Shiva Purana
स्वमलप्रक्षयाद्यद्वदग्नौ धास्यंति धातवः ॥ तत्र पापक्षयात्पापा नराः कर्मानुरूपतः ॥ २॥
Just as metals are melted in fire to remove their impurities so also sinners are put in hells in order to remove their sins.
english translation
svamalaprakSayAdyadvadagnau dhAsyaMti dhAtavaH ॥ tatra pApakSayAtpApA narAH karmAnurUpataH ॥ 2॥
hk transliteration by Sanscript