Anandakanda 2.7.37

2.7.37

koṣṭhīyantre dhamedgāḍhamaṅgāraiḥ khadirodbhavaiḥ /
triyāmadhamanād eva sattvaṃ patati nirmalam //

One should blow fiercely in a hearth furnace with charcoal derived from khadira wood; / from three watches of blowing alone, the spotless essence falls down.