Stoneware, thicker than porcelain, is heavy, non-porous and fired at high temperatures. Generally, stoneware is gray but can vary in color according to the type of clay used. Stoneware has been made since the days of ancient China. It became an American household staple in the 19th century holding food, butter, water, pickles and beer. Antique stoneware crocks were decorated with painted, stamped or incised motifs. One of the more-familiar forms of decoration is the cobalt oxide applied in the form of flowers, letters, numbers, scenes and more.
Carefully examine your antique stoneware crock before cleaning it. Check for cracks and previous repairs. Use natural daylight and a magnifying glass if needed. Water used in cleaning will further damage cracks, and early repairs may have been made with water-soluble materials which will disintegrate during cleaning.
Clean your crock, if no cracks or repairs have been found, with Orvus (a conservation soap) and water using a soft cloth. Wipe the crock inside and out. Use a small brush for corners or particularly dirty areas. For a very dirty crock, repeat the cleaning process.
Wash your stoneware crock in soap and water if wiping did not clean the crock completely. Add a few drops of mild liquid detergent to the water. Wash gently and do not soak.
Allow the stoneware crock to air dry.
Antique stoneware crocks should not be soaked in any type of liquid. Soaking leads to staining which detracts from the beauty and the value of the crock. Repair of cracks and chips should be left to a conservator. Your local museum will provide you with names of local conservators.