(photo credit: Wikipedia)
Whenever one sees a photo like the one above, one immediately thinks ancient Roman soldier, and that is correct. But the thing that makes one think that is not an awesome helmet like the Greek Hoplite, but the distinctive armor. This armor does not have an official name (Wikipedia calls it “Lorica Segmentata“), but it seems unique only to the Roman legionaries. Remember, it was law that a Roman Citizen had to join the army for a certain number of years and then get rewarded with a farm and lots of money. During the time that they were in service, these citizens were the main fighting force of the Roman army, and so had access to the best armor. (Let’s call it legionary armor.) This armor was made up of several plates that were hinged together at the joints, (later on the hinges were replaced with simple rivets) and in some places tied together with leather cord.
The Romans had access to iron, and they figured out a way to harden the plates of iron on one side only, as hardening both sides would make it too brittle. It’s actually pretty simple. What the Romans would do is they would have the already-formed plate and put a bunch of leaves (a source of Carbon) on top of the face that would be facing out from the body in the finished piece. When the metal was placed in the furnace, it would absorb the carbon and so become very hard. Which is nice to have when someone’s throwing a javelin at you.