October 19, 2016

In the last couple of post we covered making a brushable silicone glove mold with a rigid plaster mother mold. Now that we’ve created the mold, we can cast a piece. For the skull mold I’m going to show you how to use bronze powder, urethane casting resin and brown urethane colorant to create cold cast bronze piece, that is a skull that is made or urethane resin that looks like it is made of bronze.

Materials needed

Your work area

  • Clean, level work surface, covered in paper or plastic for easier clean-up
  • All materials comfortably within reach

Project steps

  • Tape the edges of the mother mold together


  • Once the edges are all taped together, place the binder clips over the tape to further secure the mother mold. Fold the silver part of the clips so they are resting against the mother mold, not sticking straight up.

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  • Balance the mold on some upside down mixing containers, cups or any thing else that will hold and stabilize the mold while casting.


  • Apply a light coating of urethane mold release to the inside of the mold. This will not only help the casting release from the mold easily, it will also help the bronze powder stick to the mold.


  • Pour the bronze powder into the mold. Pour in more than you think you will need to coat the mold. Then pick the mold up and roll it around to coat the inside with the bronze powder. You may need to place your hand over the opening of the mold to ensure that the powder gets all over the interior of the mold.

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  • Pour any excess bronze powder back into the bronze powder container.
  • Place mold back onto cups/containers
  • Measure out equal parts, by volume, of the urethane casting resin. So for this piece we did approximately 12 fluid ounces of the “A” side and 12 fluid ounces of the “B” side. Urethane is moisture sensitive before it is cured, so you don’t want to use it on a rainy day or a day when the humidity is over 50%. Because wood and paper products can also contain moisture, that’s why we use plastic containers and utensils for mixing and measuring urethanes.

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  • Add the brown urethane colorant to the “B” side and mix thoroughly. The purpose of the colorant is to match the color of the bronze powder. This way, if the powder missed a spot or we rub too hard with the steel wool, there won’t be an obvious white spot showing through. Instead it will blend into the color rest of the piece.
  • Mix the “A” and “B” sides together, taking care scrape the sides and bottom of the mixing container. Mix vigorously for about 90 seconds.
  •  Grab the plastic spoon and position it over the mold opening upside down, that is with the rounded (convex) side facing up.
  • Carefully and slowly pour the casting resin over the backside of the spoon. This will break up the pour, so to speak, avoiding a heavy direct pour that could disrupt the coating of bronze powder. Pour the urethane casting resin to the top of the mold.


  • This resin has a demold time of 30 minutes. However it is still pliable when when removed from the mold at 30 minutes. Since I will be polishing the piece with steel wool, I let this piece sit in the mold over night so it will be fully cured and hard when I pull it from the mold.
  • When it is time to demold remove the clips and tape and the mother mold. Grab the mold by the base and pull upward carefully and slowly to remove it from the piece. Don’t worry if this pulls the mold inside out, it will pop back into shape when you turn it right side out.


  • Now we have a lovely brown skull!
  • To bring out the shine of the bronze powder, rub with the steel wool, in a circular pattern, not too hard. I used a course #1, but i usually prefer finer steel wool. The course is just what I had in the shop. You want to rub firmly, to bring out the shine, but not so hard as to completely rub the layer of bronze powder off.
  • Now we have a skull that looks to be made of bronze, when in fact it is a lighter weight urethane resin.

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That’s the cold cast bronze process using urethane casting resin. There are other types of metal powders available, but the process is the same for using them.Next post we’ll be doing a two part pourable silicone block mold of a pumpkin.


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