Places started with copper
Copper is soft, workable, and melts into a liquid at temperatures achievable by a wood fire(1100 C)
Iron is hard and does not melt into a liquid in a normal wood fired kiln, it only melts if you get it up to 1500C. You need a special charcoal fired bloomery to process iron ore so you first need a good source of iron ore *and* charcoal.
Tin, similar to copper, will completely melt so all it takes is for someone to mix the two and discover that bronze is *wayyy* harder than copper making it much better suited for weapons.
It’s called iron age, but really it’s age of ferrous metallurgy – most notably steelmaking. Iron cannot be so easily reduced as tin or copper, or well you can, but it results in alloy of so [high carbon content](https://en.wikipedia.org/wiki/Pig_iron) that it’s completely unusable. Modern metallurgy has [ways](https://en.wikipedia.org/wiki/Basic_oxygen_steelmaking) to work that to steel, but ancient metallurgy didn’t so they couldn’t start with that route.
Instead they used [bloomery](https://en.wikipedia.org/wiki/Bloomery) process, which reduces iron without ever truly melting it. It produces a porous impurity filled mass or iron and working it into usable steel billet takes a lot of effort and bit of skill, even then quality is poor. There is an [easy and low tech](https://en.wikipedia.org/wiki/Wootz_steel) way to do better, but to invent it you either need to be very lucky or have some basic understanding of metallurgy.
Ancients had no understanding of metallurgy, it was all trial and error and pure dumb luck, so hard way it was for most of the world right until modern times. Making minimally usable steel is much more complicated than making minimally usable bronze.
It’s all about the temperature!
Iron ore takes some really high heat to melt, so it requires special furnace designs or extra tech like bellows to get the temp high enough to work it.
Copper and tin can be melted at much lower temperatures, and were some of the first metals being worked on with basic forges be so mixing them together was a lot easier for people to figure out / invent first before they learned to melt the tougher elements.
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