Anandakanda 1.24.14

1.24.14

puṭayedandhamūṣāyāṃ krameṇa mṛduvahninā /
ahorātraṃ trirātraṃ vā bhavedagnisaho rasaḥ //

One should calcinate it in a blind crucible gradually with a mild fire
for a day and night, or for three nights; the mercury becomes fire-resisting.