree tank, it has a strong tendency to explosively vaporize into vapor – “bursting out” and spraying all and everyone with this terribly hot and terribly caustic solution. people were killed. It is vital that the water replenishment system is properly designed and competently operated.

cold blackening:

To reduce the dangers of hot blackening and save energy, proprietary cold blackening solutions have been introduced. These work at around room temperature and use a different chemical base, making them far less dangerous. However, room temperature blackening is not a true black oxide process; rather it is the application of a copper-selenium compound. This compound is not always an acceptable substitute for black oxide as it doesn’t look as pretty and can tend to be very dirty (easily rubs off on hands and clothing).

There are dozens of letters online related to black oxide and cold blackening here. Here is a partial list of some of the early ones:

Many more threads on black oxide and room temperature blackening can be found on the site.

Where can you have it done

Many workshops offer black oxide services. Please go to our workshop directory and search for the term “black oxide”. If this does not meet your needs, post a request for the service you require on our Job Search page.

Where to buy consumables to do Black Oxide or Cold Blackening in-house?

Several national suppliers offer hot bluing and cold bluing processes. Please go to our Chemicals & Consumables Directory page and search for the term ‘black oxide’. If this does not meet your needs, submit a request for the service you need on our Products and Services Search page.

Please ask technical questions about blueing or cold blueing in our “Hotline Letters” forum.

Other Resources

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iron oxide red

Alternative names: iron oxide, red iron oxide, RIO, iron(III) oxide, Fe 2 O 3 , hematite

Description: Synthetic hematite

Remarks

Synthetic red iron oxide is the most common colorant in ceramics and has the highest iron content. It is commercially available as a soft and very fine powder made by grinding ore material or heat treating ferrous/ferric sulphate or ferric hydroxide. All irons normally decompose during firing, producing similar colors in glazes and clay bodies (although they contain varying amounts of Fe metal per gram of powder). Red iron oxide comes in many different shades from a bright bright red to a deep red maroon, these are usually denoted on a scale of around 120-180 (this number designation should be on the bags from the manufacturer, darker colors are higher numbers), however should be these different grades of pottery all burn at a similar temperature because they contain the same amount of iron. The different raw colors result from the degree of grinding.

In oxidation firing, iron is very refractory, so much so that it is impossible to produce a metallic glaze even in a highly molten frit. It is an important source of tan, reddish brown and brown colors in glazes and bodies. Iron red hues, for example, rely on the crystallization of iron in a liquid glaze matrix and require the presence of large amounts of iron (e.g. 25%). The red color of terracotta bodies comes from iron, typically around 5% or more, and depends on whether the body is porous. When these bodies are fired at higher temperatures, the color shifts to a deeper red and eventually brown. The situation is similar with medium fire bodies.

During reduction firing, iron changes its nature and becomes a very active flux. Iron glazes, stable in oxidation at cone 6-10, run off the ware upon reduction. The iron in reducing fired glazes is known to produce very attractive earthy brown tones. Depending on the chemistry of the glaze and the amount of iron, shades of green, gray and red can also be achieved. For example, ancient Chinese celadon contained about 2-3% iron.

Particulate iron impurities in reduction clay bodies can melt and liquify during firing, creating stains that can bleed through glazes. This phenomenon is a highly desirable aesthetic in certain types of pottery, when the particles are quite large, the resulting spot in the glaze surface is called a bloom.

Iron oxide can gel glaze and clay slurries making them difficult to work with (this is especially a problem if the slurry is deflocculated).

Iron oxide particles are very small, typically 100% of the material will pass through a 325 mesh sieve (this is one of the reasons iron is such an annoying dust). As with other extremely small particle size powders, agglomeration of the particles into larger particles can be a real problem. These particles can resist degradation, even a powerful electric mixer is not enough to disperse them (black iron oxide can be even more difficult). In such cases, they are broken down by sieving a glaze. However, screening larger than 80 mesh is difficult because it is not fine enough to eliminate the speckles that iron can create. Therefore, ball milling may be the only solution when mottle is undesirable.

Red iron oxides are available in spherical, rhombohedral, and irregular particle shapes. Some high purity grades are specially controlled for heavy metals and used in pharmaceuticals, cosmetics, pet food and soft ferrites. Highly refined grades can contain 98% Fe 2 O 3 , but red iron is typically about 95% pure and very fine (less than 1% 325 mesh). Some grades of gunmetal have coarser speckles and this can result in unwanted speckles in the glaze and shards (see image).

Raw materials with high iron content or alternative names: burnt sienna, crocus martis, Indian red, red ocher, red oxide, Spanish red. Iron is the main impurity in most clay materials. A low iron content is very important, for example, in kaolins used for porcelain.

One method of producing synthetic iron oxide is to burn solutions of ferric chloride (waste liquor from the steel industry) to produce hydrochloric acid (its main product) and hematite (a by-product). 100% pure material contains 69.9% Fe.

We have received some information about CaO’s ability to bleach the color of iron in bodies (as noted by Hermann Seger). This refers to a chemical reaction between lime, iron and some of the clay’s silica and alumina to form a new buff-colored silicate. He found that this bleaching effect is most evident when the lime content is three times that of iron. Of course, the presence of lime in a shard leads to its rapid softening, making it impossible to produce vitrified products.

related information

Lanxess Iron Oxide Original Container Bag – Back

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Lanxess iron oxide original container bag – front

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Iron oxide powder is available in many colors. Here are three.

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How can there be so many colors? Because iron and oxygen can combine in many ways. In ceramics we know Fe 2 O 3 as red iron and Fe3O 4 as black iron (the latter being the more concentrated form). But would you believe that there are 6 others (one is Fe 13 O 19 !). And four phases of Fe 2 O 3 . Plus more iron hydroxides (yellow iron is Fe(OH)3).

