Kautilya's Arthashastra 2.13.49

2.13.49

tāram upaśuddhaṃ vā - asthitutthe catuḥ samasīse catuḥ śuṣkatutthe catuḥ kapāle trir gomaye dvir evaṃ saptadaśatutthātikrāntaṃ saindhavikayojjvālitam //

Pure or impure silver (tára) may be heated four times with asthituttha (copper sulphate mixed with powdered bone), again four times with an equal quantity of lead, again four times with dry copper sulphate (sushkatuttha) again three times in skull (kapála), and lastly twice in cowdung. Thus the silver acted upon seventeen times by tuttha (shodasatutthátikrántam) and lastly heated to white light with rock salt may be made to alloy with suvarna to the extent of from one kákani to two Máshas. Then the suvarna attains white colour and is called sveta-tára.