Bell Metal
Srimad-Bhagavatam
SB Canto 3
SB 3.33.6, Purport:
Usually, initiation depends on the bona fide spiritual master, who directs the disciple. If he sees that a disciple has become competent and purified by the process of chanting, he offers the sacred thread to the disciple just so that he will be recognized as one-hundred-percent equal with a brāhmaṇa. This is also confirmed in the Hari-bhakti-vilāsa by Śrī Sanātana Gosvāmī: "As a base metal like bell metal can be changed into gold by a chemical process, any person can similarly be changed into a brāhmaṇa by dīkṣā-vidhāna, the initiation process".