Quality of metals

[jvrjenivs]

In another topic (Montefortine helmet development by Al-hammd) I came accross this statement

Perhaps lower grade material (the Romans were not known for having consistently high quality alloys.

This got me interested in others people view on this. In my opinion the opposite is also true. Although some items were cruelly made and had not that good alloys and metals, we also know of many high-quality workshops. Take for instance the metal workshops at Vindonissa (if I remember right, could also be Augusta Raurica) were different compartments were found in the workshops floor, where alloys were in fact sorted by quality. This at least proves to me the craftsman working in this workshop knew the differences.

Of course this get also confirmed by different metallurgy researches done over the last few years.

[Gaius Julius Caesar]

That would confirm the impression I get when looking at different artifacts.
Some look to be of high quality materials, others, less so.

[Robert]

Making copper alloys like brass and bronze was well understood, so I presume you are refering to iron objects.

I will try to explain something about iron and steel, along with alloys. Starting with the alloys; to make an iron alloy you will have to melt the iron. There is some discussion on whether or not the Romans could cast iron iow melt iron to a liquid. There are certainly shapes I have seen in tools that would imply iron casting, as forging these would have been highly impractical. Mind you, casting an object in iron does not make the object of castiron, as that has a very high carbon content of over 2 % and is notoriously brittle. But ... the metalurical knowledge was very limited in respect to the effects of additives like nickel and chroom, should these have been available in pure form. What I think happened is that natural alloys from a specific ore-site and their superior performance was noticed by the craftsmen. Iron ore contains a good number of assorted metals in small quantities, but certainly sufficent to create a natural alloy. Medium alloyed iron contains between 1,5 en 5% alloying elementens (excluding carbon). Manganese and silicium (yep, sand!) are the most commom elements for alloys (Si = 0,7 % Mn = 1,6 %) The raw material from a specific mining location could thus be recognised as being of superiour quality and traded as such. A trained craftsman could certainly tell the difference, using things like a scratch test with a tool of known hardness.

Then there is the difference between iron and steel. Steel is basicly iron with a higher carbon content or an iron alloy with other metals. Mind you, the carbon content is not above 2 %! Steel may be hardened and tempered, low carbon, unalloyed iron cannot be hardened at all. There were methodes available to the Romans to add carbon to the iron, like the very slow smouldering in a burning charcoal bed with little oxygen. This would cause a slow diffundation of carbon into the iron (case hardening). Reworking the iron in the forge and then repeating the proces caused more carbon to be absorbed and created a homogenious steel.

Mild or construction steel(max. carbon 0,17 % Mn 1,40% Si 0,045%) as we have available these days is really top range stuff compared to the materials the Romans would have had by and large. Great stuff for making swords and a lot more accurate then EN45 spring steel blades (Carbon 0.55% Mn 0.75% Si 1.75%).

[jvrjenivs]

well, actually the quoted comment was made on cupric alloys. But I indeed like to include all sort of metals in the discussion.

[Robert]

Oeps, sorry. Indeed there is a good range of copper alloys as well. What we tend to call "bronze" objects were indeed also of a wide variaty of different cupric alloys, although I would think tin and lead were the main alloying elements and the colour does tell you a bit about the possible alloy, unlike iron.

[PhilusEstilius]

I have gained great experience from having reproduced much Roman raised relief work however there are many Roman artifacts that do frustrate me from the point of view of the depth that they could achieve.
I think that the reasons are that with much of the brass sheet we have today it is difficult to get any with such a good lead content, this is where I think the ancients were so good at creating their brass and bronzes to the quality they wanted.

[Robert]

I was wondering if the metalurgical analysis Jurjen mentioned is available. Would a high lead content (20%) also make the bronze easier to cast in greater detail or thinness? I have often wondered how the casting of 1 mm thin chapes or the really detailed small figurines was achieved by the Romans without a vacume caster.

[PhilusEstilius]

I think that a lot of the very thin casting of bronze may well have been done by sand casting, such as the vine leaf decoration on things like the Vindolanda Chamfron that are even less than 1mm thickness and have three integral pins on the under surface.

[Robert]

A chape is hollow, so would have had a clay core. We know clay molds were used on scabbard slides, as one was found still in the clay mold. But for sandcasting also, the metal would have to be very "runny" to flow well enough to be able to fill a thin space. Bronze cools really fast. Which made me wonder whether or not high lead levels could account for this.

[Gaius Julius Caesar]

I believe the lead content was to allow the bronze to be cast more easily, Robert, yes.

[PhilusEstilius]

Robert.

I think this is where we need some of the casting experts to come into this topic to give us a better understanding about molten bronze.
It might be the lead content or copper or indeed the amount of tin and other metals involved in the mixture.
What I do know is that some of the fine sheet used for Roman decoration work and also for covering of other things such as wood is very thin indeed in the region of 0-5mm.
Then for small statues and more solid pieces they may well have used the centrifical method where the molten metal is thrown into the mold.