So we already know that William is vehemently anti-ivory, so King Charles’ portrait will most likely not be painted on ivory. I suppose that leaves some sort of porcelain perhaps, or another white material.
We were told Catherine's Family Order used glass so I suppose that might be the material in future.
https://www.harpersbazaar.com/celebrity/latest/a24125335/kate-middleton-royal-order-queen-elizabeth/
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