Being a blog, I can ask questions that professionals won’t go near due to lack of evidence

For instance, who made stone moulds used to cast pewter and perhaps bronze items? Having had a skim through several sources of information about stone moulds, such as the Museum of London books, the Historical Metallurgy Society journals and excavation reports, I can’t find anything about this question. Of course being a proper careful archaeologist you can’t go beyond what the artefacts tell you, and they mostly don’t come with labels or engravings saying “Made by Bob”.

Obviously carving stone is a specific skill associated with masons, but the moulds are quite small and were probably carved using small sharp tools and files. My experience of making them suggests that with a modicum of artistic ability and fine motor skills you can carve moulds reasonably well, but the complex ones for pewter badges will take quite a lot more practise and real artistic skill.

Which therefore does not mean a stone mason is required.

One suggestion, in “English Medieval Industries” (page 78 of the paperback edition) is that many of the more detailed pewter badges were cast in metal moulds, the metal being cut by seal engravers. Who would indeed have the requisite skills, but I would need more evidence that they actually used metal moulds for this sort of thing in the first place, given the stone moulds that we know of from Britain and Germany, and the fine detail you can get in any fine grained rock.

I did read somewhere that the earthen moulds for casting bells and cauldrons and suchlike in, were made by a specialist on making them, referring to some records from medieval York, but those were single use moulds and he would have made scores every year, whereas a good stone mould can make 300 or 400 pewter castings and thus last for years of use.

Certainly engraving metal moulds would be harder for foundrymen to learn and do than simply outsourcing the work to someone else, but on the other hand they might, especially in smaller towns, encompass engraving within their skill set, as it is in a way related to the finishing of small items.

Turning to “A History of British Pewter”, we find mention of moulds in wills, but unfortunately it does not say what the moulds are made of. Some will be bronze, for plates and the like, and maybe they all are, or maybe some are stone. The authors write that metal moulds were widespread in England in the 15th century, attested to by references in the London Pewterer’s records, in Welch, i, 14-15, 105, 120, 175, 179. Welch is “History of the Worshipful Company of Pewterers of the City of London, 2 vols, 1902. (Note to self, the NLS and St Andrews and Glasgow have it) They also assume that the pewterer would make his own moulds. It would be comparatively simple for a pewterer to cast their own moulds (Or got a foundry to do it, but in smaller towns they were often both the same person and workplace), and spend time finishing them off. It would certainly negate the need for an engraver.

In fact, having been spurred on by this question to read more, I have an answer in the book “Pilgrim souvenirs and secular badges” by Brian Spencer, one of the Museum of London books. It says in the introduction that “Little is known about the men who made the moulds, an operation that called for skill in small-scale precision work as well as a degree of artistry and iconographical knowledge.” It seems some records in York and Cneterbury mention mouldmakers, i.e. people who carved the moulds, whether stone or also, as is shown in the book, cuttlefish bone, but it is not always clear in the records whether they made their own mould or hired them. The introduction also mentions the use of seal engravers in mould cutting and goldsmiths and that bronze moulds for casting pilgrim souvenirs are also described.

So we have more of an answer than at the start of this post. With any luck there is more information available; I shall certainly be chasing up the excavation reports of the moulds mentioned in the book. Cuttlefish bone is one substance I haven’t really looked at, but will surely be the subject of a blog post in the future.

The rather important use of lathes by foundrymen

For 7 years now I have been thinking about all aspects of how medieval craftsmen made objects out of bronze or pewter. The method of manufacture of some objects is obvious, from the excavated stone moulds for pewter badges through to the methods of making bells, which are recorded in detail in the 12th and 16th centuries, with nothing much changing in between.

But that still leaves some items unaccounted for. Some, like small bells, are made of sheet metal or else have cores of sand and clay within a stone mould a bit like that for making bullets, but nobody has left instructions for their manufacture.

Others, like mortars and pestles, well, eventually I worked out that they were probably made the same way as bells, and lo and behold, in Biringuccio’s book it says that you make mortars, basins and other vessels which need to be hollow in the middle in the same way as bells (page 268 of the paperback Dover edition).

This means that a founder needs a lathe of sorts in order to turn the core of the mortar.

I did in fact first try to make a mortar by having a wooden pestle and a mortar made, which could then have the earth pressed around them before removing them and sealing it all together, but that proved to be harder to get right than I thought. The pestle I did produce had a lot of flash down one side because the dampl clay was not rigid enough to stay perfectly in shape when I was taking the former out, and the mortar mould has not yet been tried due to the slowness of mould manufacture and drying and issues with melting the metal.

Anyway, what did lathes look like then, of the sort I can use?

Fortunately there are pictures and descriptions, but never enough. Continue reading

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.