Smooth-Cast® ONYX™ is a mercury-free urethane resin that cures quickly at room temperature to a deep black, solid plastic. ONYX™ offers a higher heat resistance, up to 250°F/121°C, and the cured plastic can be polished. Applications include reproducing sculpture, making prototypes and potting / encapsulation. Due to it’s quick cure time and hardness, ONYX™ FAST is a good choice for creating fast cold cast bronze, brass, copper, nickel/silver, etc. effects.