Real and possible misperceptions about medieval copper alloy castings

In the rainy season I thought I’d correct a few of the odd ideas that people have about medieval casting, but the annoying thing is I can’t really find many examples of people being wrong. This is in part because the internet is not all powerful and all containing, and searching it isn’t as easy as it used to be. Some of the errors will have been promulgated off internet, in magazines or books that I have never read. Other errors have no exact source, but are what people assume based on partical knowledge.

The most common error is certainly the idea that they used sand casting, especially for cauldrons and smaller items. Apart from the lack of dumps of metal contaminated sand at excavated foundry sites, we’ve got lots of physical evidence of what they did use to cast bells and cauldrons. Yes, clay and organic muck. It survives well in the ground. For instance, at Exeter archaeologists dug up over half a tonne of clay based mould fragments from the manufacture of cauldrons, as can be seen by the shape of the pieces.

Elsewhere in the country many fragments of bell mould have been dug up, whether in London, York or elsewhere. These correlate with areas and times when the documentary evidence shows that there were bell foundries at work, and in the texts they refer to earthen moulds, i.e. not sand, but mixes of clay and other sedimentary materials.

(I just found this report on a Lincolnshire excavation that examined bell casting debris http://services.english-heritage.org.uk/ResearchReportsPdfs/104-2002.pdf)

We also have the remains of a clay based buckle mould found in London, showing that they were using it for small items as well. Since we also have stone moulds which could be used for buckles and the like, the question is, where would sand moulds come into it?

The point is that we can’t prove a negative. Nevertheless, given all the positive evidence for how bronze and brass objects were produced, why would you postulate the use of sand?

Some people do like to point to Biringuccio’s mention of it being done using sand from the Seine river in Paris, but that was not the same as modern casting sand, not using oil but river gravel, baked in a furnace, as well as burnt rams ashes and old flour, moistened with wine or urine. He certainly suggests that it can be used for small items or bells, but here we have to go by archaeological and textural evidence, and neither, in England at least, indicate the amounts of stuff that would be required. He also lists a number of powders that can be used for small castings, such as crushed brick, tripoli, vine ashes, tils calcined tin, ashes of various sorts. He writes, “The goodness of all these depends on three things; namely, on receiving the metal well, on being so fine as to be almost impalpable, and on their being made with a magistery that renders them hard and strong when they are dry.”

(Dover reprint of the Smith and Gnudi translation, paperback, page 324)

It is entirely possible that English founders sometimes used this sort of method, but also that throughout most of the later medieval period, they didn’t, and it was used more by the continental ones. Further research by myself in academic studies of continental castings and excavations of foundries would be useful. The archeologists have to be aware of all possible materials they might meet during their dig, it wouldn’t surprise me if some evidence of these powder casting methods has been overlooked at one time or another.

Another dodgy idea is that mortars were made with bell metal. Bell metal contains 22 to 24% or so of tin, the rest being copper. This immediately makes the alloy more expensive than the normal bronze used, and also doesn’t confer any advantage in strength, and makes it more brittle (It is the brittleness of bells that helps give them their wonderful sonorous sound, compared to say a bell made out of lead, which is not brittle).

The main mention of this I have is in the 2010 reprint of the 1988 Shire book on “Bellfounding” by Trevor S. Jennings, who did a lot of research into itinerant bellfounders. He states on page 4, re. a photograph of one, “Apothecaries’ mortars were often cast out of bell bronze.”

Naturally there is no reference, and the idea is likely to come from some comparatively recent ones that were made with that alloy, or from 3rd hand information.

By contrast, real period mortars are like this one:

http://www.christies.com/lotfinder/lot/a-tudor-period-bronze-mortar-16th-century-5401271-details.aspx

The lot notes say, “This lot is sold with a Metallurgy Analysis by Dr Peter Northover in which he concludes ‘The particular type of bronze used in this mortar is well known in later Medieval cast vessels in England up to the mid 16th Century but their use was probably well in decline by the end of that century.’ He also discusses similar analyses are found in North German mortars of a similar and later period. ”

Which probably means a form of leaded antimony bronze, which makes perfect sense, it being easily available and fit for purpose. It certainly doesn’t mean bell metal, and the other analyses of mortars that I have read of are closer to the usual 10% or less of tin.

Finally, I find often that some archaeological texts , papers etc, say that the clay based mould is porous once fired. This is not necessarily an error; it is something I disagree with, i.e. you could say a professional disagreement based on differing experience and interpretation of the finds. I have to say that I have never observed this in my work, because the clay coats all the organic debris, and even when it is mostly burnt out or carbonised at least, there simply isn’t much porosity. Certainly not enough to affect the final work or get rid of bubbles. I could of course be wrong here, not having been able to destructively examine some historical casting debris, but still….

For instance, the Exeter report says “The mould when dry was light and porous enough to permit the escape of air and gas through the fabric during the casting process.” (Page 41 of Devon Archaeological Society Proceedings no. 58, 2000) I have not examined the specific mould material mentioned, and of course that statement applies only to those finds. But I am sure I have read such an assertion in other places too, and I disagree.

For instance, here’s a piece of a mould from London, a cross section; note the black, reduced inner side and the red, un-reduced outer side. This may have used chaff from straw as an ingredient:

London mould side view

And some of mine, made with clay, horse dung and sand, sorry about the unfocused look but you can clearly see a lack of porosity:

my mould cross section after use

Neither have voids or enough porosity for air to get through them.

Are there any other things that people mistakenly believe about medieval and post-medieval casting?

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One thought on “Real and possible misperceptions about medieval copper alloy castings

  1. Pingback: Whewell’s Gazette: Vol. #31 | Whewell's Ghost

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