Iron Oxide Black is also known as BIO, Magnetite Powder,
In ceramics, black iron is used as a source of Fe (in preference to red iron) where its black raw colour 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 Black because the clay blend does not fire to quite as dark of a colour. Since the original Albany Slip powder was a dark grey, black iron (rather than red) is employed in the Alberta Slip recipe to match this colour better and provide the needed iron to the fired product.
The chemistry shown here is not the actual, synthetic black iron is almost pure Fe3O4. This chemistry is intended to work with INSIGHT where it is normal to define only FeO and Fe2O3.
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.