The first and venerable controversy is whether medieval, (mid to late) maille was made by drawing wire or punching discs of iron.
Now some respected authorities opine that maille couldn't have been made out of drawn iron because they didn't have the ability to draw iron wire before the 15/16th century. And oddly enough some of the 16th century stuff I've seen seems rather finer and more rounded in cross section, whereas the 15thC and older is mostly flat:
But it could have been drawn and flattened by a hammer, although that would be even more work. Certainly the rings seem pretty good for having been flattened, if they have been. Drawing the iron, presumably soft low carbon iron from a well hammered bloom through a drawplate would still have been a lot of work, requires a pretty damn good piece of steel for the drawplate itself and leaves nice striations upon the wire so produced.
Except that they don't seem to have found much evidence for said striations, partly because the maille always turns up rusted and in need of some TLC.
If you look at it sideways you can see that it is pretty much flat, with slightly rounded edges, which hey, could be a result of all that wear and tear:
Or they could have hammered the iron down into sheets and produced rings by punching. Admittedly you'd probably need a tilt hammer, fire and a few hours, but you can make a case hardened rivet punch without much trouble, whereas a drawplate requires a lot more careful work and still needs the time and effort to hammer the iron small enough to fit through a hole in the first place.
Now the interesting thing about iron is that it has a crystalline structure, a horrendously complex one because of the various elements present including carbon. The structures vary depending on what elements are within and how the iron is cooled. This grain structure then deforms when you hit it, oddly enough, suggesting that if the rings are made of sheet iron the grains should be flattened parallel to the surface, whereas if it is drawn, they will be distorted along the length of the ring. Great, all we need to do is persuade them to let me etch the surface of some rings with a nice mix of acids then we can study the grain structure under a microscope.
Ah ha, there is a slight problem...
See before 1 and between 5,6 and 7? Annealing. That means re-heating the iron to remove the stresses and strains of being horrible deformed. It also means that the grain structure from being drawn or hit flat then punched, may well be destroyed. Whoops.
This may be why I have not yet found any information about the grain structure of the rings of maille shirts, because everyone has given it up as a lost cause because they think there won't be any evidence after annealing and conservation.
Maybe. Of course some shirts weren't annealed, and others we have no idea what was done to them. It is likely that a primitive form of case hardening was carried out, which would increase the surface carbon levels towards steel, thus making the surface hard and resistant and the inside softer and more easily deformable. (Has anyone done proper cutting demos with a proper testing machine yet?)
Also it is likely that they won't let me cut a few rings open to see what the internal structure is like, thats the last place to look for evidence of the manufacturing process, but given how hot the rings have to be heated to flatten the ends and so on, it is likely that there wouldn't be any evidence left there either.
Now what would be the advantage of flat/ flat and slightly rounded rings? I don't quite know, I think they move and hold shape better than circular rings. As well notice that the outside of the shirts have the extruded metal from the rivetting, I think.
Finally, the point was made by one of the early papers on manufacture of maille, that you don't have to use a drawplate. You can use a sort of screw together set of jaws, so that the metal you pull through is squashed flat rather than round, - instead of O. This might well be the method they used, since it does away with the slightly trickier drawplate, and would allow re-hardening, cleaning and sharpening of the jaws as required. It might also lead to a slightly more complex shape of the flattened iron, as you can see from the sideways photo. On the other hand one would expect some problems with curving it into shape, quite severe problems.
So I leave you with this photo, showing that the rings on the torso are thicker than the ones on the lower arm, I assume because the ones in the lower arm would be under the protection of the upper part of a gauntlet: