Brass & Bronze in Theophilus – Understanding Medieval Copper Alloys

by Jamie Hall on February 27, 2015

Note: This article is based on Smith & Hawthorne’s translation of On Divers Arts, and the references are written [Book-Chapter], so that [III-10] would be Book 3, Chapter 10. Hendrie’s translation has also been consulted, but is not referenced directly, and care should be taken, as the chapter numbers are not quite the same! I recommend purchasing On Divers Arts; you can read a review here.

Copper and it’s alloys feature heavily in the archaeological record. Tools, weapons, jewellery and other metalwork have unique requirements, and alloys can be chosen to suit the object being made. That’s how we think about it in the modern day. The terms brass and bronze are used to refer to copper-zinc and copper-tin alloys, respectively, and a wide range of alloys are available commercially (or can be made in the workshop). Lead and other metals can also be added.lead tin zinc

Artefacts can be analysed to determine their alloy contents, and plotted on a lead-tin-zinc ternary diagram, at which point we can be certain about what they are, and use the terms brass, bronze, gunmetal, leaded copper, and so on.

Extant texts are more challenging to interpret. The availability of all alloys cannot be relied upon – the artisan might choose a technique to fit their metal, or use the local alloy for everything. They would not have a list of the alloy constituents, so they would rely on the colour, weight and other factors, when examing the metal. Those who refined and alloyed metals might have a better understanding, but the purchaser of an ingot could be using various names for the product.

In On Divers Arts, by Theophilus Presbyter, copper plays an important role in the metalworking section, and he describes uses for different alloys, such as casting and gilding. There are several words used for copper alloys, and these don’t always match the meanings found in other texts.

If we start with copper, Theophilus describes a number of alloying recipes. It seems fair to assume that the primary feedstock is impure copper, because III-67, The Refining of Copper, is intended to remove lead, so that the metal can be forged and used for repoussé and wire-making. This metal is called torridum, or “heat-dried copper”. If calamine (a zinc-bearing ore, finely powdered) is alloyed with torridum, then the resulting brass is called auricalcum, and can be used for gilding, but if coarse brass is made with unrefined copper, according to III-66, it is aes, and cannot be gilded due to the lead content.

In later chapters, the cast of bells is discussed, in some detail, particuarly III-85. Here, the term aes is used once again, but this is explicitly bronze composed of 80% copper and 20% tin. The copper might contain impurities, and the use of aes could denote a ‘coarse bronze’, containing lead. Aeramentum is also used in that chapter, in the context of the “components” of the alloy. A grasp of Latin would be really helpful right now; I’ll add it to the long list of things I need to learn!

At the very least, there are three different brasses (leaded brass, lead-free brass, and Spanish brass), two coppers (leaded and lead-free) and different copper-tin alloys for bell casting. What can we learn from this?

Theophilus must have been familiar with alloying, and with choosing the right alloy for a job. For bell casting, a bronze was needed, but for all other castings, brass was used. Lead-free brasses and coppers were forged and made into wires, and could be gilded, making them ideal as decorative alloys. Theophilus was probably working in a large-scale workshop, where heavy objects were made, rather than small items of jewellery. A secular jeweller might have had other uses for bronze, such as casting brooches, but for Theophilus, it is brass (in several alloys) which provides the mainstay. Leaded for casting, and lead-free for forging and gilding. There are only two brass tools described – a hammer for making gold leaf [I-23] and the brass brush, filum ex auricalco [III-39], which is made from wire. The hammer could be any of the brasses, but the wire is likely made from a lead-free copper alloy, because it must be forged and then drawn, and both processes depend on a malleable, ductile metal.

His expertise is less convincing when it comes to trade goods – the origin of auricalcum Hispanicum is not described, but he talks about various foreign golds in a confusing and even mystical way [III-48], either because he had little experience with them, or perhaps he was easily persuaded by merchants! Spanish brass is mentioned twice in the text – once for improving the casting properties of silver [III-30], and also for making pressblech dies [III-75]. The latter is a way to decorate thin sheets of metal, by pressing them onto a die. In both cases, lead would be a contaminant, so auricalcum Hispanicum is probably lead-free. This fits with aes for leaded alloys, and auricalcum for lead-free alloys.

