My new dilemma – where to put the ingate and vents

So I want to carve some soapstone for casting bronze havettes. I have a couple of largish pieces of stone that I just need to flatten properly on one side, to make a two piece mould. The question is where to put the ingate. Should I run the bronze down a channel and fill the mould from the bottom? That way the air is forced out the top, but I’ll need to carve an extra channel for the pointy bits. Overall this way has produced good results when I’ve done it using lost wax.

havette wilt-e0afbc one

Should I put it in the side and tilt the havette slightly so that it flows up? That isn’t too bad an approach, although it does rather hasten the filling and I think increases the chances of air bubbles.

havette wilt-e0afbc two

Should I just pour in the top and hope for the best? This is usually the worst way, unless the mould is open enough to be able to let air out all around, since the air coming out will have to mix with the liquid metal coming in.  It also leaves more messy sprue around the outside to clean off.

havette wilt-e0afbc three

I know, lets have a look at some real ones from the portable antiquities scheme.

Unfortunately, despite them being pretty good photographs, it isn’t at all clear. If I handled one myself I could have a better idea, but at the moment it isn’t obvious. They may even have been made with lost wax method, but what makes it difficult to say is that they will have been finished off with the sprue cut off and the area sanded down.

https://finds.org.uk/database/artefacts/record/id/897784

This one for instance, in order to make it work properly the hooky bits need to be filled. But it is pretty smooth all over, with perhaps a small bubble at the bottom where the hook joins the main body. The decoration is probably made using a file.

https://finds.org.uk/database/artefacts/record/id/882498

Is slightly bent, again, hard to judge what it was cast into or how it was cast into.

I like this one, it’s a bit pitted but clearly shows how pointy the spikes are.

https://finds.org.uk/database/artefacts/record/id/810783

I suspect that they were made with much larger spikes to be ground down into proper spikes. Which means that if the bronze came into the mould that way, there would be less evidence for it since it was ground away.

In the end all I can do is try a method and see how it turns out.  I am minded to go for the first and second ones, the stone is big enough to do that.

My next thing is working out how to hold the two halves of the mould together when the bronze is being poured into it.

What I did on my holiday this year – bronze casting

Once again, I was on the foundry at Kentwell hall in Suffolk.

This time, I was the master of the foundry. Unfortunately there was a slight lack of assistants, so there were 2 of us most days. Still, I managed to get a few things done and refine my understanding of the processes and equipment.

Last year, which I meant to write up and publish but never really did, involved a group of us trying to make cannon. We failed, due to lack of time and lack of appreciation of the difficulties. Replicating the techniques of 16th and 17th century foundry work, even with at least 3 of us having a great deal of expertise, was something that would have taken more than the 8 days that we had, and so we failed. Just two more days would have been enough though. Which isn’t bad, all things considered.

For those who don’t know, this is the foundry at Kentwell, first the bellows end and then the furnace end:

bellows end of kentwell foundry 2016 furnace of kentwell foundry 2016

Continue reading

Casting into powder, a method from Biringuccio

Something I’ve been wanting to try since last year is casting into powder to make small objects, which is an interesting thing to test for real experimental archaeological reasons. This is done by following the instructions of Biringuccio in his Pyrotechnia (published in the 1540’s). (page 324 for the powders, and 326 for the method for making up the boxes, of the Dover paperback edition)

The simplest way to do it for me was to make a wee casting box from sheet wood that I had lying around:

Wooden casting box April 2016

Note the dowelling rods to hold it together, and the fact that I carved the open parts out by drill and saw. This photo is of one side, with the outer piece of wood to hold the powder in, underneath the middle hollow bit.

It is big enough to make 1 large or 2 medium sized buckles, or some buttons or something. If I make it too big I’ll need lots of powder to fill it and it’ll be more of a pain to deal with. It is then bound together with string when ready to use, and you can see the pouring hole at the top, which will rapidly get burnt from the molten metal. Only I forgot the string, so held it together between two bricks when I was casting.

The archaeological aims are to see if it is possible to use the powders, and how well they do, and whether there are any obvious indications of such a method on the finished product, i.e. how much work would be required to finish it off, compared to my usual clay casting. Then there is the question as to whether the powder is damaged by the heat and thus forms a sort of ash or slag that can be identified or overlooked in a dig. It also seems to be an area of experiment that nobody else has tried, although I lack the language skills to tell if someone else abroad has done it before.

Now, the powders that Biringuccio says to use are quite varied, but you can immediately see that they are all heat resistant. Quote:

“Powders are also made of crushed brick, tripoli, vine ashes, tiles, and glazed drainpipes, or burned emery, calcined tin, straw, and of burned paper and horse dung as well as of young-ram’s-horn ashes and many other things. 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 dry.”

He also mentions another of two parts pumice and one of iron scale, pounded finely. The other powder he spends most words on is made from a burnt and pounded loam originally made from fine grained earth or gravel or river silt, mixed with wool cloth cuttings, spent wash ashes and horse dung.

