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The black color effect was achieved by means of changing the amount of oxygen present during firing.
This was done in a process known as three-phase firing and was likely accomplished with multiple firings of the pottery.
The charcoal would be dust and then mixed with animal fat to make the pigment.
Mars Black (black pigment made of synthetic iron oxides) is an opaque, more neutral black with stronger tinting power.
They began by using charcoal, and then made more vivid black pigments by burning bones or grinding a powder of manganese oxide.