This is 100% of the pure material. Notice how the iron is fluxing it more on the left, it is beginning to run. And how the reduction atmosphere amplifies the color of the iron (by changing it to the metallic state).
A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state.
In ceramics, this term is most often used to refer to kilns firing with an atmosphere having available oxygen to react with glaze and body surfaces during firing
|Oxides||FeO - Ferrous Oxide|