Oil-based clays are made from various combinations of oils, waxes, and clay minerals. Because the oils do not evaporate as does water, oil-based clays remain malleable even when left for long periods in dry environments. Articles made from oil-based clays cannot be fired, and therefore are not ceramics. Because the viscosity of oils decreases as temperature rises, the malleability is influenced by heating or cooling the clay. Oil-based clay is not soluble in water. It can be re-used and so is a popular material for animation artists who need to bend and move their models. It is available in a multitude of colours and is non-toxic.