Shatapatha Brahmana 6.5.1.6

6.5.1.6

athaitattrayam piṣṭam bhavati | śarkarāśmāyorasastena saṃsṛjati sthemne nveva yadveva tenaitāvatī vā iyamagre 'sṛjyata tadyāvatīyamagre 'sṛjyata tāvatīmevaināmetatkaroti

Then there are these three kinds of powder (dust)--(sand of) gravel, stone, and iron-rust--therewith he mixes (the clay), just for firmness. And as to why (it is mixed) therewith, it is because thereof this (earth) consisted when it was created in the beginning: thus whatlike this (earth) was created in the beginning, such he now makes it (the earth, or fire-pan).