Interesting article on Viking/ Scandinavian casting methods, compared to what I know and have done

http://web.comhem.se/vikingbronze/casting.htm

It shows a number of things. One is that you can do casting with a comparatively small and simple hearth, with bellows feeding a tuyere. This is not of course news to many people, but it does also indicate the sort of improvements in technology that took place over the following 500 years as furnaces got bigger and bigger. Yet they could make some sophisticated and high quality items on such a small fire.

The hearth is clay lined, on stones, very simple to make, and only about 6 inches long.

He mentions some of his experiences, 15 minutes to melt a crucible of bronze is about right, I’ve done it in that time on my small setup. There is a photo of a double hearth, with one suggested for the melting, and one for heating moulds. I usually do both in the same hearth, which if it is large can mean losing a mould in the charcoal. Otherwise I just put a firebrick at the bottom at the far end of the oblong hearth, and thus less charcoal builds up there with less air getting at it, which ensures a lower temperature there, ideal for moulds.

He spends some time on the moulds, going over the advantages of lost wax casting.

It seems the crucibles were made from sand tempered clay, which is always good, but you could only get one or two meltings from each one, which I say is because it is hard to get the right sort of clay, he points out that the crucible turns glassy when used. If you use a whiter high alumina clay then it doesn’t get so glassy, but a high iron one glasses up very quickly. He then says that he uses sand tempered stoneware clay, as far as I can see stoneware clays are for higher temperature firing, and thus are more heat resistant, so of course that is what to use.

Interestingly potters have evolved their own separate nomenclature that is not directly based on scientific analysis, so they say stoneware or earthenware or describe a clay by it’s type of firing and colour.

Now, one of the interesting things is that he says the mould (made from clay, sand and horse dung) should be completely burnt through, with no organic matter left. This is partly to ensure that there is no unreacted lime within the mould, from the clay. Yet he says that this is in contrast to the historic moulds, which are often poorly burnt, oxidised red on the outside and black through the middle, which can make the casting hard to do. The reasons he suggests for successful casting are that they used zinc copper alloys which were better at getting into the moulds through being less viscous and having a lower temp.

Importantly he notes that the clay mould conduct heat slowly, so you have time to take it out the fire and pour into it without rushing, which is a good point. This also permits the manufacture of finely details pieces, because the metal stays liquid enough to get into all the corners.

Clearly I have a lot of practise left to do because my castings don’t turn out the way his do. The secret is in the mould.

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Modern information that helps us understand casting practises

Exhibit A, the phase diagram for a copper- tin alloy, usually known as bronze (Stolen from wikipedia or wherever):

 

Bronze phase diagram

Along the bottom is the weight % tin, i.e. if the object weighs 1kg, and is 50% tin, there is 500g of tin. Sure, that doesn’t seem as scientific as going by mols, but then a lot of metallurgy harks back to before it was properly integrated into modern chemistry. Continue reading

Making medieval bells – part 1 (A never ending series)

So, wanting to get on with things this year I recalled that I had a couple of medieval type bells to re-create. I also have some instructions for their manufacture. Naturally my resources to make bells are limited, so I am looking mostly at the smaller sort.

However after doing some reading I thought it would be easiest to organise things to stimulate the actual making and associated blog posts by bell type and date, starting earlier.

The earliest information I have about bell making is from the 12th century On Divers Arts, by Theophilus, a work you will all no doubt be familiar with.

He gives some instructions for making two types of bells, one the familiar sort that is put in a tower and tolled, and the other that is hung up and hit with hammers. The shape of the two sorts is in fact quite different, something that I hadn’t really realised before.

I imagine this difference is down to the different acoustic demands from the bells, but more research is of course needed.

(This short article seems to point towards some answers: http://www.photomodeler.co.uk/applications/documents/bell_vibrations.pdf As far as I can gather, the bell studied vibrates in different modes, i.e. up and down, side to side, etc, at the same time, so obviously if your bell is very much different in shape it will produce different tones and such through vibrations being different)

 

 

The above photos show two bells, the one on the right definitely being medieval, the left one I think more post-medieval, made abroad judging by the inscription on it. Inscriptions on medieval bells is a whole ‘nother blog post by itself, they have been studied a fair bit. Anyway, note the very slightly different shapes, the earlier they are the more vertical the size, but both have raised ridges round the outside, allegedly from wire used to hold the mould together.

The smaller bells though, Theophilus hardly describes, instead spending time making sure you know that in order to make them sound in tune when playing, the amount of wax for each bell should be carefully controlled, and he uses the method of dividing the wax in proportions, halves, eights etc. So by volume, rather than by weight as a modern person might think of doing.

