Casting in bronze is a complicated and labour-intensive process and therefore quite expensive. At the end you get something that will last for a very long time and it looks and feels good. The entire casting process described here currently costs around €1000,- for a figurine this size. That doesn’t include any payment to the artist. A large edition size, or having the bronzes manufactured commercially can bring down the price somewhat.
The process is best done with a specialist art foundry, such as Kunstgiesserei in Munich, Germany.
The foundry workers will do all of these steps for you but they are happy for artists to take a more active part in the process. I’ve used pictures of different pieces so that I could show you all the steps in the process. The nude figures shown here are mine.
First you bring a sculpture to be cast. This is often in still-damp clay which, in the case of a figure like this, has a skeleton of strong wire “armature” and may need additional external support.
They will make a mold out of silicon with a plaster jacket in very much the same way as I described in my blog post about making a two-part mold. The red dots shown in this picture are used to position the two halves of the mold together.
In this mold, they will cast a copy of the sculpture in wax. It comes out with very rough edges, seam lines and a lot of imperfections like the wax between the hands and holes in the shoulder blade than you can see here.
Then, with the help of a heated metal tool, the wax model is corrected, you need a wax copy for each copy you plan to make in bronze. I did this myself and took almost an entire working week to correct my three small wax models (I don’t think they’d give me a job at the foundry, at least not one with an hourly wage). This stage is called “wax chasing”.
At this point many artists sign and number each item in an unobtrusive position. For example, 3/9 is the third of a series made from the same mold where the artist is making a limited edition of 9 sculptures. These bronzes don’t need to be cast at the same time but the artist commits to only casting this number, maybe with an additional unnumbered artist’s proof.
Once the wax model is as you want it, pouring tubes called “sprues” will be added. These are solid wax rods. At the top of the construction (which may well be the bottom of your sculpture) they will add a funnel of wax.
At this point, foundries usually start a slow process of repeatedly dipping the wax figures into a ceramic slip and then covering them in very fine sand, letting them dry between each dipping until they have a 1cm thick crust or “shell” all around the wax. This foundry has another method:
They make up a mixture of crushed brick and plaster and pour this into boxes.
Each wax model is placed carefully inside one of these boxes and totally immersed into the rough plaster with the funnel uppermost.
When the block starts to set, the funnel area is scraped free.
This block (or shell if the other process is used) is positioned into an oven (upside down) where it will be slowly heated for four days. This makes the wax melt completely leaving a gap where the wax was.
The still-hot blocks are set out and molten bronze (made of copper and tin) is carefully poured into the funnels left in each heated block
When the metal cools a bit, the brick/plaster is bashed away with a hammer leaving something looking like this
Now the pouring and gas-release tubes are sawn off (the metal can be re-used).
Each model is sand-blasted to get rid of the debris, all the imperfections are corrected and the surface where the tubes ave been attached are re-created. This process is called metal chasing.
The sculpture comes out this colour.
The model is now heated with a blow torch and painted with oxides and other chemicals (depending on which colour finish you want). This process is called patination.
There are lots of patinas to choose from: below you can see a glossy black, a coppery green and a brown. Once the patina is ready and the sculpture is cooled, you can mount the figure.
These nudes were made in a single piece of solid bronze but larger or thicker pieces are made hollow or in several pieces which are later welded together.
Here are the additional steps that are needed to create a hollow bronze:
At the stage of casting the wax inside the silicon mold, repeatedly swish it around and tip it out again, leaving a hollow object with an empty centre like an easter egg.
This centre is then filled with liquid plaster which sets.
Once the wax melts there’s nothing to stop the solid plaster centre just falling down inside against the outside wall. So it is held in position by hammering nails into the whole thing, through the wax and into the plaster centre. This is done after spruing. The heads of these nails stick out and are set within the supporting plaster block.
The plaster inside the form will also send out gases as it burns up in the smelting process, so you need vents which are also added to the wax form.