Adam Clarke on Isaiah 1:25

I will turn my hand upon thee - So the common version; and this seems to be a metaphor taken from the custom of those who, when the metal is melted, strike off the scoriae with their hand previously to its being poured out into the mould. I have seen this done with the naked hand, and no injury whatever sustained.

Purge away thy dross "In the furnace" - The text has כבר cabbor, which some render "as with soap;" as if it were the same with כברית keborith; so Kimchi; but soap can have nothing to do with the purifying of metals. Others, "according to purity," or "purely," as our version. Le Clerc conjectured that the true reading is ככור kechur, "as in the furnace;" see Eze 22:18, Eze 22:20. Dr. Durell proposes only a transposition of letters בכר to the same sense; and so likewise Archbishop Secker. That this is the true reading is highly probable.