Plastilin is a sulphur-free wax-based modelling material, smooth like clay, but being wax-based it never dries out, and can be used many times.
Available in 2 grades, Standard and Hard. The Standard grade is an ivory colour, softer and more malleable, so is easier to use by hand. The Hard grade is grey, and can be better for working in fine detail. Both can be softened by careful warming.