"Gold that is tested in fire"

"Gold that is tested/proven/purified in fire" and various permutations of that phrase occur in several places in the Bible.

There is some background in the phrase that is not readily available to modern readers, and I want to share that background.

What every little boy in Israel knew at the time of Christ

Every town of modest size had somebody who could "assay" and purify gold.

The problem with gold is that its value dwarfs that of silver and copper, metals that happily alloy with gold and can make a coin or ring seem to be much, much more valuable than it really is.

So how would you assay "pure" gold and if it was not pure, how would you remove the baser metals with the technologies available in the time of Jesus?

Conceptually, you would capitalize on the fact that silver and copper have an affinity for oxygen that is not shared by gold.

In modern terms, lets say you took that ring of suspect alloy and you put it on a fire-brick. Then you melted it with an oxyacetylene torch set to an oxidizing flame.

The ring would melt and form a ball of molten metal due to surface tension. If the gold was debased by copper or silver, the brilliant, reflective surface would quickly be dulled by an oxide coating that would inhibit further oxidization. The molten, golden ball that lost its luster would be proven to be debased. It was "tested by fire" and found wanting.

The process of refining that very same ball of molten, golden metal involves repeated sprinkling of flux over the ball and then scraping the ball out of the pool of molten flux. Flux dissolves the oxide coating and after the ball is rolled out of the pool of molten flux it is again exposed to the air and more baser-metal is oxidized.

Using this method might take days-and-days to fully refine the amount of gold in a simple, modern wedding band. However, that amount of gold might be the equivalent of several years wages for a common laborer. 

Key Points

The process of "being purified by fire" is not quick.

The process does not have a clear-cut "yeah, it is done" criteria.

The process is accessible to everybody with access to the most basic of commodities: Time, firewood, fire-resistant pottery, wood-ashes, snail-shells and silica (sand).

The process cannot create "gold". If there is no elemental gold in the yellow alloy, then the process will first burn out the zinc and then burn out the copper.  From the Christian standpoint, if a soul has completely rejected the promises of Christ then there is no redemption, no possibility of "purging" of the impure parts and leaving the noble metal.