Was armor heat treated back in the day?

Initially it was just beat out and mostly likely hammer hardened. The Italians, about the turn of the 15th began heat treating and showed up as mercenaries for the French wearing this stuff at the Battle of Agincourt, but the Brits prevailed. I suppose like everything else armour could be purchased in different grades and the hammer hardened stuff could still be had for less money.

Most plate was made of mild steel and had a hardness of 20 - 30 HRC, but some breast plates were found with ca. 55 HRC, they must have been quenched.

Interesting. But the video states that the technique of heat treating armor was discovered in the 15th century. However, the knowledge to heat treat iron weapons had been around for about a thousand years by this point. Surely heat treating armor is no more difficult than heat treating a sword, right? I just don't understand why at least one smith wouldn't try doing to armor what was done to weapons. Why would it take them that long to figure it out? Or, if a few people did figure it out before the 15th century, why didn't it catch on?

There's much less benefit. Wrought iron makes for a lousy sword (compared to hardened steel), but wrought iron can make quite good armour. Sure, it might dent more easily than spring steel armour, but that denting and deforming of the iron protects you - better that the energy damages your armour than damages you.

Heat treating armour can be more difficult than heat treating a blade. The piece of steel is bigger, and harder to quench uniformly, and harder to do things like auto-tempering and differential hardening by differential quenching or auto-tempering. Because the piece of steel is bigger, it's also harder to do things like flame tempering. One thing you don't want is brittle armour, so difficulty in tempering matters. Heat treatment of armour can be made much easier by laminated construction (steel-iron laminates).

While there was a lot of hardening steel armour at end-of-Medieval times, wrought iron armour made a comeback as guns became more common, and armour got thicker for bullet resistance. A lot of that 5-6mm (or thicker) bullet-resistant armour was iron. (Some was iron-iron laminates, which might provide more opportunity to absorb energy from a bullet through separation of the layers.)

But hang on... If armor was harder to heat treat because it had a larger surface area than swords, axes, etc., than why was it that heat treated armor only came about when full plate was invented? In other words, when you had armor that had the most big, one-piece plates with large surface area? Compared to, say, the 1100s, when you basically had mail and helmets. And why didn't they heat treat helmets and such? Surely a helmet constructed out of mulitple pieces would be no harder to heat treat than a large axe-head, right? Also, if heat treating armor was only developed in the 15th century, than what took them so long? What technology or knowledge about heat treatment did they acquire at that point, that made them more advanced than armor-smiths of earlier time periods?

Already by the 14th century, one sees heat-treated steel armour. Helmets hardened by quenching, hardened plates in coats-of-plates. I haven't seen enough analysis of older stuff to know how far back one needs to go.

Apart from the things already mentioned above:

A key advance might have been better understanding of tempering. A lot of Medieval tempering was done through slack-quenching (i.e., auto-tempering). This is hard to control.

Lamination/composite construction methods can make tempering much less important. Even if enough steel is available to make an all-steel sword, heat-treatment, especially tempering, can be difficult. With suitable lamination methods (e.g., sanmai, kobuse, welded-edge), you have a much larger margin for error than with a homogeneous steel blade. If it's a steel-iron composite, you can safely attempt auto-tempering via slack-quenching, and if you fail, you have a brittle edge but the iron body will stop cracks. I guess this is a large part of why we see laminated construction for swords in Europe into the 16th century, and even later in China and Japan, and into the 20th century in parts of Asia.

Alas, it's hard to do large laminated iron-steel armour plate. So you can expect heat-treatment of large armour plates to lag behind that of laminated blades. I've seen very little metallurgical analysis of small armour plates (in brigandine and lamellar). Some Japanese lamellar used iron-steel laminated lamellae, but I don't know how often they were quenched. But small plates like this are the place to look for early heat treatment of armour. The dominance of mail for much of the early Medieval period in Europe means that this is harder to find. For multi-plate helmets, you might want to rivet everything together before you harden it. But these helmets might be things to look at for potential laminated construction.

Full plate is sexy and high-tech, so that's where people have looked for high-tech stuff like heat treatment. Lack of known older heat-treated helmets etc. might be due to less attention and less analysis of pieces.

Perhaps that's why by the 16th-17th century you see less plate on the arms and legs? Also if you look at most full plate it has an extra plate covering the joint area. Perhaps that was meant to cover the joint from damage just as much as protect the actual joint opening from piercing and thrusting weapons? Either way concussion attacks are going to hurt and taking a hit or shot on the elbow is going to be pretty excruciating and incapacitating. Having a shield or the main length of your arm open and free would seem to be more beneficial than having it armored. Legs make sense especially on horseback but arms just seem less beneficial to have fully armored outside a forearm length gauntlet or shoulder pauldrons.

Speaking of later periods and firearms, seems to me that it would be quite awkward to use a musket or flintlock with a fully armored shoulder, elbow and even gauntlet. So perhaps that's also why such full suits were eradicated? I mean when it comes to firearms your main target is the chest so it would make sense 16th-17th century armorers focused on a hardened and durable cuirass and helmet over a full suit.

I’ll buy that. Hehe. In my post I said elbow although I was thinking the bend in the arm. But that was only an example to be applied to any area where several plates slide over or under others. What plate I own is small and does not work that way such as my coat of plates, helmet, vambraces, and knee protectors so I examine and get first hand knowledge. Mail is another story.


Speaking of mail, I’ve been reading Matt Eason’s recommended book “Swordsmen of the British Empire”. It’s most enlightening. I was surprised to hear mail being used by both the Indians and the Brits in the earlier part of the 19th century, other forms of armour was used as well. The Brits were hard pressed for protection from the tulwars and British swords were lacking mostly due to the poor cutting edges caused by the steel scabbards. Hence the thrust was advantageous although in debate. Or to put it another way, the English reluctance to adapt and change, maintaining the attitude if it was good enough for dad it’s good enough for me.