smooth cast onyx fast black liquid plastic casting resin - trial unit smooth-on smooth-cast onyx is a mercury-free urethane resin that cures quickly at room temperature to a deep black, solid plastic. onyx offers the convenience of a 1a:1b by volume mix ratio and has a very low viscosity, so it is easy to mix and pour. pot life: 2.5 mins; cure time: 10-15 mins. , shore hardness: 80d and offers higher physical properties and higher heat resistance vs. other general purpose resins. consult the technical bulletin for more information. read the msds sheet before using. adult supervision required. keep out of the reach of children.