Alternate Names: Ferric Oxide, Red Iron Oxide, RIO, Iron(III) oxide, Fe2O3, Hematite
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Synthetic red iron oxide is the most common colorant in ceramics and has the highest amount of iron. It is available commercially as a soft and very fine powder made by grinding ore material or heat processing ferrous/ferric sulphate or ferric hydroxide. During firing all irons normally decompose and produce similar colors in glazes and clay bodies (although they have differing amounts of Fe metal per gram of powder). Red iron oxide is available in many different shades from a bright light red at a deep red maroon, these are normally designated by a scale from about 120-180 (this number designation should be on the bags from the manufacturer, darker colors are higher numbers), however in ceramics these different grades should all fire to a similar temperature since they have the same amount iron. The different raw colors are a product of the degree of grinding.
In oxidation firing iron is very refractory, so much so that it is impossible, even in a highly melted frit, to produce a metallic glaze. It is an important source for tan, red-brown, and brown colors in glazes and bodies. Iron red colors, for example, are dependent on the crystallization of iron in a fluid glaze matrix and require large amounts of iron being present (eg. 25%). The red color of terra cotta bodies comes from iron, typically around 5% or more, and depends of the body being porous. As these bodies are fired to higher temperatures the color shifts to a deeper red and finally brown. The story is similar with medium fire bodies.
In reduction firing iron changes its personality to become a very active flux. Iron glazes that are stable at cone 6-10 in oxidation will run off the ware in reduction. The iron in reduction fired glazes is known for producing very attractive earthy brown tones. Greens, greys and reds can also be achieved depending on the chemistry of the glaze and the amount of iron. Ancient Chinese celadons, for example, contained around 2-3% iron.
Particulate iron impurities in reduction clay bodies can melt and become fluid during firing, creating specks that can bleed up through glazes. This phenomenon is a highly desirable aesthetic in certain types of ceramics, when the particles are quite large the resultant blotch in the glaze surface is called a blossom.
Iron oxide can gel glaze and clay slurries making them difficult to work with (this is especially a problem where the slurry is deflocculated).
Iron oxide particles are very small, normally 100% of the material will pass a 325 mesh screen (this is part of the reason iron is such a nuisance dust). As with other powders of exceedingly small particle size, agglomeration of the the particles into larger ones can be a real problem. These particles can resist break down, even a powerful electric mixer is not enough to disperse them (black iron oxide can be even more difficult). In such cases screening a glaze will break them down. However screening finer than 80 mesh is difficult, this is not fine enough to eliminate the speckles that iron can produce. Thus ball milling may be the only solution if the speckle is undesired.
Red iron oxides are available in spheroidal, rhombohedral, and irregular particle shapes. Some high purity grades are specially controlled for heavy metals and are used in drugs, cosmetics, pet foods, and soft ferrites. Highly refined grades can have 98% Fe2O3 but typically red iron is about 95% pure and very fine (less than 1% 325 mesh). Some grades of red iron do have coarser specks in them and this can result in unwanted specking in glaze and bodies (see picture).
High iron raw materials or alternate names: burnt sienna, crocus martis, Indian red, red ochre, red oxide, Spanish red. Iron is the principle contaminant in most clay materials. A low iron content, for example, is very important in kaolins used for porcelain.
One method of producing synthetic iron oxide is by burning solutions of Ferric Chloride (spent pickle liquor from the steel industry) to produce Hydrochloric Acid (their main product) and Hematite (a byproduct). 100% pure material contains 69.9% Fe.
This is what about 10% iron and some titanium and rutile can do in a transparent base glaze with slow cooling at cone 10R on a refined porcelain.
Since iron oxide is a flux in reduction, overglaze iron based pigments run if applied to thickly
This is cone 6 an oxidation transparent glaze having enough flux (from a boron frit or Gerstley Borate) to make it melt very well, that is why it is running. Iron oxide has been added (around 5%) producing this transparent amber effect. Darker coloration occurs where the glaze has run thicker. These are all simple mechanisms, which, once understood, can be transplanted into other glazes. This glaze is also crazing. This commonly occurs when the flux used is high in K2O and Na2O (the highest expansion fluxing oxides). K2O and Na2O produce the brilliant gloss. They come from feldspars, nepheline syenite and are high in certain frits.
The recipe contains 6% red iron oxide. The chemistry is high alumina (from 45 feldspar and 20 kaolin), zero silica silica (4:1 Al2O3:SiO2 ratio) and 20% calcium carbonate. The remainder is a little talc and calcium phosphate. The reduced iron is fluxing what would otherwise be a very matte surface. Reducing the iron percentage to 4% produces a yellow mustard color.
