PLEASE NOTE:
If you are mixing up a Steven Hill glaze that calls for "High Purity Red Iron Oxide," he gets his materials through another source - US Pigment. Steven has said that his Red Orange glaze will NOT turn out as expected if you use a different iron. Steven went on to tell me that using the US Pigment version of the high purity red iron oxide is not as critical in the Juicy Fruit glaze recipe.

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.

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.
- Digital Fire ceramic materials database