Shatapatha Brahmana 14.1.3.16

14.1.3.16

atha yadubhayata ādīptā bhavanti sarvābhya evaitaddigbhyo rakṣāṃsi nāṣṭrā apahanti tasminpravṛjyamāne patnī śiraḥ prorṇute tapto vā eṣa śuśucāno bhavati nenme'yaṃ taptaḥ śuśucānaścakṣuḥ pramuṣṇāditi

And as to why they are kindled on both sides: he thereby repels the Rakshas, the fiends, from all the quarters. Whilst this (pot) is being heated, the (Sacrificer's) wife covers her head, thinking, 'Lest this one, when heated and glowing, should rob me of my eyesight,' for it indeed becomes heated and glowing.