Minerals and Metals in Ancient India v1.c19.s6.t1.1

v1.c19.s6.t1.1

The NBP sherds are highly glossy, surface ranging from grey or brown to black in different shades. the paste is well levigated as discussed under PGW. Hegde (1987) has proved that red ochre was used to paint PGW, and baking around 600-700°C in a reducing atmosphere produced the black marks of magnetite. Red ochre was ground with clay to produce the burnished black slip on the Black Slipped Ware (BSW). In the NBPW stage, red ochre must have been ground with clay and sajji maṭṭi, the vitrefying substance, which produced the shining glaze during baking. The ware was baked twice: first at 600-700°C, then the paint was applied and the second baking was done at a higher temperature may be 800-1000°C in reducing atmosphere. The slip glaze shows much higher proportion of Na₂O and CaO compared to the composition of the core of NBPW (vide Table 11.4). High percentage of sodium in the glaze imparts to it a higher coefficient of thermal expansion than that of the cores, resulting in its cracking and peeling in course of time.