Shaped block of baked clay used for construction. Usually obloid in shape with plane faces and parallel edges; fine uniform texture with no fissures, air bubbles, or pebbles; some are deep-salmon colour from the iron content of the clay. Small proportions of CaCO₃ and silt may be added for strength. The type of clay preferred has a low water content, to limit shrinkage on drying, and is high in organic matter to reduce fuel costs, e.g. the Jurassic Oxford Clay of England. Furnace, or refractory, bricks are made from kaolinite, etc.