What is "l'Sig"?
Rashi: It is waste mixed with silver or gold. One must melt it in fire to refine it.
Malbim: I explained in Yeshayah (48:10) "Tzerafticha v'Lo b'Chesef Becharticha b'Chor Oni" that one who refines impurities in a cauldron of silver, he discards the impurities and keeps only the silver. However, one who refines impurities in a cauldron of Oni, which has no silver, he removes only the worst impurities, and leaves the esteemed metals, even though they are not like silver. So if Hashem refines a generation with Tzadikim and Resha'im, He eradicates the Resha'im and leaves the Tzadikim. If none are Tzadikim, He selects those who are Tzadikim compared to the Resha'im. They are considered like silver compared to the absolute worst. Since all of Yisrael were like Sig to Me, I put them in a furnace to refine them, and then the [better] Sigim were like silver. So was the refining at the time of Galus Yechanyah. He took out of them "he'Charash veha'Masger 1 " (Melachim II, 24:16) and the best of them, and exiled them to Bavel. They were silver compared to the Sigim that remained in Yerushalayim.
Surely this refers to the majority of them. They were absolute Tzadikim among he'Charash veha'Masger, e.g. Mordechai, Daniel, Chananyah, Misha'el and Azaryah? (PF)
It is written l'Sug, and we pronounce it l'Sig!
Radak: Aleph, Hei, Vov and Yud are interchangeable.
Why does it mention these metals?
Radak: They are inferior to silver ? copper, tin iron and lead. If they were all in a furnace to melt them, so they were to Me dross, and not silver. When I melt them, like a craftsman melts these metals, a Kli made from their mixture is inferior to a Kli from one metal alone. If it breaks, it can be fixed only if he melts it again. Therefore, Yisrael are compared to mixed metals.
What is "Kor"?
Rashi: It is a furnace in which they refine gold.
What do we learn from "Sigim Kesef Hayu"?
Radak #1: The word Samuch to Kesef is omitted. It should say Sigim Sigei Kesef.
Radak #2: It is as if it says Sigim b'Chesef; the prefix Beis is omitted, like "ha'Nimtza Veis Hashem" (Melachim II, 12:11) and many similar verses. A small amount of silver was mixed with them. They make some Kelim from metals mixed with Kesef, e.g. cymbals to make noise. Yisrael had few Chasidim - "Kevutzas Kesef u'Nechoshes u'Varzel" (20). It says "Bechanani ka'Zahav Etzei" (Iyov 22:10), for Yisrael were not generation after generation like gold, rather, like silver.
Radak #3: Now they are dross; they were silver in the past. It says silver, and not gold, for [silver] is cleaner than all metal Kelim.
Radak citing Targum Yonasan: They were like the waste of silver.
