This is Cool

While excavating an ancient smelter site in Georgia, they came across hematite (iron oxide, basically black rust) and slag, and thought that they had found a very early iron smelting facility,

They had not.  The hematite was used as a flux to improve copper extraction.

If this is not how iron smelting was discovered, it is at the very least incredibly adjacent to the discovery of iron smelting:

Research from Cranfield University sheds new light onto the transition from the Bronze Age to the Iron Age, showing how experimentation with iron-rich rocks by copper smelters may have sparked the invention of iron.

The work reanalyzed metallurgical remains from a site in southern Georgia: a 3000-year-old smelting workshop called Kvemo Bolnisi. During the original analysis in the 1950s, piles of hematite (an iron oxide mineral) and slag (a waste product of the metal production) were found in the workshop. Finding those iron oxides, the original excavators thought the workshop was an early iron smelting site.

However, new research shows that those assumptions were wrong. Rather than iron, workers at Kvemo Bolnisi were smelting copper using iron oxide as a flux -- a substance added into the furnace to increase the resulting copper yield.

These discoveries give weight to a long-discussed theory that iron was invented by copper smelters. This evidence shows that ancient copper metalworkers experimented with iron-bearing materials in a metallurgical furnace, which was a crucial step towards iron smelting. 

This made my day.

I love ancient tools and ancient technology.