This post is by way of an introduction, giving some background for what you will see over the coming year.

Now, for the question.
Remember, the molten bronze is at around 1200°C.
That’s as hot as a blowtorch. So what can contain the heat of a blowtorch? Not a lot, most metals will melt, pretty much everything you use in your kitchen would break, shatter, blow up or just melt.
Fortunately it turns out that baked clay and sand are good against heat. Sand for instance, silica, silicon dioxide, usually needs over 1600°C to melt.
However if you mix up clay, sand and dung or other organic matter, you get something like this:
buckle in mould one side of
There are many recipes in the 16th century technical literature.
Birringuccio recommends the use of cloth clippings with the clay and Cellini adds the refinement of letting the cloth clippings and clay sort of rot together over time, rather than using them fresh. Biringuccio also mentions the use of clays with washed ashes, coarse sands, or add rust or finely ground iron scale, and others use dry dung of a horse, donkey or mule, even cow dung or with finely cut straw.
Or to summarise, the precise organic substance doesn’t matter and can be varied as per the clay you use and what you are making.
I use horse dung, clay and sand, mixed in fairly random proportions, until the mould is strong, stable and capable of holding together when dry. If there is too little clay it won’t. If there’s too much organic matter, it won’t hold shape properly, and if too much sand it won’t either.
Making good moulds depends very much on touch, i.e. how the substance feels when you squeeze it into or out of shape. With enough experience you could make the right mixture up rapidly and consistently to ensure full coverage of the mould pattern with the same consistency of clay.

After firing they look like this:
two moulds for belt decorations feb 2013
Note the redness, due to iron oxide in the clay, whereas inside the moulds everything is black, as you will see later. The outside has been exposed to oxygen, the inside hasn’t.

Of course medieval moulds weren’t fired to ceramic sorts of temperatures, those around 800°C when clay turns into ceramics like mugs and plates. Instead they would be fired high enough to burn out the organic matter, leaving lots of holes for gasses to escape and allow a little more give in the mould. In fact I wonder how much of the organics was actually burnt out in use; we can’t tell easily from the surviving objects. It is also clear from my own examination of mould material that a lot were very high in sand, a mix of sand and clay with some chaff or similar chopped organics. This would be more suitable for larger objects, since it is a rather solid mixture but without a very smooth surface.

There’s potentially a lot of work to be done on what surviving mould material was made of, the size of sand and gravel used, the organic component and why it all varied, but it is difficult to analyse and of less pressing concern than what alloys were used. There is a massive backlog of objects needing analysis using modern methods so we know better which copper alloys were used and in what circumstances (there’s even a Shire book which makes the old and erroneous claim that mortars were often made out of bell bronze, which was proven wrong nearly 20 years ago), let alone the difficult and perhaps impossible quest to work out all the variations of their making.

Of course I would argue that we can’t completely understand the history of objects unless we can recreate them, which is what I am doing here. We can often find evidence for why something was made, what it was made from, who made it for whom, but there are finer details of technology which come out most clearly when you explore the making of objects, which will hopefully become clear over the posts I make this year.