In 2002, an assay was done on some of the armour in the Palace Museum in Malta. This was carried out by D. Vella and C. Degringny, of the Diagnostic Science Laboratories of the Malta Centre for Restoration; M. Grech of the University of Malta, faculty of Engineering, and A. Williams of the Conservation Department at the Wallace Collection in London.
Most of the armour was northern Italian in provenance, and the purpose of the survey was to determine what form of corrossion control would be most applicable to protect this armour. The pdf file is quite long, several megabytes of download, and can be found in its entirety here...
I have taken a bit of the summary from this assay because it is pertinent to the knowledge of how to make armour...which of course, is what this blog is all about. This is only a summary, mind you, and you should go to the above link for the whole story. I am reasonably certain that this abstract falls under the "fair use in publishing" and of course, I am attempting to give all credit where credit is due.
The first part is a table of the results of the assay.
This was table #2 in the study. They had surveyed ten separate armours, and discovered that all but one item were all 0.1% to 0.3% carbon. Of those, three are considered to be "wrought iron", and six of the remaining items were "low carbon steel", nearly iron. They had all been "hot forged", and were either "air cooled" or "rapid air cooled". None had been quenched in either the traditional water quench or in a "slack" quench, in which the item would be quenched in oil or molten lead.
One item, a pauldron was "hot short" due to phosphorus in the alloy...such an armour should not have passed a "proof" test. Dr. Williams points out that although metal containing phosphorus is fairly common in the day, it was almost never found in armour because it would fail in combat. He feels that the armourer must not have been aware that the armour was "hot short" or he never would have gone to the trouble to make it.
One of the pauldrons was made from not bad steel, .06% carbon by weight, which is similar to modern spring steel. Interestingly enough, even with the better quality of steel which went into it, they did not harden-temper the steel to take advantage of the high carbon. Instead, the armour maker let the steel cool in air. We know this because it was pearlitic steel. Fast quenching would have produced Martensite.
All the armour was laced with slag inclusions...likely from the ore (bog iron?) and from the elongation of the glassy slag, we know it was forged hot. Not like the armour I make here in this shop, which is universally worked cold, with some annealing from time to time to keep it from work hardening. These slag inclusions will instantly identify the armour as old, as opposed to, say, a Victorian era reproduction.
Out of the ten pieces, seven were what we would call "steel" and three were "wrought iron". The wrought iron was the most brittle, and it follows that brittle armour is inferior.
So there you have it...an assay of ten of the armours on display at the Palace Museum in Malta. Seven were steel, three were wrought iron, all were forged out while red hot, and none were quenched in water, lead, or oil.