Polyurethane rubber molds are used for casting plaster or cements. Polyurethane is commonly referred to as simply "urethane". Ceramic slip casting molds are cast in urethane rubber molds. Urethane is less expensive than silicone rubber and has excellent durability and long library life. These molds can be either solid "block" molds or a thin "skin" mold with a rigid shell. It comes in a range of hardnesses to meet many different mold-making needs. Flexible and semi-rigid parts can also be cast in urethane rubber that can be pigmented in an almost endless variety of colors.