GunnerX

If it's really a problem they could just make cast bronze tools need to be finished (work-hardened) on a copper anvil.

Arsenic is the hardening agent as opposed to tin, possibly in addition to zinc or tin. It is also effective at fairly low percentages like 2% arsenic. It should also be somewhat lighter in colour. I don't recall where exactly it was historically used, but basically if the local ore sources were sufficiently "impure" it was possible. A more reliable alternative, though possibly anachronistic, would be to add realgar or oripiment to your molten copper. But I don't think either of those exist in the game at present. Again probably a lot more trouble to code then it's worth though, aside from possibly realgar, oripiment additions.

Should be possible by simply replacing log piles in a charcoal furnace with coal blocks.

Yep, black bronze is actually a jewelers metal, softer then pure copper, and shouldn't be a Tier 2 metal to begin with but meh... Also Rose Gold. On the other hand tetrahedrite has frequent impurities of arsenic, silver and iron. I'm not 100% up on my ancient metallurgy, but arsenic bronze should be feasible, and if notheing else, the Chinese used copper-nickel alloys in weaponry. So those might be possible replacements. Edit: No, let's just say amateur mineralogist. My brother is also studying material science and metallurgy, so I get to talk steel micro-structure with him and stuff like that.

Most of the early (copper, bronze era) ores are Sulphides or Carbonates. The sulphur and carbon are relatively easy to turn into SO2 and CO2 via roasting. Thus the pit kiln. High heat and excess carbon (charcoal) is needed to reduce iron oxides to an elemental form, thus the bloomery. Cassiterite is also an oxide, but doesn't have irons bonding energy and is more easily reduced. Further, it will reduce even more easily in the presence of free sulphur or carbon from copper ores like malachite and tetrahedrite. Technically the native ores should be processable simply through simple mechanical means (pulverizing with a hammer). The native ores are smelted because melt and pour is easier then to work (a la those iron blooms) every single ore into a pure nugget, then welding each of those nuggets into a usablel bar. Limonite is actually hydrous (wet) hematite, and can be roasted to remove the water prior to proper smelting to save on high carbon fuel (charcoal or coke). While it would be kinda cool to add things like only being able to pit kiln cassiterite in the presences of suplhur and carbon bearing ores or through the use of a charcoal pit furnace, and/or increase charcoal cost on non-pit kilned limonite. I don't think it would add enough to overall gameplay to justify the extra coding time.

As an American who has studied Computer Science and Engineering, I will tell you metric is just as bad as SI. Screw the decimal system. The Babylonians had it right. Base twelve, or even better, 60, are far more useful. Heck, I could even live with binary. Plus, that way everyone gets to learn new measurements and it's fair all around.