History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c12.26

c12.26

The tempering materials used with clay were mica, sand and lime. These were often used singly and rarely in combination. Mica is a very common substance found in the pottery wares as it betrays itself by its lustre. It is a common ingredient in the sand of the Indus Valley, from which it can be readily obtained by levigation. Its presence in clay facilitates the working of the latter on the wheel, and the drying of the pottery piece without cracking It, therefore, served as a useful tempering material. Lime is present in the fabric of both small and large Harappan jars Curiously enough it has been found to occur often in lumps of quite appreciable sizes. In fact, extra large fragments of lime have been flaked off the surface of some of the jars through the swelling due to slaking. These large particles of lime, in the opinion of Mackay, were mixed up with clay accidentally from the floor at the time of making the pots. A similar use of lime as a tempering material has been found in the wares of ancient Baluchistan, in some of the wares of Al Ubaid, Jemdet Nasr and in many of the predynastic wares of Egypt Opinions may differ as to the value of lime as a tempering material But with certain clays it seems to be an important ingredient and from the earliest times it has been mixed with them for this purpose.