Here’s what can happen with iron oxide based overglazes

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Because iron oxide is a strong reducing agent, iron-based pigments can run poorly if applied too thickly.

How do black, red and yellow iron additions compare in a glaze?

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Example of 5% black iron oxide (left), red iron oxide (middle), and yellow iron oxide (right) added to G1214W glaze, sieved to 100 mesh, and fired to cone 8. The black is a bit darker, the yellow has no color? Do you know why?

FeO (iron oxide) is a very strong flux

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This 10R conical glaze, a Tenmoku with approximately 12% iron oxide, demonstrates how iron becomes a flux during reduction firing and produces a glaze melt that is much more fluid. When oxidized, iron is refractory and does not melt well (this glaze would be completely stable and much brighter in an oxidation firing at the same temperature on the dishes).

Does iron oxide stain a dolomite body red? nope!

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These fired rods are the L4410P low temperature clay body (replacing the traditional 50% talc with 40% dolomite and 10% nepheline). These wands are fired from cone 5 through cone 06 (top to bottom). The shard contains 4% red iron oxide, which would normally be enough to produce a bright red fired colour. But of course, the dolomite kills his development. A better option is to use the L4170 plastic terracotta (or its L4170B cast version).

Yellow, black and red iron oxide in a polished burning body at cone 6 oxidation

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Plainsman M340 Buff Cone 6 stoneware. 3% iron was added to each of these. The yellow iron (left) is clearly not as concentrated (and not blended in as well). The black (middle) gives a maroon color.

The multitude of things that iron oxide can do in reduction

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Iron oxide is an amazing glaze addition in the reduction. Here I added it to the transparent base G1947U. It produces green celadon with low percentages. Still transparent where thin, 5% produces an amber glass (and the iron shows its power to flow). 7% brings opacity and tiny crystals form. The color is black 9% where it’s thick, 11% where it’s thin or thick—this is “Tenmoku territory.” 13% have shifted it into an Iron Crystal (what some would call Tenmoku Gold), 17% is almost metallic. Behind, iron crystals grow on top of others. These samples were of course cooled in a large reduction furnace, the mechanism of crystallization would be much harder if cooled more slowly.

What 1% iron oxide does in a talc, cone 10R mattifies

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The body is Plainsman H450. Both have a black engobe (L3954N) that is applied to the inside and halfway up the outside during the leather curing phase (the insides are matt glazed with Ravenscrag GR10-C Talc). The outer glaze on the left contains 1% iron added to the base matte recipe. The one on the right has no iron. Notice how different the glazes are over the black engobe.

Reduction high temperature iron crystal glaze

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About 10% iron and some titanium and rutile in a transparent base glaze with slow cooling can do this at cone 10R on a finished porcelain.

Iron red high temperature reduction glaze

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This recipe, our code 77E14A, contains 6% red iron oxide and 4% tricalcium phosphate. But the color is a product of chemistry. The glaze is rich in Al 2 O 3 (from 45 feldspar and 20 kaolin) and low in SiO 2 (the recipe contains zero silica). This calculates to an Al 2 O 3 :SiO 2 ratio of 4:1, which is very low and usually indicates a matte finish. The iron oxide content of this is half of what is typical of an Beyond Tenmoku iron crystal glaze (those that have enough iron to saturate the melt and precipitate as crystals during cooling). The color of this is also a product of a type of iron crystallization, but it occurs in a low-silica, high-alumina melt where phosphate and alkalis are present. Reducing the iron content to 4% produces a yellow mustard color (we called it “Red Mustard” for that reason).

Cone 10 reduction fired transparent glaze with 12% iron oxide

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4% iron oxide in a clear glaze. Unshielded. The result: burn marks.

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Iron oxide is a very fine powder. Unfortunately, it has poor agglomeration and no amount of wet mix seems to break up the clumps. However, if you push the glaze through a sieve, in this case 80 mesh, they will be reduced in size. Ball milling would remove them entirely. Other oxide dyes have the same problem (e.g. cobalt oxide). Stains disperse much better in muds.

Agglomerates of iron oxide particles produce strong specks

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5 different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The spots are not due to the particle size, but to differences in the agglomeration of the particles. Glazes using these iron oxides obviously need to be sieved to break up the lumps.

Comparison of fired glaze spots of different brands of iron oxide

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Five different brand names of Iron Oxide at 4% in G1214W Cone 5 Clear Glaze. The glazes have been sieved to 100 mesh, but any remaining stains are still due to agglomeration of particles, not particle size differences.

What a difference the iron oxide makes in these two Konus 6 glazes

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The top two samples: Bayferrox 120M. Bottom two samples: Huntsman #1115. Two glazes on the left: 4% iron in bright G2926B base. Right two glazes: 4% in G2934 matte base. Firing the Cone 6 used a drop-and-hold schedule.

Adding iron to a clear glaze has eliminated the microbubbles!

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The glaze at right is transparent, G2926B, on a body with dark firing cone 6 (Plainsman M390). On the left is the same glaze but with 4% red iron oxide added. The trapped microbubbles have disappeared and the color is deep and much richer. It’s not clear how this happens, but it’s a kind of “fining up” and certainly beneficial.

2% Iron Oxide in a lustrous low temperature glaze gives better color and less haze

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Both pieces are the same body, Plansman L215. Both are branded on cone 03. Both are glazed using the G1916Q recipe. The glaze on the piece at left contains 2% iron oxide (and sieved to 80 mesh). Each grain of iron (which is refractory in this situation) serves to collect the microbubbles so they can move through the glaze layer. Also notice how much richer the color is on this piece. The piece on the right contains no added iron oxide. It’s not as red and not as transparent. Incidentally, these two jugs are glazed on the bottom and were fired on stilts.

