I've made a plaster version of a piece of coal! It's interesting - the same form but the material has changed - does the form define a piece of coal - no the material - but the form is important and so a plaster version can be confusing. The detail in the plaster is fantastic - and when cast in porcelain, what will the values be?
Ideas include different clays, clays with feldspar creating the shine that you get on coal, different oxides, cutting pieces in sections - maybe hiding narratives, or creating surfaces for printing historic images or capturing wording that relates to the story of south Wales. I don't know - have to explore!