Rasahridayatantra 2.7.2

2.7.2

antaḥpraviṣṭatalabhāṇḍavadanaṃ jalamagnanijamukhaprāntā /
upariṣṭāccipiṭaghaṭī deyodaraṣoḍaśāṅgulaviśālā //

The mouth of the lower vessel is entered inside, with the rim of its own mouth submerged in water; / above, a flat earthen vessel should be placed, sixteen aṅgulas wide at the belly.