Kautilya's Arthashastra 2.13.47

2.13.47

tapanīyaṃ jyeṣṭhaṃ suvarṇaṃ surāgaṃ samasīsātikrāntaṃ pākapattrapakvaṃ saindhavikayojjvālitaṃ nīlapītaśvetaharitaśukapattravarṇānāṃ prakṛtir bhavati //

Pure and glittering gold is tapaníya. This combined with an equal quantity of lead and heated with rock-salt (saindhav'ika) to melting point under dry cowdung becomes the basis of gold alloys of blue, red, white, yellow (harita), parrot and pidgeon colours.