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Vannochio Birringucio, the late 15th/ early 16th century foundryman, wrote a book about all forms of casting and related work he had done and how to do them. It is important as one of the earliest comprehensive works left by an actual artisan, a real practitioner, rather than by some over educated alchemist. Biringuccio doesn’t like alchemists, and spends many pages excoriating them.
Anyway, amongst much important and useful information, he mentions a number of different recipes for making moulds for casting copper alloys into.

From these recipes, I have tried clay and horse dung, clay, dung and sand, clay and chopped wool, and they all worked very well, withstanding the heat when being baked, and the molten metal when it is poured into them. But he also writes, on page 219 of the Dover paperback, “There are some others who mix various earths with it; others, wash ashes; and some, coarse sand.”

So I wanted to try wash ashes and clay, although it has to be said that Biringuccio is not very keen on the use of pure clay, preferring clay that has organic matter in it. Nevertheless I thought it would be interesting to see what pure clay and ash does. Fortunately I had a great deal of ash that had been washed already, in order to get the lye from it. What was left is a mix of calcium carbonate, various metal oxides, tiny bits of unburnt charcoal and anything else that isn’t water soluble, which means no potassium or sodium oxides. Mixing it all together was quite hard, but I ended up with a surprisingly smooth paste. When squashed flat, it held its shape well, the only problem was it’s lack of stickiness, so it was a little hard glueing the two halves together.

I pressed a buckle into it to see how well it took the impression, the answer is that it does hold it rather well. I hoped it would leave a nice smooth surface on the finished product, since I want to try and match the very well finished medieval artefacts.
In the end though, the casting didn’t work properly because the metal wasn’t hot enough to flow well. The small bit of metal that did get inside was at least quite smooth, but that was probably less to do with the mould material and more the fact it was so small it had frozen without touching the walls.
So here it is, note the layers of white stuff that is ash which isn’t properly mixed with the clay. The inside is grey, due to lack of oxygen, and the outside is red, due to the iron in the clay. The larger piece shows the inside of the mould, with the curve of buckle at the top left; the piece at the top right is a through section of it.
ash and clay mould for casting

It worked, and I think in the future I’ll try using more organic rich clay and see what sort of result that gives. In fact I need to experiment with the sort of clay you find in rivers anyway, fortunately there’s a good source not far from me. Stay tuned for more secrets of the medieval foundryman.