One noteworthy feature of tBrass Furnacehis text, and many others, is that metallic zinc is not mentioned, and the prevailing opinion is that it was not generally available. For this reason, zinc-bearing ores (like calamine) or foundry waste (cadmia) were used to get the zinc into the alloy, and it was clearly a difficult task. Zinc forms an oxide when heated, and it boils at 907°C, so melting copper alloys to casting temperature would loose some zinc, even if a reducing environment was used. Theophilus must have had access to the ore, because he uses it for making brass, but he makes no mention of cadmia, which suggests that he is unaware of it, or that his own brass-founding is not on a large enough scale to generate an appreciable amount. Compare that to tin, which is mentioned more than a dozen times, as a metal in it’s own right, and as a feedstock for making bronze.

As always, we can’t draw too many conclusions. The text is presumably the work of one artisan, and represents a liturgical metalworking tradition that might be limited to a geographical area, or even a single monastary. The dependence on brass might reflect a local source of calamine via trade or mining, while the occasional use of bronze (for bell casting) was probably caused by necessity. If anyone has a dataset of compositional analyses for European metalwork in the 11th and 12th centuries, I would be very interested to see what patterns emerge.

This text, and others like it, do suggest that the modern perception of alloys and their function cannot be relied upon. It is not enough to say “brass is better for machining”, or “the zinc was there to help with casting”, because that assumes modern alloys and (reasonably) modern techniques for processing them. The medieval artisan was at the mercy of geography and history, and with a little effort, we can understand the limits that were placed on them, and how it impacted on their work.

Text references: Brass hammer – Gold leaf [I-23]; Brass brush (filum ex auricalco) – Polishing the Gilding [III-39], How Brass is Gilded [III-68], Repoussé Work Which Is Chased [III-78]; Brass – The Cast Censer [III-61], The Chains [III-62], Studs [III-76], The Copper Wind Chest and It’s Conductor [III-84]; Coarse brass – Making Coarse Brass [III-66]; Gilded brass – How Brass is Gilded [III-68]; Brass wire – Polishing the Gilding [III-39], Iron (and Overlaying and Inlaying with Gold and Silver) [III-91]; Brass powder – Milling Gold for Books and Casting the Mill [I-28], How Gold and Silver Are Applied in Books [I-29]; Brass impurity – Refining Silver [III-23]; Spanish Brass – Casting the handles for the Chalice [III-30], Work Pressed on Dies [III-75]; Spanish Gold – Spanish Gold [III-48]; Bell metal – Copper [III-63], Casting Bells [III-85], The Scale of Sizes for Small Bells [III-86]; Calamine – Copper [III-63], Making Coarse Brass [III-66]

{ 2 comments }

Tom Stramiello March 8, 2015 at 20:32

I am surprised there was no mention of Cadmium or “Zinc Fever”. They may not have been aware of Cadmium toxicity

Jamie Hall March 9, 2015 at 12:56

Theophilus discusses problems with mercury fumes, but doesn’t mention anything else (if I recall correctly). Zinc and cadmium fumes would certainly be a problem, but the techniques used might have made them less of a problem.

Archaeological studies show small amounts of cadmium in brass objects. Cadmium has a lower boiling point of 767C, so the smelting process would presumably drive off the cadmium before the zinc was lost. Brass production was done on various scales. Theophilus uses powdered ore for his brass, other texts use cadmia, which is scraped from the linings of brass furnaces, according to Dioscorides. That could contain cadmium, but it would principally be zinc.

I haven’t seen any mention of metallic zinc or cadmium – the smelting operations are done with copper, to produce brass, and the process might be repeated several times, to get enough zinc into the alloy. That suggests processes which do not drive off excessive amounts of zinc in the form of zinc (and cadmium) oxides, and wouldn’t generate clouds of fumes.

I would expect long term problems for any medieval metalworker (and a lot of modern ones too!), but maybe they hadn’t identified the precise causes of them.

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