Art Clay Silver 650 20gm Clay.This can be modelled in the same way as regular water-based clay. Once opened, it is susceptible to the air and must be kept moist in order to retain its pliability and workability. Use a container of water and a paintbrush to moisten any areas of the clay surface that begin to crack while working. This clay can be fired in a kiln, on a gas hob or with a torch. The clay fires at 650°C and shrinks about 8-9%.