Kautilya's Arthashastra 2.13.8

2.13.8

rūkṣatvād bhidyamānaṃ tailagomaye niṣecayet //

Impure gold is of whitish colour. It shall be fused with lead of four times the quantity of the impurity. When gold is rendered brittle owing to its contamination with lead, it shall be heated with dry cowdung (sushkapatala). When it splits into pieces owing to hardness, it shall be drenched (after heating) into oil mixed with cowdung (taila-gomaye).