Iron Oxide sucks up glaze bubble clouds at Cone 6

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These two mugs are made from the same dark-firing stoneware (Plainsman M390). They have the same clear glaze, G2926B. They are fired to the same temperature in the same firing schedule. But the glaze on the left has 4% added iron oxide. On a bright burning body, the iron stains the otherwise clear glass amber (with speckles). But on this dark burning tone it appears transparent. But amazingly, the bubble clouds have disappeared. We did not test further to find the minimum amount of iron required for this effect.

There’s so much iron in a box of the cleanest china you can make!

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The recipe: 50% New Zealand kaolin, 21% G200 feldspar, 25% silica and 3% VeeGum (for Cone 10R). These are the cleanest materials available. However, it does contain 0.15% iron (mainly from the 0.25% found in New Zealand kaolin, VeeGum chemistry is unknown, I assume it contributes zero iron). A 50 pound box of Pugged would contain about 18,000 grams of dry clay (assuming 20% ​​water). 0.15% of 18,000 is the 27 grams of iron you see here! This mug is a typical Grolleg-based porcelain with a standard raw bentonite. A box of these contains four times as much iron. Enough to fill the cup half full!

Adjusting the color of a natural clay with an iron oxide blend

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The freshly cast piece at left obverse is a medium temperature plastic stoneware body. Its color comes from a natural ferrous clay in the formula. However, this red clay is becoming much more expensive and difficult to source due to truck availability and cross-border issues. We’re investigating the addition of Iron Oxide to a mix of Buff Burning materials (which can be adjusted to match the working and burning characteristics of the original body). An addition of 3% iron oxide produces the same burnt color. But the raw color also needs to be matched. The answer is a mixture of red:yellow:black iron oxides. The 3% iron addition in the rear center is a 50/50 mix of red and yellow iron oxides, definitely too red. The right front piece is a 40:50:10 mixture of red:yellow:black iron oxides. This is getting closer, for the next try we will try more black and less red.

Cone 10 Reduction, home of an amazing oxide: iron

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It is a powerful glazing flux, variegator and crystallizer, a dye of many characters in bodies and glazes, and a stain remover like no other. And it’s safe and cheap!

How do metal oxides compare in their degrees of fusion?

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Metal oxides with 50% ferrous frit 3134 in crucibles at cone 6ox. Chromium and rutile are not smelted, copper and cobalt are extremely active smelters. Cobalt and copper have crystallized as they cool, manganese has formed an iridescent glass.

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Red iron oxide is the most common colorant in ceramics and has the highest iron content. It is commercially available as a soft and very fine powder made by grinding ore material or heat treating ferrous/ferric sulphate or ferric hydroxide. All irons normally decompose during firing, producing similar colors in glazes and clay bodies (although they contain varying amounts of Fe metal per gram of powder). Red iron oxide comes in many different shades from a bright bright red to a deep red maroon, these are usually denoted on a scale of around 120-180 (this number designation should be on the bags from the manufacturer, darker colors are higher numbers), however should be these different grades of pottery all burn at a similar temperature because they contain the same amount of iron. The different raw colors result from the degree of grinding.

In oxidation firing, iron is very refractory, so much so that it is impossible to produce a metallic glaze even in a highly molten frit. It is an important source of tan, reddish brown and brown colors in glazes and bodies. Iron red hues, for example, rely on the crystallization of iron in a liquid glaze matrix and require the presence of large amounts of iron (e.g. 25%). The red color of terracotta bodies comes from iron, typically around 5% or more, and depends on whether the body is porous. When these bodies are fired at higher temperatures, the color shifts to a deeper red and eventually brown. The situation is similar with medium fire bodies.

During reduction firing, iron changes its nature and becomes a very active flux. Iron glazes, stable in oxidation at cone 6-10, run off the ware upon reduction. The iron in reducing fired glazes is known to produce very attractive earthy brown tones. Depending on the chemistry of the glaze and the amount of iron, shades of green, gray and red can also be achieved. For example, ancient Chinese celadon contained about 2-3% iron.

Particulate iron impurities in reduction clay bodies can melt and liquify during firing, creating stains that can bleed through glazes. This phenomenon is a highly desirable aesthetic in certain types of pottery, when the particles are quite large, the resulting spot in the glaze surface is called a bloom.

Iron oxide can gel glaze and clay slurries making them difficult to work with (this is especially a problem if the slurry is deflocculated).

Iron oxide particles are very small, typically 100% of the material will pass through a 325 mesh sieve (this is one of the reasons iron is such an annoying dust). As with other extremely small particle size powders, agglomeration of the particles into larger particles can be a real problem. These particles can resist degradation, even a powerful electric mixer is not enough to disperse them (black iron oxide can be even more difficult). In such cases, they are broken down by sieving a glaze. However, screening larger than 80 mesh is difficult because it is not fine enough to eliminate the speckles that iron can create. Therefore, ball milling may be the only solution when mottle is undesirable.

TOP 10 WAYS TO DECORATE WITH OXIDES

You’ve probably heard of oxides, but you’re probably wondering how they are used.

Then you are exactly right here! Here are the 10 best ways to experiment with oxides.

