Rasahridayatantra 7.1

7.1.1

grāsaṃ na muñcati na vāñchati taṃ ca bhūyaḥ kāṃścidguṇānbhajati bhuktavibhuktimātrāt /
yajjīryate pracurakevalavahniyogāt tasmādviḍaiḥ suniviḍaiḥ saha jāraṇā syāt //

It does not abandon the swallow, nor does it crave it again; it gains certain properties just from consuming and processing it;
since it is calcined by the intense application of fire alone, calcination should therefore be done with dense viḍas.