These pieces are made primarily of epoxy resin, a two-part liquid which, when mixed, heats up and slowly hardens. It must be built up slowly in layers or the batch will get smoking hot and melt the mold. Like porcelain, resin can be worked in a variety of ways depending on the degree of liquidity or solidity. Different resins have different degrees of viscosity, speed of cure, durability, etc. Many are naturally amber in color.