Spread oxides on greenware, biscuit and/or glaze. Do some slip and add some oxides to create colors. Mix well to get more even colors. Don’t mix too well to get more random, blotchy colors. Brush Oxid Wash over an unfired glaze, then fire. Be very careful when handling as you can smear the oxide. Brush on oxides, then apply glaze. It’s best to dip or spray to avoid brushing off the oxide (and if you dip, it’s best to set some glaze aside so oxide doesn’t contaminate your entire batch of glaze). In general, the heavier the oxide wash, the more it bleeds through the glaze. Mix Ball Clay with your oxide/water. This gives a better consistency and softens the color. Brush on a few different oxides, overlapping in areas. Sgraffito. Brush on oxide. Once dry, scrape through the oxide with a sharp tool to reveal the clay underneath. Cover with transparent or translucent glaze. Or do the same with oxide over unfired glaze. Squirt wax on the surface and brush the oxide wash over it. Use other masking techniques, such as B. torn, wet newspaper and smear an oxide wash over it. Scatter different oxides on a newspaper. Place leather-hard objects on top of the oxide mixture (e.g. a piece of tile). Or use a piece of styrofoam or a sponge to pick up the oxide and transfer it to your piece. Keep the pattern when it lands or smear it around. In this case, you can add glazes during the one-time firing. Otherwise brew and apply glazes; The oxides still interact with the glazes when fired together.

Remember that oxides are strong dyes, so a little goes a long way. In a solution you’ll probably only want around 2-8% or you’ll end up with black.

Be sure to use a respirator or mask when handling the dry oxides, and remember that using oxides like this gives unpredictable but sometimes beautiful results. Test and note!

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The terms oxide and stain or “pottery stain” are often used interchangeably by potters. If you are new to the exciting world of ceramic pigments, you may have wondered what oxides and stains are. Perhaps you’ve been pondering the difference between oxides and stains and haven’t gotten any wiser. This article looks at some of the properties that oxides and stains have in common and how they differ.

Crude or simple oxides are metal elements combined with oxygen. They are ground into a powder and used in pottery as a coloring agent, among other things. Ceramic mordants are a mixture of metallic and ceramic oxides and colorants that have been heated, quenched and ground.

It is often said that ceramic paints are simply more refined or processed versions of raw oxides. Here’s a closer look at the difference between oxides and mordants used in pottery and ceramics.

What are Oxides in Pottery and Ceramics?

Oxides are used in pottery and ceramics for a variety of reasons. They can be used as fluxes, glass formers and colorants.

Fluxes lower the temperature at which a ceramic body melts when fired. Some examples of flux oxides are lead, sodium or zinc.

Glass formers are oxides such as silica. As clay or glaze melts in the furnace, they contribute to the formation of glass.

Both fluxes and glass-forming oxides can also change the color of the substance to which they are added. Typically in ceramics, coloring oxides can be added to glazes, underglazes, slurries or directly to clay bodies.

Oxides are raw materials. They are binary connections. This means that they consist of only two elements. They consist of at least one oxygen atom and one other element. Some oxides contain more oxygen than others, which affects their behavior.

In metal oxides, the other element is metal. This means that the metal oxides used in pottery and ceramics are basic elements.

Although raw oxides are fundamental elements, they impart different properties to glazes. For example, oxides can determine how tough a fired glaze is or how much it shrinks or expands during firing. You can not only influence the color of a fired glaze, but also whether it is matte or glossy.

Because metal oxides are raw materials, their behavior can be less stable than heavily processed materials. Therefore, the color of a metal oxide can differ greatly from its color after firing. This is why glazes often have a different color when they come out of the kiln.

What is the difference between oxides and stains:

The main difference between plain oxides and stains is that ceramic stains are a more refined version of raw oxides.

Ceramic stains are usually made from a combination of metal oxides and ceramic oxides and colored dyes. Metal oxides are derived from metal elements. In contrast, ceramic oxides are made from non-metallic minerals that have been crushed or ground into a fine powder.

The metallic and ceramic oxides are mixed and heated to the point where they melt and fuse. These are then quenched and ground into a powder.

This process is essentially how frit is made in pottery. Frit in glazes makes glazes more stable. They also have lower melting temperatures and fewer defects. In addition, frit produces smoother surfaces and brighter colors (source).

Some of the benefits of using ceramic paints

Pickles and oxides are very similar, but there are some advantages to using pickles. Some of these benefits are as follows…

1. Spots produce more predictable, repeatable colors

Ceramic stains are sometimes referred to as fried dyes (source). Because ceramic patches have undergone a fritting process, they are generally more stable in use than raw metal oxides. Therefore, the color of the unfired ceramic stain is very close to its color after firing.

Another reason ceramic stains do not change color when fired is that dyes are used in their manufacture.

Due to the frying process and the addition of coloring agents, the color of the pickles can be bright. The difference between oxides and stains in this regard is that plain oxide colors look more muted and organic.

2. Stains are less toxic and less soluble

Frying also helps make substances non-toxic and less soluble. This gives pottery stain the advantage of being safer to use in pottery.

However, some manufacturers of ceramic colorants state that they cannot guarantee the food safety of their products. Pottery materials such as glazes, underglazes, and stains may be non-toxic at the point of sale.

However, how safe they are after being used on a piece of pottery depends on other factors. These factors include, but are not limited to, which materials are combined with and how they are fired.

3. Stains come in a variety of colors

Because ceramic stains are made by mixing a number of oxides with colorants, the range of colors available is wide. Some of the colors available as a ceramic stain would be difficult to create by mixing pure oxides.

Basically, the manufacturer has done the hard work to create the specific color you might be looking for.

Penguin Ceramic Patch – View on Amazon

What is the difference between using oxides and stains?

In short, oxides and colorants can be used in very similar ways. They are both very versatile ways to color pottery.

