Rasarnava 4.58

4.58

pravitatamukhabhāgaṃ saṃvṛtāntaḥpradeśaṃ sthalaracitacirāntarjālakaṃ koṣṭhakaṃ syāt
bakagalasamamānaṃ vaṅkanālaṃ vidheyaṃ śuṣiranalinikā syānmṛnmayī dīrghavṛttā //

The furnace should have a wide mouth, an enclosed interior, and an internal grate set in place for a long time. / A bent tube equal in size to a crane's throat should be provided, and there should be an earthen hollow tube, long and cylindrical.