Kautilya's Arthashastra 2.13.16

2.13.16

tataḥ śulbakākaṇyuttarāpasāritā ācatuḥsīmāntād iti ṣoḍaśa varṇakāḥ //

When the streak of pure gold (made on touch-stone) is of the colour of turmeric, it is termed suvarna. When from one to sixteen kákanis of gold in a suvarna (of sixteen máshakas) are replaced by from one to sixteen kákanis of copper, so that the copper is inseparably alloyed with the whole mass of the remaining quantity of the gold, the sixteen varieties (carats) of the standard of the purity of gold (shodasavarnakáh) will be obtained.