Śatapatha Brāhmaṇa ("of a hundred paths") 14.3.2.21

14.3.2.21

atha taṃ copaśayāṃ ca piṣṭvā mārtsnayā mṛdā saṃsṛjyāvṛtā karotyāvṛtā pacatyutsādanārthamatha ya upaśayayordṛḍhaḥ syāttena pracaret

Thereupon, having pounded that spare (clay), and mixed it with powdered clay, he makes (a pot) in proper form, and bakes it in proper form for the purpose of the 'setting out'; and let him perform (the sacrifice) with whichever of the two reserve (pots) may be firm.