Some potters use oxides and mordants just suspended in water. However, some simple oxides such as cobalt and chromium oxide and some ceramic patches are quite refractory. This can lead to adhesion problems. If the oxide or stain is simply suspended in water, it may not fully adhere to the ceramic.

Even if you put a glaze on it, the dye can prevent the melted glaze from sticking to the surface. This can cause the icing to creep, causing the icing to bunch up and roll off the surface. As a result, oxides and stains are quite often mixed with a flux such as Gertsley borate.

Sufficient flux must be used to allow the spots and oxides to melt and become suspended from the glaze. However, the mixture shouldn’t melt so much that the dyes bleed into the glaze.

A simple rule of thumb is to mix the pigment you are using with an equal amount of flux.

Some of the ways that oxides and stains can be used:

Oxides and stains are versatile. Any type of colored pottery has been stained with one or both mordants or oxides. Here are some ways they can be used…

Applied to greenware or biscuit?

Some potters spray or paint them onto bone-dry clay and allow the dampened surface to dry again before firing.

However, it is more common to apply them to biscuit-fired clay. They can be applied with a brush as a wash or dipped. If you dip into the oxide or stain, make sure it’s mixed well before you dip.

When the powder is mixed with water and settled, the oxide or stain will concentrate at the bottom. Therefore, when you dip the sponge cake into the liquid, the pores of the sponge cake mainly absorb water. This will prevent the oxide or stain particles from being absorbed instead. As a result, the pigment layer becomes thin and washed out.

Once the piece has been dipped, wiping the surface of the ceramic will highlight its texture. Wipe to remove stains or oxides from raised surfaces. At the same time, the dye is pressed into the biscuit pores in the recessed areas. This intensifies the color and gives you a good contrast between dark valleys and lighter peaks.

Alternatively, applying the stain or oxide evenly will create an opaque colored finish.

To glaze or not to glaze?

Stains and oxides can be fired alone, or a clear glaze can be applied over them. Oxides interact differently with each other. So when you apply a colored glaze, you can get unexpected results.

Slips, underglazes and glazes

Coloring oxides and stains can be mixed into slips, underglazes and glazes.

Mixing with clay bodies

They can also be mixed with a clay mass. Some coloring oxides are not suitable for this, others are specially designed for this use. Mason’s stain is a good example of the types of pottery stain that can be used to color clay.

They are simply pinched to create a clay body color of your preference. This is great for Nerikomi (aka Agateware).

Check out my YouTube channel for a full step-by-step video of making colored clay.

Stains and oxides on the glaze?

If the stain or oxide has been mixed with a flux, it can be painted over an unfired glaze. This is known as the majolica technique. When the piece is fired, the stain and flux mixture fuses with the glaze.

With this technique it is advisable to use a white base glaze. This is because oxides can interact with the oxides in colored glazes, creating unwanted colors.

Problems using simple oxides in pottery

Since raw oxides are not fried, they tend to be soluble and toxic.

Some raw oxides like copper and zinc are volatile, meaning they evaporate easily (source). When they vaporize, they give off toxic fumes. Toxic fumes from oxides such as manganese are dangerous to inhale (source). In addition, the fumes can be absorbed into the inner surface of the oven.

Because simple oxides are less stable than ceramic stains, they are more likely to react to oxides in glazes. Such oxides can affect the color of the glaze and produce undesirable colors. For example, chromium oxide reacts with tin in a glaze, turning a white glaze pink (source).

Problems with ceramic stains in pottery

Because of the problems sometimes encountered with raw oxides, potters are often advised to use ceramic stain.

Stains are not without limitations, however, and here are some of the issues to consider:

The bright, uniform, and predictable colors are not to everyone’s liking. Some potters find them dull compared to the more organic variegated earth tones of a raw oxide.

Because ceramic stains are time-consuming and labor-intensive to produce, they can be expensive compared to pure oxides.

Although ceramic stain firing results are more predictable, they cannot be guaranteed. As with oxides, the final color of a ceramic paint is affected by how it interacts with the glazes or underglazes used. Some ceramic stains are not compatible with certain glazes, so the chemistry between the two must be considered.

Porcelain stains are more stable and generally considered safer to use than raw oxides. They have been prefired and are often mixed with other materials that make them less leaching during glaze firing.

However, they contain metal oxides and the glaze can leach metals from the stains when fired. Therefore, it is recommended to test ceramics, which are not only used for decorative purposes, in the laboratory.

It’s relatively easy to get your pottery lab tested. There are organizations you can send a sample of your glazed pottery to. They will then run tests on that sample to determine if the glaze is safe to use.

Final Thoughts

In a way, the difference between oxides and stains in ceramics is pretty straightforward. Stains are simply oxides that have been processed to make them more stable and provide a wider range of colors. However, oxides have multiple uses in ceramics, while mordants are mainly used as colorants. Also, the chemistry behind the behavior of oxides and stains is complex and perhaps best left to the chemists among us. That would be an interesting venn diagram – I wonder what the overlap between potters and chemists is like!

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All Glossary

food safe

There is growing awareness among potters about the food safety of glazes. Be skeptical of food safety claims made by potters who cannot explain or show why.

details

In recent years, potters and small manufacturers have become aware (or have had to be aware) that ceramics and pottery are not as inert as they once thought. There are a multitude of potential health effects for users of the goods they manufacture. These include flaking of glaze splinters (which could be swallowed), harboring bacteria (in glaze cracks and pores of bodies or engobes), possible fractures or cracks in the goods in the event of sudden heating (e.g. from hot coffee), overheating of the Ware microwave ovens (with risk of burns) and leaching of metals or other elements from the glaze surface (by hot, acidic or basic liquids).

