We aren't exactly sure. Although we do have some ideas. Check out this previous thread where two moderators argue the origin of bronze and iron smelting

Personally I favor the accidental discovery theory. The first ingredient you need for bronze is copper. The first copper we find worked is native copper. It's a lovely green rock that will deform when you hit it with another rock. At some point somebody discovered that heating copper made working it easier. Maybe somebody left their axe or necklace too close to the fire. Maybe a firepit was built using a rock of native copper in the ring and somebody discovered it became semi-malleable. Or maybe somebody discovered the thermal transfer properties of native copper and used a rock as a pan. Lots of maybe.

The earliest forms of bronze we have discovered are arsenic bronze. Copper often occurs with small amounts of arsenic. But some mineral formations have larger amounts of arsenic. If some of the high arsenic minerals were accidentally in with the copper when it was heated they got bronze which would be shown to be a superior metal in most respects (it didn't need to be heated to melting just heated enough that the arsenic cooked out of the other rocks and was able to combine with the copper about 600 degrees celsius would do it. That's hot but not insane it can be achieved in a campfire/bonfire. In fact a very hot bonfire can actually reach 1100 degrees celsius which is enough to melt copper.

Again I emphasize that we don't really know if this happened I'm simply suggesting it could have happened.

The next phase is moving from arsenic bronze to tin and zinc bronze. Tin and copper rarely occur anywhere near each other and so would have to be brought together and then smelted. I think this was also quite possibly accidental. There is a rock called [arsenopyrite](https://uwaterloo.ca/earth-sciences-museum/resources/detailed-rocks-and-minerals-articles/pyrite#Arsenopyrite FeAsS) It is known to occur near both native copper sources and the most common source of tin cassiterite. There is some evidence to suggest that arsenopyrite was used in the smelting of arsenic bronze. I think it possible that cassiterite was mixed with arsenopyrite when mining the latter. The cassiterite ended up in the furnace and the rest is bronze age history.

The process is quite difficult to get right but remember trial and error was around long before "The Scientific Method". People made observations based on available information and refined their methods based on what they observed.

"There are no mistakes. Only happy accidents." - Bob Ross

Copper and tin smelting is a very straightforward path of discovery. Copper and tin ores are distinctive, and interesting. They would naturally have been collected. And if you happen to place them in a fire, even one of fairly modest heat, you can quite easily produce small amounts of liquid metal. This is because the process for smelting copper and tin more or less just requires producing a reducing environment (which is easily done with Carbon Monoxide production inside of a charcoal fire) and then melting the reduced metal that is produced, which tends to naturally separate it from impurities.

Discovery of bronze is slightly more complex, but arsenic often exists along with copper naturally, and as mentioned tin would have also been something that people were naturally drawn to due to the lustrous nature of the ores. The trick is that tin ore is much less common than copper ore, but once someone somewhere does a few experiments with bronze they'd pretty quickly discover some of its advantageous properties. It looks like bronze working technology spread in a couple different waves.

Iron working is interesting because even though the basic techniques for iron working are well within the technical capability of people who are capable of working bronze, it's a very different process and a different set of skills. Copper and Tin are often melted during smelting. If you try that with Iron you end up with a very inferior product. What you need to do instead (if you don't have a blast furnace) is to maintain a reducing (CO) environment inside a chimney at a fairly high temp (around 1000 deg. C) in order to weld/sinter little bits of iron together while keeping silicate impurities and slag molten. This creates a "bloom" or sponge of iron while the slag and impurities drain out of it or are kept molten within it. At the appropriate time the hot iron sponge is removed from the fire and then hit with hammers to squeeze out the remaining molten impurities, and to even out the composition.

After which, iron is typically worked at sub-molten temperatures, by heating it up, beating it with a hammer to work it into different shapes, fold it, etc. This contrasts sharply with the techniques of bronze-work which revolve around melting, mixing alloys, and casting. And that's a major reason why the bronze-age lasted as long as it did and why iron working took a considerable amount of time to develop, because it's a non-intuitive leap from bronze casting (though there are other issues as well, such as the power structures of trade).

hi! this question is worth x-posting to /r/AskAnthropology.

Also, you may be interested in the FAQ.. I don't think it will answer your questions directly, but may have some tangential value

• Mineral resource extraction