מַצְרֵ֣ף לַ֭כֶּסֶף וְכ֣וּר לַזָּהָ֑ב וּבֹחֵ֖ן לִבּ֣וֹת יְהוָֽה׃
鼎為煉銀，爐為煉金；唯有永恆者熬煉人心。
Rashi
A refining pot is for silver A vessel in which they refine silver and which is made from burnt ash on earthenware is called מצרף, a refining pot.
Rashi
and a furnace is for gold A vessel in which gold is refined, a vessel in which gold is purified, like a broken pot, is called כור, a furnace.
Rashi
A refining pot is for silver, and a furnace is for gold The refining pot is made to refine silver for its work, and so is the furnace to purify the gold, but [peoples’] hearts—the Holy One, blessed be He, purifies them and tests them and knows their thoughts.