Casting a bust in a two-part mold

WP_20150503_15_59_44_ProOK so now I have the mold ready it’s time to cast a bust in plaster of Paris.

When I first made a cast I was surprised at how time-consuming the whole process was. I’d imagined just sloshing in the wet plaster, letting it set, taking the hardened model out of the mold and then just pouring another one. Although those are the main steps, and that works fine for casting ornaments or rough objects, it turned out that the precision needed for an artwork translated into a very slow and fiddly process with days of repair work at the end shaving off imperfections and filling holes left by tiny air bubbles.

I realised that casting a life-sized bust in a two-part mold would be trickier than just an open-backed relief. I couldn’t find instructions for this on the Internet.

  • Do I put the halves of the mold together and fill it up from underneath?
  • Or do I cast two halves and then stick them together?

Well, I ended up needing to use both methods to get a finished product and this is what I’ve described below.

  1. Wash the silicone mold, making sure to remove all the traces of clay, and dry it.
  2. Smear a very very thin layer of vaseline all around where the plaster will touch. Make sure that you wipe it off again especially where details such as hairs should show.
  3. Make sure the inside of the mother mold is free of debris, and that the silicone mold sits snugly in the mother mold with all the register keys properly in their places.
  4. Make up a small batch of plaster, don’t try to make up all the plaster you need, it’s better to do it in small batches. (For more detailed plaster instructions go to plastermaster.)
    1. I’ve found that a plastic bag inside the bucket makes it easier to clean afterwards.
    2. Put cold water into your container.
    3. Now, with dry hands quickly sprinkle handfuls of dry plaster into the water. Keep doing this quickly until the handfuls you put in don’t sink quickly but form islands that stay on the surface and slowly darken with damp from the edges like here in the picture.bucket_plaster
    4. Now wait and watch. It should take a two or three minutes for the islands to completely darken with dampness. Don’t touch it during this time. Once the island is completely damp, put your hand gently into the bottom of the bucket and very gently wiggle your fingers from side to side to stir without incorporating air bubbles.
  5. Pour some into the the hollow of the head and gently make sure it reaches all the little corners and pockets (use a cheap disposable paintbrush or your fingers). You can tip and roll the mold half to help the plaster reach to the edges of the mold as well as into hair wisps, nostrils, ears and eyes.

    Air bubbles in the plaster cause holes in the finished cast that are difficult to repair.

    Avoid holes like these in the surface of your final cast by making sure there are no bubbles, particularly in the nooks and crannies of hair strands etc.

  6. Gently smooth the plaster up the sides of the head and over the neck onto the chest, a thin layer will stick each time and the plaster shell on the inside of the mold will slowly become thicker.
  7. Once the first thin layer is spread as evenly as possible and starting to set, make up another batch of plaster and repeat the process.
  8. Get some damp strips of hessian, dip them in the plaster and lay them inside the mold, making sure to avoid the edges or anywhere the hessian could touch the outside surface of the finished model.
  9. The bust will not be filled completely, but hollow. You are aiming for an even thickness of about 5cm. You will need several small batches of plaster for this. Subsequent batches can be put on faster because it will stick more easily to the plaster that is already starting to harden.
  10. Clean off the silicon edge and create a smooth slightly inward-sloping ledge all around the edge of your bust where the two halves will join. This wedge-shaped space will be where you glue the two halves together. Make sure the edge angles right up to touch the edge of the silicon. The plaster edge must be lower than the silicon it is next to.


    The edge angles slightly down from the edge of the silicon. The plaster layer is not thick enough yet and the edge needs to be neatened up very carefully.

  11. Once you have both sides of your head, each filled with a 5cm layer of plaster with a smooth clean angled edge all the way around, you can position the pieces very carefully together to make sure you have a very smooth neat fit without gaps or humps. Spend a lot of time making this edge right, it will save you time later.
  12. Now make up a small amount of plaster and make a soft sausage with it.
  13. Dampen the edges on both halves of the head.
  14. Lay the sausage all the way around the edge of the head and (get someone to help you) position the two halves together making a sandwich and squishing the sausage of plaster.sandwich
  15. If the plaster is already too solid, or the edges too porous, you won’t be able to squish the pieces together properly and you’ll end up with something like this where the only solution is to saw it in half and start again.
  16. Now, if you’ve been able to position the parts correctly, and press the two halves together, you can strap them together and prop them up inside a box.boxed_up
  17. Pour more wet plaster into the bust, tip and roll the molds so that the liquid plaster gets into and supports the seal between the two halves.
  18. Pour and spread the plaster to make a strong smooth standing surface for the bust.
  19. The resulting bust can be taken out very carefully after a few hours.Be careful when you peel off the silicon mould.
  20. Then it’s time to correct imperfections by scraping very carefully with a sharp knife, and by carefully filling the divide between the two halves so that you can no longer see where the line is, as well as filling any bubble holes.

I used very white plaster here, which gets whiter and whiter as it dries completely over then next few weeks. I may consider adding dye to it next time, rather than trying to stain it afterwards which is pretty tricky. The making of a cast in concrete may become a different blog post.




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