To obtain the subject metal salt for use in ceramic siding materials, or the like, by homogeneously mixing a neutralizing agent with waste clay used for treating oil-and-fat followed by addition of a lipase-contg. liquid to neutralize the fatty acids and hydrolyze the oil-and-fat almost simultaneously.

This clay-contg. fatty acid metal salt useful for ceramic siding materials or as a water repellent for pulp cement boards is obtained by the following process: a mixer is charged with waste clay used for treating oil- and-fat, a neutralizing agent such as calcium oxide and/or calcium hydroxide is then added to the waste clay followed by homogeneously mixing the blend, an aqueous solution or dispersion of lipase is subsequently dripped into the mixer in two minutes under agitation followed by further mixing to effect the hydrolysis of the oil-and-fat and the neutralization of the fatty acids almost simultaneously. During the process, the lipase-contg. liquid is absorbed into the mixture, and the contents after halting the agitation are converted to a granulated product about several mm to 1 cm in granular size, and the granules are then ground with a grinder and dried.