CHĀNDOGYA UPANISHAD · 4.17.7

तद् यथा लवणेन सुवर्णं संदध्यात् ।
सुवर्णेन रजतं रजतेन त्रपु त्रपुणा सीसं सीसेन लोहं लोहेन दारु दारु चर्मणा ॥

tad yathā lavaṇena suvarṇaṃ saṃdadhyāt ।
suvarṇena rajataṃ rajatena trapu trapuṇā sīsaṃ sīsena lohaṃ lohena dāru dāru carmaṇā ॥

As one binds (softens) gold by means of lavana (borax), and silver by means of gold, and tin by means of silver, and lead by means of tin, and iron (loha) by means of lead, and wood by means of iron, or also by means of leather,