Refractory clays consist of natural clay materials, primarily clays from the kaolinite group, along with fine-grained micas and quartz, and may also contain organic matter and sulfur compounds. Refractory materials are ideal for the construction of furnaces, barbecues, and fireplaces, which must maintain their strength and structure at high temperatures. Depending on the application, these materials must withstand thermal shock, be chemically inert, have low thermal conductivity, and a low coefficient of expansion.