Rasarnava 12.162-

12.162-

gandhakaṃ lohadaṇḍena ekaviṃśatibhāvitam
yuktaṃ lohamanenaiva jambīrarasasaṃyutam //
sabījaṃ sūtakopetam andhamūṣāniveśitam
bhūgataṃ māsamekaṃ tu tāraṃ kāñcanatāṃ nayet //

Sulfur macerated twenty-one times with an iron pestle,
joined with this very iron and combined with lime juice,
endowed with seed, accompanied by mercury, placed in a blind crucible,
[and] kept underground for one month, turns silver into gold.