With that said, let's get on to mail itself. It was used by Romans, who probably encountered it while fighting Celts in France and England; there are rusty masses found in Celtic graves from roughly 400 BC which seem to be mail. It was probably invented in the northern France/England area at this time. On the other end, Richard ffoulks has mentioned Pakistani tribesmen who were using it as fighting armor circa 1910. This gives it an active lifetime of roughly 2000 years, in the European theatre. It was also developed in the Asian domains, often as a way of connecting small plates into a flexible shirt.
Mail's primary attributes are: it is highly flexible, it is ablative, and it is easy to repair after damage. The flexibility makes it not only easy to make, but convenient for fitting multiple people. (Plate armor was usually individually fitted.) While mail may not look ablative, and people will say that it doesn't do anything to stop impact damage, they are wrong. It is not as effective as a plate of metal, but because of its mass, its flexibility, and occasionally its deformation, it soaks up a lot of energy. Repair of mail is simple; it's no different from manufacture. Holes are rapidly patched, and no welding is strictly necessary. It's also slinky as hell and almost impossible to stop touching.
Mail's primary disadvantages are: it is heavy for its protectiveness, all the weight is concentrated on a very few points of the wearer, it has a VERY high surface area and rusts quickly, and it is not as protective against crushing blows as plate armor. The tendency towards rusting means that we've a scant few pieces from antiquity to study, and virtually nothing from before 1400 AD, although much of the extant mail was obviously made from older materials which were cut up and remade.
The most common version is called 4/1 or four-in-one; each circular link intersects with four others, to form long sheets of metal rings. There is a warp and woof to this; it stretches differently in different directions. This is known as anisotropic behavior. It is usually made of steel; in antiquity it was sometimes of bronze. These metals are hard enough to stand up to not only direct impact but also the wear of rings against each other.
The wire for mail is traditionally mild steel, which tends to bend under impact, absorbing energy, rather than shattering, as hardened steels do. There is no consensus on when wire-drawing through hardened drawplates was developed; extensive evidence shows that it was considered new technology in the 1300's, but many pieces of wire in jewelry from far before that show the distinctive lines of drawn metal, rather than hammered wire that was occasionally used in very old mail.
Rings were made by wrapping the wire around a mandrel, then cutting them one by one, either by a cold chisel or by a sawing process. Current practice is to make butted mail, which means the ends of an individual ring are pushed into close proximity to each other. The ring stays closed from springiness. Mail used in serious fighting was traditionally riveted, welded, or punched. Riveted mail had the ends of rings overlap; they were hammered flat, a hole was punched through, and a triangular rivet was forced through the hole and hammered down on the point end. Welded mail was simply forge-welded into one piece. Punched mail was somewhat more rare; half the rings in a run were punched out of plate steel or tube, and they were connected with either riveted or welded mail. Often, repair rings were carried to battlefields for rapid repairs; these were visually identical to current keyrings. Ring size varied from roughly 16 mm in diameter, with 14 gauge wire, for horse armor, all the way to rings less than 3 mm in diameter, using 24-ga wire.
Shirts of mail (called hauberks or haubergons) usually weigh between 7 and 25 kilograms, depending on the size and extent of the coverage desired. Many contemporary accounts in Mallory or Marie de France specifically mention the use of very light-weight shirts being worn in several layers. Beneath these were worn heavy quilted shirts, quite thick, as padding. The number of rings is directly dependent on their size; my work generally has about 10,000 rings linked, for a typical shirt, while a haubergon in a museum in Vienna contains 200,000 rings (and you cannot stick a pin through it.)
