a smelting furnace: the metalworker's fire that melts metal down so the pure part can be separated from the waste
This is not a furnace in the modern sense of a machine that heats a building. It is the metalworker's fire, built hot enough to turn solid metal into liquid, so that the useless material can be lifted off and what remains is pure. All nine places the word appears trade on that single picture.
Because such a fire both destroys and improves, the biblical writers run the image in more than one direction:
- In the proverbs it is a plain fact about how testing works, and the point of the comparison is that the LORD tests hearts the way fire tests gold.
- In Ezekiel the picture turns dark, because Jerusalem itself becomes the furnace and its people the scrap metal about to be melted down in God's anger.
- In Isaiah the furnace is the hardship God's people lived through, and they come out of it refined rather than ruined.
Three passages give the word its most famous use, calling Egypt an "iron furnace" that Israel was brought out of. Iron demands the fiercest heat of any metal the biblical writers worked with, so the phrase is a way of saying that no slavery burned hotter. Readers have long differed over how far to press it: whether Egypt was meant to purify Israel the way a furnace purifies metal, or whether the image simply measures how brutal those years were.
Several Hebrew words sit close to this one and should not be blurred together: the household oven for baking bread (תַּנּוּר), the kiln known for the thick smoke it throws up (כִּבְשָׁן), and the small crucible (מַצְרֵף) that stands beside this word in Proverbs, where the crucible takes the silver and the furnace takes the gold.