This last one is a little time consuming to make, so will be made later.

This post is about the use of calcined tin, which was some purchased from a chemical supplier, and of a fine, floury consistency. Biringuccio above said to add a magistery to them, by which he meant the magistery of salt. Fortunately I tried burning salt in a silica rich environment last year, so added some salt solution made from that salt. (More on the magistery of salt in another post)

Now, onto the trial.

Making up the bronze to be cast was simple enough, so I will ignore that.

Instead, the interesting and complex bit was making the mould to cast into. Firstly, I made the tin oxide damp with the magistery of salt solution, but it ended up a bit lumpy. I tried pressing it into the mould, which was easily enough done, but it was never of a smooth consistency. Ultimately, when I was trying to press the buckle into it I came to the conclusion that I should have dampened it a lot more. Biringuccio wrote:

“…that has been slightly moistened for moulding, as I told you, so that by pressing with the ends of the finders and with the hand it holds together as well as possible.” (Page 326)

But there is a fair bit of room for error here, I think it holds together at a wider range of dampness than the actual wetness that would make the moulding turn out best.

Nevertheless I soldiered on. The first photo shows the buckle on the powder:

Casting powder buckle in frame April 2016 Continue reading

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.

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

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.

What I hope to do at Kentwell 2015 next month

This year (If all goes well) I am being a foundryman at Kentwell hall, in the year 1588. This is a good opportunity to try various things out, most of which I shall blog about afterwards. So I shall be sure to take a lot of photographs.

Things I intend to do/ show:

Finish my stone pewter spoon mould

Cast pewter buckles which will be used to try and make the next items

Cast bronze and brass buckles, purse bars, scabbard chapes, and pestle and mortar

Experiment with dry powder casting as mentioned in Biringuccio

As well as helping out with attempts at making small cauldrons or a cannon or a bell.

The trick will be preparation, so the week before I shall be mixing up lots of muck for moulds and trying to finish off a couple of pewter buckle moulds. Once there I shall also be discussing foundrywork with a couple of people who know a lot about it, and trying to keep the newbies who don’t, right. And get the younger ones working the bellows.

Yes, I am aware that having a foundry at a 16th century manor house in the countryside isn’t really accurate. It’s one of these irritating compromises that helps keep both the public, the site owner and us re-enactors happy and interested. Ideally we’d have a replica town, but that requires some millions in investment to make happen.

(And don’t say that early medieval association or group has managed it, because I’ll just laugh at you)

Finishing the casting off so it is nice and shiny

A quick foreward to this post – I originally did some “lets see what happens when I try some methods of finishing castings off that I think they might have used” last year, about 5 minutes worth. That convinced me that using files and stones was possible. Then of course I realised it would be a good idea to do some research as well, using my well stocked library. Hence this post, which follows on nicely from the previous one.

So, now you have a nice cast object. It is still discoloured and a bit rough from the casting process. What do you do?
To answer this I turn to the usual two printed sources that bracket my period of interest, Theophilus and Biringuccio.

The 12th century monk Theophilus describes the use of files to finish a cast silver chalice off before engraving it.
More interestingly, he writes, page 175 (Of the Dover paperback edition), after casting a bell and bracing it on the lathe, “…the bell can be turned and smoothed all over with a sandstone.” Which is surely good evidence for the use of a natural and often freely available precursor to sandpaper! You can get different types of sandstone too, some fine, some coarse, which would give you great ability to smooth it out properly.

Earlier, he writes of the making of burnishers, which are for finishing the work, they are like scrapoers but more rounded. They are still in use today for smoothing out the surface of a metal object by plastic deformation.

You should also, (page 93) have files made out of pure steel, hardened with a form of carburisation. Large, medium sized, four cornered, three cornered and round. The descriptions are not so clear, but he does refer to making finer files “.. work whic h has previously been filed with other coarser files should be smoothed.”

Obviously I need to expand my toolkit, get some more files and burnishers as well as different grades of sandstone.

Biringuccio writes on page 377, regarding the finishing of flasks, saltcellars etc, “With rasps and scrapers and other cutting tools they are smoothed, polished and made beautiful.” I get the impression that the finishing of metal items is so well known about and widespread that he doesn’t see any need to go into details, whereas the art of casting in bronze and such is tricky and difficult and he wants you to know all about it (And that he knows all about it).

I have an interesting book on English pewter, called “A history of British Pewter” by John Hatcher and T. C Barker, published in 1974. It has an outline of the manufacturing process, and it is clear that you used the lathe for it as told by the period sources, with several types of iron tools similar to those used by wood carvers for burnishing the surface. Interestingly though you also hammered some types of wares, like plates.
But it obviously only covers pewter.

