There doesn't have to be any fat left by the time it met any clay in order for the ash of fat (edit to add: or other facets of using fat) to give different properties to ceramic and so be used in the production of ceramic....there wouldn't be any fat left by the time it met any clay.
There isn't any bone left by the time it meets any clay in the making of bone china. It is ash of bone by that time. It is still a use of bones.
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