1 often use different colored clay slips to decorate mugs and goblets All of the pieces I create are food safe in the materials used, and are microwave safe. However, some pieces that have been decorated with slip have nooks and crannies that food could get caught in and result in bacteria forming. Caution and common sense should be used, depending on the piece. The casserole dishes I’ve created are also oven safe, only if put into a cold oven to start. Most any ceramic item, unless specifically created to withstand it are susceptible to thermal shock. You shouldn’t put a cold dish into a hot oven or a hot dish immediately into a cold environment, to extend the life of the pot temperature changes should be gradual. You should have no problems with pouring hot coffee into mug, or filling a teapot with hot water for tea, but a dish in an oven needs to be gradually brought up to temperature, and also shouldn’t be subjected to temperatures above 400˚ F.