Anandakanda 2.6.24

2.6.24

tanmadhye drāvitaṃ nāgaṃ śuddhaṃ secyaṃ tu saptadhā /
mahiṣasyāsthicūrṇena supākaṃ mūtrasecanāt //

Pure lead melted within that should be quenched seven times.
With the bone powder of a buffalo, it attains good calcination by quenching in urine.