The Egyptians made kilns to place their clay pots in for firing. The kiln was lined with a kind of insulation brick that was made from a mixture of straw and clay which had been dried in the sun. Later, the ancient Egyptians used a finer clay with a high quartz content for their delicate pottery.n medieval times sand was mixed with clay to make cooking pots strong enough to be placed over an open fire. Today, for the same reason, casseroles used for baking are made from clay mixed with grog which is a ground-up fired pottery.