Questions on the metallurgy of helmets

[Aetius Helvius Merula]

I'm curious about something. It seems, from what I've gleaned from the web, historical accounts, and re-enactor kit, that helmets (is Galaeii the word?) in the Republican era seemed to be made mainly of brass or bronze. While Imperial ones were made from iron or steel. Why is that? It seems significant that milites would have lorica hamatas, steel gladii, and iron spearheads, yet they'd still have their heads protected with softer metals.

What I'd like to know is, do brass or bronze have some quality that makes it better to have on your head than your body? Or is it a question of economy? I'm no expert, but I know that iron is more extensive and costly to make than brass and bronze. Or simply infrastructure? If there isn't a standing army, as in Republican times, then it may be there wasn't a standing industry to be making all that steel.

And that leads to another question. Would it have been steel? I had the impression that steel was an early medieval innovation, rather than Roman. Don't know why, maybe it' something I read.

[Celer]

Salve Merula,

Brass/bronze is just a hard as wrought iron. The idea that brass was an inherently weaker metal is simply wrong. Iron in its natural state has a Vickers harness rating of 100, so does brass/bronze. Add a bit more tin to the mixture and anneal the brass and it doubles the harness to 200 VS. Roman iron, like all iron of the period (except for Wootz steel from India) was impure with many inclusions in the metal, so it was softer than many modern steels. As a result the relative performance between brass and iron helmets would be much the same. I don't know of any studies that show that iron helmets were hardened.

Brass is easier to work into a bowl than iron, but it is much more expensive. It did continue to be used to make helmets into the C3rd, but iron helmets seem to have been much more common. With the increased emphasis on low cost and simplicity in the C4th, iron was used entirely for helmet production. Robert may correct me, but as far as I am aware there are no late Roman helmets with brass shells, only with brass skins.

Hope this helps,.

Celer.

[Paullus Scipio]

It should also be remembered that Iron, whilst common in Northern Europe, was rarer in the mediterranean world. It is not until Rome expands northward that they acquire plentiful sources for iron, and learn better iron working techniques from those superb iron workers, the Celts ( one field of technology where the 'Barbarians' were superior to the 'civilised' people.)
As with most things historical, the explanation is not simple, but rather a combination of a number of factors.

[Peroni]

There is also an example of an Auxiliary Infantry B in the Cardiff museum that is made of almost pure copper!

http://i20.photobucket.com/albums/b242/ ... -B_02a.jpg

http://i20.photobucket.com/albums/b242/ ... -B_02d.jpg

It came from the de Walden collection (same source as the IG-J in Caerleon and the Montefortino at Cardiff.

[Matthew Amt]

Avete!

Bronze and brass are different alloys with different properties, and the properties of each will vary according to their content. Bronze is copper and tin, while brass is copper and zinc and was called "orichalcum" by the Romans. Brass seems to have first appeared in the mid to late 1st century BC--before that all copper alloys were bronze. But bronze continued in use for domestic items, since orichalcum seems to have been reserved for coinage and military items (by the Romans, at least). The Romans also used what is now called "gunmetal", copper alloyed with both tin and zinc, though it is not always certain whether they made it deliberately or were simply mixing recycled brass and bronze. All three alloys could also have lead added, particularly cast items.

Bronze can have anywhere from 1 or 2 percent tin up to more than 15 percent. Low-tin bronzes are softer and more malleable, more easily hammered out into sheet metal for helmets and such. High-tin bronze is very hard and strong, and if it is hammer-hardened (such as the cutting edges of bronze weapons in the Bronze Age) it is harder than most ancient steels. Only modern tool steels are harder.

Brass also varies in hardness depending on its zinc content, but I don't know what its exact properties are. I get the impression that it never gets as hard as bronze and is overall more malleable. So the Romans may have liked it simply because it was easier to make into helmets and fittings.

Bronze was generally more expensive than iron, because copper and tin deposits were rare and distant. Iron was plentiful in many areas so it was readily available. Bronze and brass can be cast to most any form, while iron has to be hammered out from a bloom or billet. But iron can be hammered while red hot and is easily forge-welded, whereas bronze and brass can only be worked cold and have to be annealed frequently as the metal work-hardens, to avoid cracking. Cold-worked iron can also crack, but it's easy to forge-weld the crack shut or weld a patch over it and keep on hammering, and you'll never be able to tell when the work is done. But if a piece of bronze cracks while hammering, it's ruined, unless you want to rivet a patch over it. Slag and other impurities are not nearly as big a deal in a piece of iron as they are in a piece of bronze, when forging. So it can be much quicker and cheaper to make iron armor and weapons, even if they are metallurgically inferior to good bronze ones.

Even though the Iron Age began around the founding of Rome, the Romans (and other cultures!) continued to use bronze and then brass for helmets and armor right through the second century AD, nearly a thousand years. Yes, iron helmets were typical in the early Empire, but brass ones were still very common. I don't think there was any significant difference in their cost or function. It was probably just fashion! Note that some items such as lorica segmentata and mail were almost always iron, while belt parts, paterae, and other things were almost always copper alloy. Iron was also perfectly common for civilian tools, nails, wagon fittings, etc., from very early times.

Most modern authorities play it safe and refer to anything of bronze or brass as "copper alloy" unless the item in question has been analyzed and its content is known. Be aware that most older sources simply use "bronze" or "brass" in the same way, generically to mean any copper alloy, without knowing the actual content, so don't take them as gospel!

The Romans did indeed have steel, most of it used in sword and dagger blades. Note that they did not have different words for "iron" and "steel" and they of course couldn't know the molecular differences. Between pure wrought iron and "steel" there is a range of carbon contents, and Roman armor and helmets were often what we now call "steely iron", having a good carbon content but not always quite enough to rate as "steel" in modern terms. Much of it was basically equivalent to a modern mild steel. They DID know how to get the results they wanted.

It's a complicated subject, but I hope that answers a few questions!

Valete,

Matthew

[Robert]

Metalurgical analisys of the cavalry helmets found at Nijmegen (late first century) show the metal was folded about 8 times before being hammered into a helmet. They have an display at the Valkhof at present. That's getting a very good hardness! Way over any brass helmet.

[Gaius Julius Caesar]

Quote of Mathew Amt -

"Bronze can have anywhere from 1 or 2 percent tin up to more than 15 percent. Low-tin bronzes are softer and more malleable, more easily hammered out into sheet metal for helmets and such. High-tin bronze is very hard and strong, and if it is hammer-hardened (such as the cutting edges of bronze weapons in the Bronze Age) it is harder than most ancient steels. Only modern tool steels are harder. "

I was quite surprised when I saw a demonstration on a TV program demonstrating an iron sword against a bronze sword, where the iron one came out second best. This is as good an arguement for bronze helmets as any! (except the cost aspect) 8)

[Aetius Helvius Merula]

Thanks, Matthew and everyone else who answered. I have more experience working in wood than metal, but as a hopeful machining student, I should start correcting that soon. I asked to get an idea on kit. I think putting it together is going to be a long process, and I'd like to make it myself because I like the DIY approach. (and because I ain't rich)