Bone Ash. Bones, when calcined in open fire, lose all their organic matters and part of the carbonic acid gas they contain, by which their weight is diminished about two thirds. The residue is a dry, friable, and white mass, of the original form of the bones. Pulverized, the powder is grayish white. It consists of basic phosphate of lime, with some lime, fluoride of calcium, carbonate and sulphate of soda, and phosphate of magnesia. The sulphur of the sulphate comes from the cartilage. Prepared from the bones of cattle, the proportion of phosphate of lime is about 90 per cent.; from human bones, about 86 per cent. Other matters may be removed by dissolving in hydrochloric acid, and precipitating by ammonia, when the phosphate of lime and a very small quantity of phosphate of magnesia alone are left in the solution. Bone ash, ground to powder, is made into a paste with gum water, or beer and water, and moulded into the form of cups, called cupels, which are used in the process of cupellation. This is separating silver or gold from lead, by melting the alloy of the metals in the cupel, and subjecting it to the action of a current of air, which oxidizes the lead, converting it into litharge.
This is absorbed by the bone ash as fast as it is produced, till the precious unoxidizable metal is at last left pure and alone in the cupel. The operation is conducted in the same manner on the large scale and in small assays. When carefully prepared, and freed from foreign matters by levigation, bone ash is called burnt hartshorn, and is used for cleaning jewelry.