Shiva Purana
इति नाग्निरसौ दुष्येत्ताम्रसंसर्गकारणात् ॥ नाग्नेरशुचिसंसर्गादशुचित्वमपेक्षते ॥ ४४ ॥
Thus the taste of fire is not contaminated by contact with copper. It does not require impurity from unclean contact with fire.
english translation
iti nAgnirasau duSyettAmrasaMsargakAraNAt ॥ nAgnerazucisaMsargAdazucitvamapekSate ॥ 44 ॥
hk transliteration by Sanscript