Perhaps the most public and serious issue is leaching. Be skeptical of claims made by potters unfamiliar with the subject. Frostings are traded online and will soon lose any non-working labels they may have had. Potters who make tableware with a transparent or white coating on food surfaces are the ones most aware of this issue. Those who make tableware whose food surfaces are variegated (with no transparent overglaze), very dull, heavily crystallized, or excessively melted (and therefore run) are the least likely to be unaware of this issue.

Although a ceramic glaze is similar to glass, it can actually disintegrate over time when in contact with liquids if its chemistry is not balanced and stable. This is not a real problem if the glaze is white or transparent and does not contain any harmful elements (e.g. feldspars, kaolin, silica, calcium carbonate, talc). However, if it contains metal oxide dyes, barium, lithium or lead these pose a problem. It is possible that a glaze containing all of these will be stable and not dissolve enough to be a hazard. But if the glaze doesn’t have balanced chemistry, it can certainly leach metals into foods and beverages.

What does balanced mean? It means normal. Consider some obvious questions:

-Every potter knows that 0.5 or 1% cobalt oxide can produce a strong blue in almost any glaze. What if a recipe contains 5% cobalt? This is a red light, the extra cobalt could very well fall out of the melt during cooling and crystallize rather than becoming part of the glaze matrix.

-Does the glaze form crystals on its surface when fired? Crystals in glazes are often unstable and leach out.

-We expect 3% lithium carbonate in a recipe. What if there is 10%? What if there is 30% barium instead of 5%? Or 20% manganese dioxide instead of 5%? These are obvious problems and some icing fails at this simple assessment.

-What if there is only 1% cobalt in a glaze base that melts a lot (and runs off the ware if applied too thickly)? Probably this glaze contains too little SiO 2 and Al 2 O 3 (the former is the glass and the latter the stabilizer, all functional glazes must have these).

-What if the frosting is dull and doesn’t even melt properly? Any dyes in it will leach out.

-What if a glaze contains 60% feldspar instead of the normal 30%? Feldspar is not dangerous, but it is the base of the glaze into which dangerous metal oxides can be added to give color. If the host glaze into which the colors are added is not balanced, they will likely leach out.

A special class of glazing dyes are mordants. These powders were assembled by mixing metal oxides with stabilizers and firing at temperatures in excess of what a normal glaze would be fired to. The cooled sintered material is then ground to a powder. In theory, these stain particles do not dissolve in the glaze melt, so safety issues are mitigated. However, some colorants produce very bright reds, yellows and oranges, they are very useful as these colors are difficult or impossible to achieve with metal oxides alone. These patches are also much less stable than others and have lower temperature limits. If used above the recommended temperature, some of the toxic metals they contain (e.g. cadmium) will dissolve in the glaze melt and become leaching hazards. For example, the cone 6 may be outside the range of conventional encapsulated dyes.

Balance is also sought in the chemistry of a recipe. For example, Insight-live.com can display the oxide chemistry of the recipes you enter. But caution is advised. Many people say that safe glazes have a specific chemical profile, and that one can recognize a leaching propensity simply by looking at a chemical formula (to see if certain oxides are out of bounds). There’s only some truth to that. For example, we might expect to see 4.0 SiO 2 in a medium temperature glaze, but if a recipe contains only 1.5 there is a good chance that a stable glass was not formed. Of course, high percentages of oxides like BaO, MnO, PbO, Li 2 O would be potential problems, but the high percentages of their starting materials would be obvious in the recipe anyway. The point is that absolute limits on all oxides (to prevent leaching) cannot be specified, the chemistry is much more complex than that. Also, it would drastically limit the range of acceptable compounds to make glazes. Remember that leaching is easy to test (see the links on this page) and anyone can do it. Therefore, a better use of chemistry is to provide instructions for changes when a glaze fails a leaching test. Still, it makes sense to be aware of obvious chemical irregularities (compared to common basic recipes for the same temperature). You can find articles and videos on this page.

Crazing is often considered a visual advantage by potters, but for safety reasons crazing is unacceptable. The cracking significantly weakens the fabric and provides sites for water to penetrate and saturate the porosity of the body beneath. The cracks harbor bacteria and provide vulnerabilities where thermal shock can spread them. Simple craze awareness is a powerful action anyone can take to improve the safety of the goods they manufacture. Hairline cracks occur when glazes are under tension and are stretched onto the goods. This is fixed by adjusting the recipe to reduce thermal expansion (many articles and videos on this site show how to do this).

Simple methods of physical examination are affordable for everyone. For leaching, a slice of lemon left on the icing (or some pure lemon juice) overnight will show differences in color or surface texture as leaching occurs. A simple visual inspection reveals hairline cracks or tremors. If a piece of crockery is put in the dishwasher every day, hairline cracks will almost certainly occur if the glaze is under tension. An immersion test in ice water and boiling water is even better to show if these problems will appear over time.

related information

Does copper cause ceramic glazes to leach?

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These are four cone 6 glazes of different chemistry. They have different melt flow abilities. They are soaked in lemon juice overnight (halfway up). None show signs of surface alteration. All contain 2% copper carbonate. If the copper were increased, particularly to the point where it becomes metallic or crystallizes, the leach test would likely have different results. So if you use copper judiciously (in moderate amounts), there’s a good chance you can make a glaze that resists leaching.

We fight the dragon that others don’t even see

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There are thousands of ceramic glaze recipes floating around the internet. People dream of finding the perfect one, but they often only think about looks, not usability, function, safety, cost, or materials. This resistance to understanding your materials and glazes and learning to take control is what we embody as dragons. You can use the resources on this site to repair, customize, test, and formulate your own glaze recipes. Start with your own account on insight-live.com.

