I recently tried casting in bronze using a process called lost wax casting, we were using ceramic shell moulds. Making the moulds is quite a long process, especially compared with other methods, however the moulds are great at capturing intricate detail, you have more freedom to work in a three dimensional form, and the cast metal is really clean, so its really great! Before you can start making the mould you need to put the wax item you want to cast onto a tree. By this I mean a wax tree so you end up with a mould of multiple items all branching off from a main funnel where the molten metal can be poured. Sadly I forgot to snap a picture of just the wax trees so instead, here is a picture of the trees with just their first layer of shell on so you can see what I mean.
Off shoots are needed to allow excess metal and air to escape, these are the bits that look a bit like drinking straws sticking out of the medal.
To make the mould, the wax tree is first dipped into slurry then coated with a fine ceramic dust and left to dry. This process is then repeated using a medium ceramic powder until you have built up around 7 or 8 layers with the final few being made up of a coarser ceramic crumb. With the mould complete the wax needs to be removed, this is done by firing the moulds in a kiln to melt/burn all of the wax out.
To remove the mould you need to first quench it in water to cool it down a bit, the sudden change in temperature helps to weaken and crack the mould. The rest of the mould is removed with a hammer and chisel, the first time I watched this being done was pretty worrying. I was half expecting to be handed a battered lump of metal, but the bronze is a lot stronger and more durable than I expected.
Next in line was sandblasting the last few stubborn bits of ceramic shell off.
The last thing left to do is to remove the sprews (‘tree branches’). The first time round me and most of the other jewellery students slaved away with a hacksaw and files to sort our medals out, which for most of us took the best part of a day. Then came medal attempt two, where I was shown the grinder, sand belt, and giant dremel which transformed the day long cleaning up process into a 20 minute task!