An Encyclopaedia of Indian Archaeology v1.c19.s6.2

v1.c19.s6.2

The soil used for making brick contains clay and such minerals as quartz, mica, feldspar, calcite, kaolinite, etc., as clay results from weathering of eruptive feldspathic rocks. The properties of good clay depend upon its plasticity which is greatly affected by impurities. The degree of plasticity of clay depends on its mineralogical and granulometric composition, on the shape and nature of the grain and on the content of soluble salts and organic impurities. The plasticity may be enhanced by adding highly plastic clays and lowered by introducing quartz, sand, slag, coal fines, organic impurities like husks of grains, etc. So rice-husk found in ancient bricks was mixed to lower the plasticity of clay. In the *Yajurveda-Saṁhitā* and the *Śatapatha Brāhmaṇa* the hair of goat, very fine sand, rock-flour and iron filings are enjoined for mixing. In the highly plastic alluvial clay these admixtures were required to bind together the non-plastic materials and strengthen the brick. Similarly the *Baudhāyana Śulba-sūtra* prescribes the mixing of clay with *ukhya-bhasma*. Clay mixed with iron filings when fired gives a red colour with excessive oxygen in the kiln, while a deficiency of oxygen lends a dark brown or even black colour. Lightweight high-porosity brick is produced by mixing with clay substances like saw-dust, coal-dust, chalk, dolomite, ground mark, etc. which are capable of releasing carbon dioxide during burning. To reduce the load of the roof on the Ramappa temple (A.D. 1203) at Palampet in Andhra Pradesh in stone the *śikhara* is made in brickwork, the bricks being so light as to float in water.