Iron Oxide Black
The Iron oxide black used in ceramics is generally this synthetic form (the natural equivalent mineral magnetite contains 5-15% impurities). Synthetic Iron oxide black is much more expensive than the natural finely ground material (-200 mesh) but if there are good reasons for its use and percentages in the product recipe are low enough the cost may be justified. In ceramics, Iron oxide black is used as a source of Fe (in preference to red iron) where its black raw color and its better distribution properties are needed. For example, Alberta Slip is a recipe of raw clays and minerals intended to duplicate Albany Slip. The recipe calls for a small amount of iron oxide because the clay blend does not fire to quite as dark of a color. Since the original Albany Slip powder was a dark grey, Iron oxide black (rather than red) is employed in the Alberta Slip recipe to match this color better and provide the needed iron to the fired product.
Synthetic Iron oxide black is fluffier and lighter than synthetic iron oxide red(a bag of Iron oxide black is much larger than a bag of red). It is a very fine powder, 100% will easily wash through a 325 mesh screen. Synthetic Iron oxide black does not agglomerate as badly as iron oxide red, thus it disperses in glaze slurries better (thus avoiding fired speckle). You can determine which form you have by washing a sample through a 325 mesh screen, if there is residue it is natural magnetite.
The exceedingly fine particle size of iron oxides makes them very messy to work with, they stain the skin in a manner than only soap can remove even though they do not dissolve in water.
High purity, low heavy metal content grades of Iron oxide black are available. All forms should have 90% or more Fe3O4. Iron oxide black is also used as a colorant for a wide range of non-ceramic products.
Most synthetic magnetites are made by some type of chemical precipitation (0.2-1 micron particle size). However a high temperature dry process can be used to convert synthetic hematite into synthetic magnetite (thus the greater cost). The resultant product of this process has a slightly larger particle size (2-10 micron). 100% pure material would contain 72.3% Fe.
This article comes from digitalfire edit released