Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.1.6

1.2.1.6

tam madhyamena kapālenābhyupadadhāti | devā ha vai yajñaṃtanvānāste 'surarakṣasebhya āsaṅgādbibhayāṃcakrurnenno 'dhastānnāṣṭrā rakṣāṃsyupottiṣṭhānityagnirhi rakṣasāmapahantā tasmādevamupadadhāti tadyadeṣa eva bhavati nānya eṣa hi yajuṣkṛto medhyastasmānmadhyamena kapālenābhyupadadhāti

On it he places the central potsherd. For the gods, when they were performing sacrifice, were in fear of a disturbance from the Asuras and Rakshas. They were afraid lest those evil spirits, the Rakshas, might rise from below them. Now Agni (fire) is the repeller of the Rakshas, and for this reason he thus places (the potsherd) on it. The reason why it is just this (coal) and no other (on which the potsherd is put) is, that this one, having been consecrated by the (above) sacrificial formula, is sacrificially pure: that is why he places the central potsherd on it.