Copper can destabilize a glaze and make it soluble

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A close-up of a lustrous Konus 6 glaze with 4% added copper carbonate. The lower part is leached in lemon juice after 24 hours. This photo has been adjusted to expand the color space to emphasize the difference. The drained area is now dull.

This leaching mug requires a liner glaze. Serious!

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Three Konus 6 commercial bottle glazes were layered. The beaker was filled with lemon juice overnight. The white spots on blue and rust spots on brown are leached! Why? Glazes require a high flowability of the melt in order to create such reactive surfaces. While such are usually leached out, manufacturers have been able to tune the chemistry of each to be resilient. But the overlaps mix well (because of the fluidity), they’re new chemistries, less stable. What is leaching? Cobalt! Not good. What else? We don’t know, these recipes are secret. It’s much better to make your own clear or white liner glaze. Not only can you apply it and get very even coverage, but you know the recipe, are in control, and can customize it to suit your body.

What can you do with glaze chemicals? More than you think!

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There is a direct relationship between the way ceramics fire and their chemistry. These green panels on my Insight Live account compare two icing recipes: one glossy and one matte. Understanding their simple chemical mechanisms is a first step in gaining control over your glazes. To fix problems such as crazing, blistering, pinholes, settling, gelling, clouding, leaching, creeping, marking, scratching, powdering. To replace frits or incorporate available better or cheaper materials while keeping the same chemistry. For setting melting temperature, gloss, surface finish, color. And spotting weak points in glazes to avoid problems. And for creating and tweaking base glazes, to work with difficult colors or stains, and for special effects that depend on haze, crystallization, or chroma. And even creating glazes from scratch, using your own native materials in the highest possible percentage.

A good matte glaze. A bad matte glaze.

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Comparison of melt flowability between two Konus 6 matte glazes. G2934 is a MgO saturated boron flux glaze that melts to the right degree, forms a good glass, has low thermal expansion, resists leaching and leaves no cutlery marks. G2000 is a widely used Cone 6 recipe, it is fluxed with zinc to create a surface network of microcrystals which not only frosts the icing but also opacifies it. But it forms a bad glass, runs too much, the cutlery stains heavily, discolours easily, cracks and probably isn’t food safe! The G2934 recipe can be google searched and shows well how the high MgO matting mechanism (from talc) produces a silky surface on the oxidation of cone 6, just like the reduction of cone 10 (from dolomite). However, it needs an addition of tin or zirconium to be white.

Why are these crazy lines so dark?

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This is an example of serious crazing in a glaze. The lines have darkened with the use of the bowl! That means the color is organic, made from food. That can’t be healthy.

A matte and a gloss liner glaze

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Left: Ravenscrag G2928C matte liner glaze on the inside of the mug. Right: A clear high gloss. The mat must be soaked in the oven long enough to ensure that it develops a functional surface, especially on the underside. Mattes aren’t always the best choice for food surfaces, but you can do it if you mix in enough glossy glaze to make it smooth enough not to leave cutlery marks.

Commercial glazes on decorative surfaces, our own on food surfaces

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These 6 cone-shaped porcelain mugs are hybrid. Three coats of a commercial glaze on the outside (Amaco PC-30) and my own liner glaze poured in and out on the inside (G2926B). If commercial glazes (made by one company) fit earthenware or china (made by another company) without cracking or chipping, that is pure coincidence! So use them on the outside. But for indoor food surfaces, make your own or mix. If you know the recipe, you can adjust the thermal expansion. And the degree of melting. And the application properties. And you can use high quality materials to get balanced chemistry. The place to start understanding your glazes, organize testing and development, and document everything is with an account at Insight-live.com.

How much rutile can a glaze take before it becomes unstable?

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The 80:20 Base Alberta Slip Base becomes oatmeal when oversaturated with rutile or titanium (left: 6% rutile, 3% titanium; right: 4% rutile, 2% titanium, right). This oatmeal effect is actually the excess titanium crystallizing from solution in the melt as the oven cools. Although the visual effects can be interesting, the microcrystalline surface is often prone to cutlery stains and washouts. This is because the crystals are not as stable or durable as the glaze’s glass.

If your glaze can handle more silica and melt just as well, add it!

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The Cone 6 G1214M glaze on the left melts well. Can it benefit from silica addition? Yes. The one on the right adds 20% but still melts, has better coverage, is shinier, has better washout resistance, is harder, and has lower thermal expansion.

connections

What are iron oxides?

Iron oxides are skin care and cosmetic ingredients used to color formulations. Iron oxides are used in three basic tones: black (CI 77499), yellow (CI 77492) and red (CI 77491).

Iron oxides consist of iron and oxygen and have been used as colorants in cosmetics since the beginning of the 20th century. Iron oxides occur naturally, for example rust is a type of iron oxide. Red Iron Oxide can be naturally derived from the mineral hematite; yellow iron oxides come from limonites such as ochre, sienna and umber; Black iron oxide is extracted from the mineral magnetite. However, the iron oxides used in cosmetics are mostly synthetic. There are a total of 16 different iron oxides used in cosmetics. In addition, iron oxides for use in cosmetics are found in paints, coatings and colored concretes.

What are the best skin care products of 2022?

