Kautilya's Arthashastra 2.14.35

2.14.35

dṛḍhavāstuke vā rūpe vālukāmiśraṃ jatu gāndhārapaṅko vā tapto 'vatiṣṭhate //

In a compact and hollow piece (ghana-sushire rúpe), small particles of gold-like mud (suvarnamrinválukáh) or bit of vermilion (hingulakalkah) are so heated as to make them firmly adhere to the piece inside. Even in a compact piece (dridhavástuke rúpe), the waxlike mud of Gándhára mixed with the particles of goldlike sand is so heated as to adhere to the piece. These two kinds of impurities are got rid of by hammering the pieces when red hot.