Erin emailed me this chart of colours which a fellow enthusiast had sent him. Mild steel sitting in an oven for a half hour.... this is what he found...
degrees color of steel
430 very pale yellow
440 light yellow
450 pale straw yellow
460 straw yellow
470 deep straw yellow
480 dark yellow
490 yellow brown
500 brown yellow
510 spotted brown
520 brown purple
530 light purple
540 full purple
550 dark purple
560 full blue
570 dark blue
640 light blue
My results varied a fair amount...I found that the steel I used did not really start to change colours until fifty minutes had gone by, then it was very quick after that. The actual temperature of the oven was not necessarily as important as the length of time it sat in there changing colours. I needed an oven cooking at a fairly high temperature...around 550 degrees...to get good results.
Different sheets of steel will behave differently, and the colours vary quite a bit. Like dye lots in fabric....no seamstress would make each side of a cote hardie from different bolts of cloth, and no armour maker should make complex coloured armours from different sheets of steel. Modern steel is made largely from melted scrap, the content of which varies remarkably from smelt to smelt.
My well thumbed copy of Bealer's "The Art of Blacksmithing" mentions these colours, but of course to be used to create the combination of hardness and toughness which make for a good chisel, knife, or sword. He suggests that the smith should forge out a thin piece of the chosen steel, and quench it properly, (that word "properly" involves a huge testing procedure itself!) then laying the metal on a hot block of metal, watch the colours as they progressed back from the hot spot. Then with files and "try" stones, he would see how tough or soft or brittle the metal would be at each colour. In the days of very inconsistent metallurgy, this was the only way he could create a tool which had the desired characteristics.
Armour in its own way has all the need for consistency, albeit different requirements make for different treatments and materials. I have seen the assays of metal from various armouries, and believe that medieval steel was so different from modern steel that almost nothing is the same. Well, maybe its tendency to rust....
Mr Edge from the Wallace Collection in London tells us that late period armour had a high carbon content, which allowed it to be hardened, and that very expensive armours were layered with high carbon steel on the outside backed up with low carbon steel. I can only presume these were made by forge welding two plates together, possibly right at the ingot level.... two ingots, one relatively high carbon and a second cheaper low carbon ingot would be forge weled into one single ingot, and then said ingot would then be hammered out by water powered hammers into a plate. (Arms and Armour of the Medieval Knight, Edge and Paddock, page 134)
That being said, I do not believe that very late period medieval armour was made from high carbon steel in order to make it lighter and more effective. I will, however conceed that a better steel gradually replaced the dirty iron over the 15th and 16th centuries. By better, I mean a bit more carbon content, a bit less slag inclusions, better over all. Armour which was too hard would be in danger of cracking. I have never seen a broken armour nor have I heard of a broken armour. Therefore I will go out on a limb and state that they ("they" being Milanese armourers, the ones I am passing familiar with) usually did not make armour from metal which would break. I have however, seen several breastplates with holes in them from various impliments of destruction....the ones which were holed by British muskets in Malta for instance (during trials in the 18th century) show holes consistent with a composition of pretty much pure iron.
Obviously there are broken armours out there, because lets face it, anything in time will break. I am referring to armours which will have broken in use because they were made from steel which, like a sword, will break before bending.
The use of malleable steel or wrought iron would also allow for two things....easy repair of holes and dents, a reduction of cracks and stress fractures, and something like a crumble zone. That is to say, the creation of a dent will absorb an impact to a surprisingly large extent. I remember a couple of fights I walked away from because the people didn't feel the impact of my (simulated) sword, and on one occassion a fella came up to me later to apologise for not accepting my blow because his helm had caved in. The crumpling of his helmet absorbed so much energy that he felt it was "light". Since this happens with modern simulated combat, and quite frequently too, then I am certain that it happened in real combat, and that an armourer would pick and choose his materials to allow this life saving phenomon to occur. That material is not high carbon steel.
Thanks Erin for sending me that chart.