Rasarnava 11.182

11.182

bhāvayedviṃśatiṃ vārān yavaciñcārasena tu
tena kalkena saṃlipya nāgapattraṃ prayatnataḥ
karṣvanalena vipacet yāvattat cūrṇitaṃ bhavet //

one should triturate it twenty times with barley-tamarind juice;
having carefully smeared a leaf of lead with that paste,
one should cook it with a fire of charcoal until it becomes powdered.