Unfortunately he doesn’t say exactly how the bells are made, one assumes the same way as the larger ones. Theophilus also points out that you have to be sure to use only the wax for the bell and add extra wax for the yoke by which it is hung and the vents for the mould. Which is very useful advice.

The translators of Theophilus helpfully include a 12th century drawing showing such small bells in use, from MS B 18, fol. I, St John’s College, Cambridge. This can be seen here, the bell player in the top left:

http://www.joh.cam.ac.uk/library/special_collections/manuscripts/medieval_manuscripts/medman/A/Web%20images/beast.htm

Usefully, someone on twitter also linked to this:

http://www.metmuseum.org/visit/visit-the-cloisters/in-season/2015/sounds-of-the-cloisters-a-new-refectory-bell

A 13th century German bell used in a refectory to signal the time to dine. It looks quite thick and heavy, but surely sound good when struck. I note that the yoke at the top has a seam on it, suggesting to me that it was made a little carelessly, when they smoothed the two parts of the mould together, or else it was not made of wax originally but a wooden or other pattern was used, which was then removed before the casting.

So the thing to do in the off season is play about with the wax I have and see what I can make, bellwise, starting with such bells.

Incredible sets of medieval moulds found in France

A few years ago at Mont San-Michel, a lot of stone moulds for pewter casting were found.
Here is the link

http://www.inrap.fr/preventive-archaeology/Events/Virtual-exhibitions/Virtual-exhibitions/Making-Pilgrim-Badges-at-Mont-Saint-Michel/The-site/p-1475-lg1-The-site-and-the-discoveries.htm

They are 14th and 15th century pilgrims badges, often showing St Michael, or the Virgin, or a sword. The detail and artistic skill is wonderful to see, far above my own. 260 fragments were found,

Quote:
“This excavation is exceptional in that it has revealed a workshop for the making of pilgrim badges, the first to have been found in France to date. Exceptionally, the activity is clearly shown by the objects found, the cast-offs, and the archaeological structures, even if the latter are incomplete.
It was not only a workshop for the making of finished objects for pilgrims, but also for the making of moulds for this activity, as indicated by the numerous waste products, rough mould shapes and reworked examples. The whole chain of production was on one site.

However, it is not clear why certain faultless intact moulds were abandoned, especially when it is clear that this type of object was considered to be precious and was transmitted from generation to generation. “

The above indicates a problem similar to that I have mentioned to do with the Coventry moulds, that is, why were they dumped? Many of the Coventry ones are not obviously damaged. What the article doesn’t specify is how faultless the dumped moulds were. It is entirely possible that still usable moulds would have been dumped when fashion changed. Or maybe one part of them broke.

It is interesting though that the moulds were made on the site they were used. What is unclear from the historical records and finds in the UK is whether the pewterer carved his own moulds. I think they may have done so in some occasions, but in others purchased them from stone carvers, but I am not aware of any definite evidence either way. It has been suggested that engravers did the carving, and even that some carved bronze moulds for pewter casting, although none of them have survived. Somewhere more isolated like Mont San Michele would not be conducive to a division of labour, whereas in a town it would be easier to find a good carver.

The website says:
“The quality of the engraving, characterised by great attention to detail, bears witness to the work of craftsmen in full possession of their art, but who were also dependent on the wishes of the monks of the abbey. Written sources inform us that the moulds were the property of the monks and were made at their demand. The monks then entrusted them to metalworkers for the manufacture and sale of the badges, an important part of the profits being paid back to them.”

which still leaves it a bit unclear who the monks had make the moulds and sits oddly with the fact that the casting and the mould making happened in the same workshop.

The photos of moulds that they have made available indicate the usual methods of manufacture, but what is interesting is that they are made of schist and limestone. The limestone is probably from the same source as the Caen limestone, but the schist is not so easily located, perhaps coming from Breton quarries. According to the website the most detailed carving is in the finest grained schist, which makes sense. But oddly the geology book I just consulted says that schist is a moderately metamorphosed rock, made up of very platy minerals, such as mica, which in a schist are large enough to nearly be visible to the naked eye. Which would to my mind make it not very good for carving. If it is so fine grained that it isn’t obviously so wavy and platy, then it isn’t a schist. Certainly some of the Coventry moulds are highly metamorphosed mudstones and the like.

They have the usual holes for locating pins, vents for air etc. One has a clear saw mark in it, proving that they cut the stone up using saws.