Polymer plates are used in letter press and can still be purchased online (e.g. boxcarpress.com). Just create the artwork in a vector graphic drawing program and upload it to their website. Press it into a thin slab (using some sort of oil as a parting agent) and then attach that using slip to the item. After the piece is bisque fired apply the color and wipe it away from the high spots using a sponge. This is a celadon cone 10R glaze (about 3.5% iron oxide) on a buff firing reduction stoneware with G1947U transparent liner glaze. The brown in the logo is a tenmoku.
Fired on a porcelain in a gas kiln.
Cone 10 reduction fired crystallizing kaki glaze (about 12% iron oxide).
4% red iron oxides from two different manufacturers in a cone 6 ravenscrag slip based glaze. The one of the left has a coarser grain size.
This cone 10R glaze, a tenmoku with about 12% iron oxide, demonstrates how iron turns to a flux in reduction firing and produces a glaze melt that is much more fluid. In oxidation, iron is refractory and does not melt well (this glaze would be completely stable on the ware in an oxidation firing at the same temperature, and much lighter in color).
5 different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The specks are not due to particle size, but differences in agglomeration of particles. Glazes employing these iron oxides obviously need to be sieved to break down the clumps.
Five different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The glazes have been sieved to 100 mesh but remaining specks are still due to agglomeration of particles, not particle size differences.
An example of how iron stone concretions contained within two clay bodies (a white and brown stoneware) blossom and produce speckle at cone 10 reduction.
Example of 5% black iron oxide (left), red iron oxide (center) and yellow iron oxide (right) added to G1214W glaze, sieved to 100 mesh and fired to cone 8. The black is slightly darker, the yellow has no color? Do you know why?
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
It is a powerful glaze flux, variegator and crystalizer, a colorant of many characters in bodies and glazes and a specking agent like no other. And it is safe and cheap!
Iron oxide is an amazing glaze addition in reduction. It produces celadons at low percentages, then progresses to a clear amber glass by 5%, then to an opaque brown at 7%, a tenmoku by 9% and finally metallic crystalline with increasingly large crystals past 13%. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.
The recipe: 50% New Zealand kaolin, 21% G200 Feldspar, 25% silica and 3% VeeGum (for cone 10R). These are the cleanest materials available. Yet it contains 0.15% iron (mainly from the 0.25% in the New Zealand kaolin, the VeeGum chemistry is not known, I am assuming it contributes zero iron). A 50 lb a 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 using a standard raw bentonite. A box of it contains four times as much iron. Enough to fill that cup half full!
Why the difference? The one on the right (Plainsman M370) is made from commodity American kaolins, ball clays, feldspars and bentonite. It looks pretty white-firing until you put it beside the Polar Ice on the left (made from NZ kaolin, VeeGum plasticizer and Nepheline Syenite as the flux). These are extremely low iron content materials. M370 contains low iron compared to a stoneware (less than 0.5%) that iron interacts with this glaze to really bring out the color (although it is a little thicker application that comes nowhere near explaining this huge difference). Many glazes do not look good on super-white porcelains for this reason.
This is common with high iron clays, they lighten dramatically during the last few hundred degrees of cooling in the kiln.
Iron oxide is a very fine powder. Unfortunately it can agglomerate badly and no amount of wet mixing seems to break down the lumps. However putting the glaze through a screen, in this case, 80 mesh, does reduce them in size. Ball milling would remove them completely. Other oxide colorants have this same issue (e.g. cobalt oxide). Stains disperse much better in slurries.
The glaze on the right is a transparent, G2926B, on a dark burning cone 6 body (Plainsman M390). On the left is the same glaze, but with 4% red iron oxide added. The entrained micro bubbles are gone and the color is deep and much richer. It is not clear how this happens, but it is certainly beneficial.
These two mugs are the same dark burning 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 light-burning body the iron changes the otherwise transparent glass to amber colored (with speckle). But on this dark burning clay it appears transparent. But amazingly, the bubble clouds are gone. We have not tested further to find the minimum amount of iron needed for this effect.
Out Bound Links
'Terra Cotta' (Italian for 'cooked earth') is red ...
Iron(II) Sulphate, Iron(II) Sulfate
Iron(III) oxide, hydrated iron oxide, iron(III) hydroxide, yellow iron oxide
Fe occurs in a number of natural forms, it is abun...
The mineral form of iron(III) oxide. Used for the ...
Fe2O3 decomposes to Fe3O4. Releases oxygen.
A kiln atmosphere which is deficient in free oxyge...
An effect created by firing a clay containing high...
In Bound Links
Fe3O4, Black Iron Oxide, BIO, Magnetite Powder, Iron(II,III) Oxide