Common clay is now mined in every state in the United States, except Alaska and Rhode Island, according to U.S. Bureau of Mines Minerals Yearbooks . The states producing more than 1 Mt were Alabama, California, Georgia, Illinois, Indiana, Iowa, Louisiana, Maryland, Michigan, Missouri, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Texas, and Virginia. Texas was the leading state in production, and North Carolina was second. Fired test pieces are tested for such properties as shrinkage, warping, cracking, permeability, modulus of rupture, and crushing or compression strengths. Color and soluble salt content are also important for some products.
Common clay is dug from open pits.
Pits must be near plants and the digging must be cheap, because both the raw material and finished products are heavy, and the margin of profit is ordinarily low. Most products made from these materials are processed and marketed much in the manner described for refractory clays. The long range trends in demands for some products made from common clay can be expected to keep pace with the growth in the gross national product, but demands for others are likely to fall short of this growth rate. Demands for clay and shale required for portland cement and lightweight aggregate are increasing, and this trend is likely to continue. The rise in fuel costs, which has been forecast, seems likely to be a major problem faced by the industry.