What is the grammatical form of "k'Hituch"?
Radak: Hituch is a noun. When it is complete (with Nun, the first letter of the root) it is k'Hintuch, the same grammatical form as "k'Hindof Ashan" (Tehilim 68:3), just there it has a Cholem, and here a Shuruk.
What is the comparison to melting silver in the furnace?
Radak: Just like silver melts in the furnace, you will melt in Yerushalayim.
Above it mentioned other metals. Why does it mention here only silver?
Radak: (a) It melts after 1 tin and lead. I.e. [the better people] will be eradicated via hunger, plague and sword later ? not in a short time. (b) Initially they were silver.
Malbim (21-22): (a) [You will melt by yourselves, not via Hashem;] even so, you will melt totally, like silver in a furnace, which melts easier than iron (refer to 22:22:3:1*). (b) Via this refining, "you will know that I am Hashem", who oversees and punishes, and you will repent. This was the purpose!
The temperature needed to melt pure silver is higher than that of tin and lead, but less than that of iron. An alloy of mixed metals melts at once; smelters would remove the other metals, and leave the silver. (PF, heard from Jacob Levin)