Another source is the treatises of Benvenuto Cellini, of the mid or later 16th century. Page 57 of the edition I have talks about finishing a work of art, some sort of cast and soldered together object, and he wrote “I took some four or five hard pointed stones, which are sharp at the ends and thicken upwards in the manner and I used them with some well powedered pumice stone. The object of using these stones is to take out the markes of the steel tools, the punches, chisels, files and suchlike and to give it a fine uniform surface; and last, but not least, a brilliancy of colour which would not be so easy if the marks of the steel tools (And the skin they make) were not obliterated.”

Page 94 tells us about the cleaning up of a large silver statue. He used pumice to clean and smooth it, then heated it to red hot and put it in a blanching solution of water with tartar and salt. Then it was scrubbed and when the silver started going white it was put in another vessel with clean water, to wash the blanching solution off. After that it was ready and clean for gilding.

Between them all that gives me enough information to improve my displays and production processes. But as usual, practise is required.

For contrast, here’s a photo of a later medieval bronze cooking cauldron from Dublin. Note that it isn’t really well finished off and clearly shows the flashing line where the outer mould was split in half to enable it to be taken off the mould it was shaped on.

Dublin late medieval cauldron

The importance of finishing your casting off – unanswered questions about medieval bronze castings

Looking at real surviving bronze objects from the medieval period leaves me amazed at how good they are. The thinness, the smoothness and so on. I have never been able to replicate it using clay based moulds, so far.

The question is, why?

Here’s a pair of spectacle buckles I have made by casting into clay based moulds:

Two spectacle buckles unfinished

They both have, to a greater or lesser extent, a pockmarked surface.

In other objects you can see what look like real bubbles, which may well be from oxygen dissolved into the hot metal. It’s my fault for not poling the metal, that is, putting a green stick into it to burn out the oxygen, but most o the casting I have done has been very close to the the right temperature.

So that’s definitely part of it. If the mould and metal are both very hot, then it will fill the mould completely, and if the metal hasn’t much oxygen in it, then there won’t be many bubbles.

This would explain some of the poor surface finish. And I have seen medieval bells with bubbles at the surface a millimetre across, often several grouped together, so they obviously had the same problem.

Now, here are some actual medieval belt buckles, found in the remains of a real 13th century or so workshop in Dublin:

Dublin buckles unfinished long one Dublin buckles many at once Dublin buckle unfinished state

Sorry about the picture quality, the photos were taken through a glass case. What you can see, hopefully, is that there are unfinished castings with some very rough surfaces. Which immediately makes it clear that a lot of fettling goes on with the unfinished castings. This was also suggested to me by a member of the public at Chatelherault a couple of years ago. I also wonder if the state of the sprue attached to them indicates the type of mould they were cast in. Pewter objects usually have a clear, sharply defined sprue with diamond cross section, because that is how the channels are cut in moulds. These by contrast are squarer in cross section, and so I wonder if they were made in clay based moulds. It is known that buckles and such were made in multi-layered clay based moulds such as one found in London, so they would have had the same difficulties I did with the quality of final product.

Another option is that they were cast into stone moulds. These would, looking at pewter castings, give very smooth, detailed surfaces. The only slight problem there is that there’s no real evidence for it. So few moulds survive, and I’ve not come across mention of stone moulds in wills or the like. Moreover, they wouldn’t last for so many casts, because 1100C Bronze or brass is hot enough to cause major thermal shock and a lot of expansion and contraction. Besides, most moulds I’ve seen have lead plugs, which might get melted out by the heat from the cooling bronze. I’m going to be doing some experiments this year to show it one way or another.

Yet another way to ensure a good quality, smooth surfaced casting is by making the mould up using very fine clay slip around the wax original, before adding the coarser clay. I haven’t succeeded in doing that method myself, but have spoken to people who have and they say it works well, even better if you add some fine carbon in so that there is a reducing atmosphere inside the mould, which means you get a nice shiny non-oxidised surface finish.

From Biringuccio, the early 16th century founder, there’s evidence for the use of casting using sand based recipes or fine powders, but of course such stuff wouldn’t leave obvious traces in the archaeological record, unlike the tonnes of clay based moulds which came from bell and cauldron moulds. Or perhaps some archaeologists have overlooked it? I have yet to try such substances, but they might also not have been much used in Britain compared to the continent. He does mention the use of multi-layer moulds like that found in London, and his instructions for making bells are almost certainly the same as how they were made in England, so it does seem worth trusting his descriptions and recipes.

The final option is that the way I am doing all parts of the work isn’t right. Hence the experiments with different mould materials that I shall carry out.

Before that though the obvious thing to do is do a quick and dirty test clean up of one of my own castings.

So here is a buckle, cast in a clay mould from pressing the original into the damp clay.

Before treatment with files:

as cast spectacle buckle

After treatment:

filed spectacle buckle

What is immediately clear is that with a fairly small but finely cut file you can get a nice shiny smooth finish very quickly, with only a few strokes, and that will take out most of the unevenness of the as cast surface. Further work could also give each of the edges a sharp angle to it. So it is likely that some of the perfection of buckles is down simply to good fettling by whoever did it.

But as always, more experimentation and research required.