Iron oxides are strictly regulated by the US Food and Drug Administration. The US Food and Drug Administration’s Code of Federal Regulations for Iron Oxides states: “Synthetic iron oxides are produced in a variety of ways, including the thermal decomposition of iron salts such as ferrous sulfate to make red wines; precipitation to produce yellow, red, brown and black; and reduction of organic compounds by iron to produce yellow and black.”

the degradation iron oxides the good: iron oxides are used to enhance the color of cosmetics and skin care products to enhance their appearance. The not so good: Iron oxides are not used to improve the condition of the skin and only serve to enhance the appearance of the formulations. For whom is it suitable? All skin types except those with a proven allergy to it. Synergistic Ingredients: Works well with most ingredients. Note: There is a difference in impurity levels between the natural and synthetic forms of iron oxides. Synthetic iron oxides are generally considered safer than natural forms. Why are iron oxides used? Iron oxides act as colorants in cosmetics and skin care products. They are the main pigments used to adjust skin tones in foundations, powders, concealers and other facial makeup. Iron oxides can also be found in eyeshadow, blush, powder, lipstick, and mineral makeup. Iron oxides are available in three basic shades: black (CI 77499), yellow (CI 77492) and red (CI 77491). There are also different shades of brown of iron oxides, but these are just mixtures of the three colors mentioned above. Iron oxides are opaque and have excellent lightfastness, however yellow and black iron oxides are sensitive to high temperatures. Iron oxides are also moisture resistant, so they don’t bleed or smudge easily. Iron oxides have excellent “holding power” which means the product will last a long time without the need for reapplication. Natural or Synthetic: Which is Better? Iron oxides are considered safe when used in cosmetics and personal care products as they are non-toxic and non-allergenic. Iron oxides are well tolerated even by sensitive skin. Although iron oxides are synthetic ingredients, they are still commonly used in products marketed as natural or organic. This is because the synthetic versions of iron oxides are actually safer than the natural versions, which often contain impurities. For example, oxides formed in a natural, uncontrolled environment are often contaminated with heavy metals. This shows that just because an ingredient is natural does not always make it safe. Are Iron Oxides Safe? The Cosmetic Ingredient Review (CIR) expert panel, a group responsible for evaluating the safety of cosmetic and skincare ingredients, has deferred its review of iron oxides as the safety has been evaluated by the U.S. Food and Drug Administration. All color additives used in foods, drugs, and cosmetics in the United States must be FDA approved and listed in the Code of Federal Regulations. The FDA only approves paints after a thorough review of all safety data and publication of the basis for their approval in the Federal Register. References: Bernstein, E., Sarkas, H., Boland, P., & Bouche, D., 2020. “Beyond sun protection factor: An approach to environmental protection with novel mineral coatings in a vehicle contains a blend of skincare ingredients” , Journal of Cosmetic Dermatology, 19(2), 407-415.

Mineral Resource of the Month: Iron Oxide Pigments

from US Geological Survey Wednesday June 20, 2018

Michael J. Potter, Mineral Resources Specialist at the U.S. Geological Survey, has compiled the following information on iron oxide pigments.

Iron oxide pigments, which can be natural or synthetic, have been used as colorants since early humans began painting cave walls. Natural pigments are derived from several iron oxide minerals: Red pigments are derived from hematite. Yellow and brown pigments – ochre, sierra and umber – are extracted from limonite. Magnetite provides a black iron oxide pigment. Micaceous iron is a special form of hematite that occurs in thin, metal-grey platelets or flakes. Synthetic pigments are manufactured under controlled conditions so particle size, distribution and shape can be accurately reproduced, resulting in superior uniformity, color quality and chemical purity.

Iron oxide pigments are relatively inexpensive materials that resist color changes from exposure to sunlight, have good chemical resistance, and are stable under normal environmental conditions. The pigments are mainly used in paints, coatings and building materials such as concrete products, mortar, paving stones and roof tiles. Natural pigments are used in primers and primers where color consistency is less critical, while synthetic pigments are used in top coats where color consistency is important.

Micaceous iron imparts unique properties to paints and coatings as the flaky particles align themselves to resist the ingress of moisture and gases. These coatings can prevent corrosion and rusting of metals and also resist blistering, cracking and peeling.

Deposits of iron oxide pigments occur in many countries but have only been significantly developed in a few. Countries historically known for producing iron oxide pigments include Cyprus, France, Iran, Italy and Spain. Countries with new significant production include India, Spain and Honduras.

Iron oxide pigments are also produced during steel production. When steel is treated with hydrochloric acid to remove surface oxides, the acid is regenerated to be recycled and iron oxide is produced. Regenerated iron oxides are used in a variety of filters, inductors, and transformers in household electronics and industrial equipment, as well as in flexible magnets, generators, speakers, and electric car motors.

New developments in the synthetic iron oxide pigment industry in recent years include granular forms of iron oxides and new versions of nano-sized materials used in computer drives and high-performance speakers, as well as in biology and medicine, including nuclear magnetic resonance imaging.

For more information on iron oxide pigments, visit minerals.usgs.gov/minerals.

PRODUCTION AND CONSUMPTION OF IRON OXIDE PIGMENTS

The total world production of iron oxide pigments (natural: 13 percent and synthetic: 87 percent) was about 1.4 million tons in 2006.

China was the world’s leading producer in 2006, accounting for 49 percent of the world’s total production.

In 2007, estimated total US production was 50,000 tons, worth about $50 million.

In 2006, the estimated global market for iron oxide pigments was US$1.1 billion.

FUN FACTS

Natural iron oxide pigments have been used in art for tens of thousands of years, ever since humans created the 32,000-year-old cave paintings at Lascaux, France.

Iron oxide pigments are used as colorants for ceramic glazes, glass, paper, plastic, rubber, and textiles, as well as in cosmetics and magnetic inks and toners.

Micaceous iron coatings have been used for high performance applications in harsh environments including industrial tanks, refineries, chemical plants, oil rigs and bridges and even on the Eiffel Tower.