Some day I’d like to have a look at these moulds.

Latten maille and the uses of brass in war

And lots of other latten in war gear.

I was browsing through “The medieval inventories of the Tower armouries 1320-1410”, a PhD thesis by Roland Thomas Richardson (Which can be downloaded from the British Library ETHOS service), when I found various mentions of latten. Latten in that period was a copper alloy with mainly zinc and other elements.

Quoting from “Medieval English Industries”, page 82,

“The contract for the mid 15th century effigy of Richard Beauchamp in St Mary’s Church, Warwick, specifies “the best latten”, and analysis has shown this to be a copper-zinc-lead-tin alloy: 84.3% Cu, 9.4% Zn, 2.2% Sn, 1.4% Pb, and the remainder including nickel, iron, antimony, arsenic and silver.”

It is attested as being hard as bronze, and shiny.

Thus one of the uses mentioned by Richardson from the inventories, is to decorate the edge of maille shirts, which you see worn by quite a few re-enactors, the shiny brassiness showing up well against the darker iron or steel. (page 39 of the thesis)

Cap a Pie, who sell maille for re-enactors, sell individual rings for making your shirt or standard look prettier: http://www.capapie.co.uk/index.php?route=product/category&path=59_75

Weirdly though, in the Tower inventories and accounts, there is mention of latten maille shirts! A shirt made completely of latten would not be as strong as iron or steel, so I wonder why. (Pages 41 and 44 of the thesis)

The other use on armour is as a latten border, presumably some sheet riveted onto the iron vambrace (page 78 has a photo)

Latten also crops up as horse armour.

Oddly it is also used, e.g. on page 110, to make fletchings for springald bolts! I presume that they were better able to cope with the stress of use? The other use related to springalds is in 7 nuts of latten, purchased for making 7 springalds. I think the nuts they mean are the ones that hold the long brass rod which is turned to pull back the arms of the springald. Thus they would have been large, strong nuts. (page 111)

Page 167 – There is mention of 10 moulds of latten for casting lead balls for firing from guns. Ten is quite a lot and would enable fairly rapid production, although each mould would surely take a minute or two to cool down when cast into, due to the lead taking a long time to cool.

The title though is a bit of a cheat – what they called brass usually included bronze, the latter word not being around in medieval times. So cannon were made of brass, only they meant the strong 10% tin-copper alloy. There is several mentions of them in the inventories, and usefully, of the people who made them.

Page 166 mentions a small gun of copper purchased from John ‘brazier of Cornhill’ in 1361, and of course by copper they must mean bronze, since only that could take the strain of the gunpowder. Technological terms then were not as definite as they are now, although at other places in the accounts the guns were said to be made of bronze. A brazier was basically a foundryman, someone who cast in bronze or indeed brass, into moulds, whether of stone or clay or suchlike.

The earliest accounts I know of from Scotland of such practise are in the reign of James IV, the 1490’s, when a potter made a mould for casting a gun in. A potter being someone who made metal cauldrons, i.e. pots. There isn’t that much evidence surviving for potters in Scotland, but there must have been some.

So, it is amazing what turns up in peoples research. I urge you all to search through ETHOS to find PhD theses of interest.

Trying to make a lead spindle whorl and mould

For several years now I have been trying to make various replica pewter and lead objects, one being a spindle whorl. Not perfect copies cast from an original in silicone, but copies of the sort that a foundryman might make after seeing an object. Of course the process of making the object can also teach you something about the manufacturing capabilities of the time and confirm or disprove how you think it was made.

Spindle whorls are the weight which keep the spindle turning when you are using a drop spindle to make thread, as was common throughout the medieval period and into the 17th century. Excavated examples include ones made of wood, pottery and lead and obviously I am interested in the latter.

Of course nobody seems to have written anything about how they were made, except it is obvious that they were cast. I have examined 5 myself, and the portable finds database gives access to photographs of many others, of several main types amongst the hundreds of examples.

E.g. :
https://finds.org.uk/database/artefacts/record/id/730362

The PAS finds 1,737 medieval lead spindle whorls, which is probably a small fraction of those found over the years, nevertheless it indicates a huge number of them.

Unfortunately the first attempt I made 5 years ago now involved casting plaster and a spindle whorl:

large spindle whorl

It didn’t really work, casting plaster is trickier to use than you think, especially when the whorl is of such an odd shape.

A friend lent me a smaller simpler whorl several years ago, and I’ve been carving since then.

This is it side and top view. Note how well rounded it is, which might be due to wear or the method of casting: Continue reading