Periodically, the solution may have to be replenished. In the process, the iron stains that are usually bound in with the calcium salts are removed along with the calcium. It is a slow but effective treatment. Soaking calcareous-encrusted sherds in a 5 percent aqueous solution of sodium hexametaphosphate has been used to remove calcareous deposits. Care must be taken, however, since a solution of sodium hexametaphosphate has a tendency to soften the paste of the sherd more readily than the calcareous encrustation.
Calcium sulfate is very difficult to remove from pottery.
To test for the presence of calcium sulfate, drop dilute nitric acid on the deposits, then add three drops of 1 percent barium chloride solution. These can be dissolved slowly by immersing in 20 percent nitric acid. As the sulfates dissolve, sulfuric acid is produced, which cancels out the reaction of the nitric acid. The nitric acid must be changed often. This technique is not generally recommended, however, and mechanical cleaning is preferred.