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Casting in Bronze

standing_figureCasting 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.

  1. 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.clay_gymnast
  2. 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.mold
  3. 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. roughwax
  4. 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”.
  5. 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.
  6. 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.wax_with_channels
  7. 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. makingslop
    • 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.
  8. 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.meltingwaxoven
  9. 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 blockpouringmetal
  10. When the metal cools a bit, the brick/plaster is bashed away with a hammer leaving something looking like thisbronze_tubes
  11. Now the pouring and gas-release tubes are sawn off (the metal can be re-used).cut_off_bear
  12. 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.
  13. The sculpture comes out this colour.before_patina
  14. 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:

  1. 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.
  2. This centre is then filled with liquid plaster which sets.
  3. 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.
  4. 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.


Casting portrait busts – Making a two-part mold

finished portrait busts in damp clay

finished portrait busts in damp clay

The double portrait busts are finished in clay and instead of firing them in the kiln, I’d like to cast them.

They are not dried out, but still softish damp clay. Damp clay won’t stick to plaster or silicone.

When I was satisfied with the bust modeled in clay, I started to make a mold, this is a bit like a jelly mold and will allow me to make more than one reproduction of the busts.

The fine detail of the portrait will be copied using a flexible silicone mold. Silicone is a sort of soft synthetic rubber. This forms a thin layer that can curve under slightly, going into the hair, ears, eyes and nostrils. When the silicone layer is removed it stretches so it can be taken off without damaging the cast.

To hold this floppy silicone layer in place there is a jacket of hard plaster, often called a mother mold. The plaster mother mold has spaces that fit special bumps on the silicone, these are called register keys and ensure that the silicone layer is positioned correctly inside the mother mold.

I’ve used this same process when making a relief of a bull inspired by a Picasso lithograph.

Toro relief in plaster

Toro relief in plaster

For a bust, the mold is made in two halves, producing a front and back, each with a flexible silicon layer and a rigid plaster outer layer, these two halves will fit together.

Day 1

  1. The first step is to position the bust, face up, on a steady table, making sure the back of the head and neck are properly supported.
  2. Then you need to build up a clay edge all around the bust, build this up in clay of a different colour so you can tell the difference and don’t accidentally start scraping at your original bust (I used grey clay for the edge on a dark brown bust). Another tip is to lay the bust on cling film so that a thin layer of cling film (saran wrap) is between the two clay colours, let a half centimeter of this lay on top of the clay edge.
  3. Choose a line, going from the arm, up the outside of the shoulder, up the neck and over the head to the other side. It’s best to choose a route that doesn’t have too much detail, the line doesn’t have to be totally straight. This line will be where the two halves meet. The clay edge needs to be quite wide because it’ll have various register keys and the sides of the plaster mother mold on it. Make the edge very very smooth on top and bring the edge of it right up to the bust.
  4. Once it’s ready, scoop some rounded register keys into it. Some people make a smooth furrow all the way around the edge, I just scooped several 4cm long spaces out. These will let the two silicone halves of the mold position exactly with each other and with their respective mother molds.
  5. Now make up some thin silicone and dribble it in a fine stream over the face, neck and chest. It will continue to flow for quite a while and end up pooling on the edge and in the register key spaces. The silicone may also dribble onto your table or floor, don’t try to wipe it while it’s liquid, just wait until it has set and then peel it off.
  6. Get a block of clay, flatten the top and create about 20 conical spaces in the clay, fill these with some of the silicone and leave them to set overnight. These will be little conical register keys made of silicone, they will position the silicone mold inside the plaster mother mold.
Bust with silicone layer

Here you can see the grey clay edge, the brown clay of the original bust and a thin layer of silicone over all of it, pooling in the register keys (indentations in the grey clay edge).

Day 2

The next day you can trowel over a good layer of thickened silicone making sure to fill in any overhangs so that the rigid plaster mother mold can slide on smoothly without catching.

Position the conical register keys into the silicone around the outside of the bust (and on it too) and let it set.

The wet silicone is very sticky but can be patted smooth with a sponge covered in cloth that has been dipped into a strong solution of washing up liquid and water. Make the silicone layer as smooth as possible.

Day 3

First cut some register keys into the outer edge of the clay edging. These will be to position the two plaster mother molds onto each other. Use a sharp knife to slice through the silicone and down about a centimeter into the clay. Make these register keys very smooth with bevelled edges.

Register keys

Here you can see two types of register keys: At the top are two dips for aligning the silicon back and front layers to each other and at the bottom of the picture is the cut-away for positioning the mother mold halves on each other.

Now make up some plaster and smooth it over the top of the silicone layer. The layer of plaster should be 2-3 cm thick and should have some hessian strengthening in it. Make it nice and thick at the edges and be sure to fill those freshly cut register keys.

Make sure the plaster is nice and smooth on the outside otherwise you’ll scratch yourself when handling the mold once it’s dry.

As always with plaster, have a bowl of water for hand and tool washing, dump this on the garden when you’re finished, never into the sink or toilet.

Voila, one side done!

Day 4

Turn the whole thing over to expose the back of the head. Peel off the clay edging. Wipe everything clean of the clay. 

Smear a thin even layer of vaseline over the exposed silicone surface and the exposed plaster in the plaster register keys which are now protruding. The silicone mustn’t stick to the next layer of silicone, the plaster mustn’t stick to the next layer of plaster.

Now go back to step 5 on day one and repeat the entire process.

Back of the silicone mold before making the plaster mother mold

The back half of the mold is ready for its plaster jacket, the mother mold. Note the different types of register keys.

Here you see the back half once it’s ready for the mother mold. The silicone is cleared from the plaster register keys at the edges, the conical silicone register keys are set into the silicone around the mold to position it into the mother mold and the long humps are along the divide to match the two silicone halves together, the surface of the thickened silicone has been patted down.

Once the back of the mold, including the mother mold is set, it’s time to scrape away at the sides until you can see the divide all the way around, now gently but firmly prise the halves of the plaster mother mold apart. You’ll have a mother mold on one side and all the rest on the other. Slide your fingers between the two silicone layers all the way around the edge then gently peel back the silicone.

The clay bust can be discarded, the mold is ready for casting.