Here are some images and basic instructions on how to tackle a 3 part plaster mold for doing slip cast pottery. I created this way back in 2014 and posted to a guild that I belonged to, but I thought it would be good on my own site! My own original sculpture and work.
As for the spelling. Both are correct, sort of. It is mould in the UK and mold in the U.S. We usually default to the British spelling for many words, but I’m not sure in this case. So I’ve spelled it mould in the title of this article but mold throughout the post. I lived for 14 years in the UK, so I’m often thoroughly confused as how to spell anything or even what country I am living in.
Thank you to Angelo diPetta for his advice on how to cast this piece.
Here is the original sculpture. This is going to be a vase with an opening in the top for flowers (or other items) to go into. I’ve had a few rude comments about it, but when it’s full of flowers it’s a hand giving you a bouquet. Behave.
Here the model is being blocked off in order to protect the areas that won’t be included in the first half of the casting. The hand looks to be holding something, but that is the shape that will end up as the third piece to be cast.
Here the area is being measured to calculate the amount of plaster that is used.
Always measure using cm. L x W x H for a rectangular volume or Pi x r squared x H for a cylindrical volume – always use centimetres. This is the required plaster mix. Multiply the number by .6 in order to get amount of water required.
Example: 30cm x 20cm x 10cm = 6000 cubic cm or 6 litres of plaster mix, then 6000cc x .6 = 3600cc or 3.6 litres of water. You could also multiply 6 litres by .6 and get 3.6 litres. This works because 1cc = 1ml. Depending on how much the model displaces the plaster you will have little or more plaster remaining.
This is the cottle board all set up. It’s a bit large for this project but it does the trick.
To see an article explaining cottle boards, visit Ceramic Arts Daily. They have a video about how they work and how to make adjustable cottles. These ones are made of recycled plastic of some sort. But you can use wood as well with equal results. Just be sure to seal the sides with mold soap concentrate so your plaster doesn’t stick to the boards and to make them easier to clean afterwards.
The piece is set up and ready for the plaster. Ensure that there are no gaps in your cottle boards, that you have a pour spout. In this sculpture the pour spout will be from the bottom due to the shape of the sculpture. That is what the styrofoam cup is for.
In this particular sculpture, the part that will be cut out for the entrance to the vase is lower than the highest part of the piece (i.e. the inside of the fist of the hand), so it was decided to make the pour spout at the bottom. After the piece is cast, this can be filled in. The part that is coming out of the hand is for the third piece of the mould.
Mix your plaster and pour it carefully into the cottle. Let the plaster set up and then you can remove the boards, turn over the piece ready to do the second part of the mold.
This is the piece after the first part of the mold is done. Carefully take away the scrap clay that you used to block off the other side.
Here is the piece cleaned up and ready to cast the second half. You must seal the plaster with a resist so that the second half of the mold doesn’t stick to the first side. Or you will have one piece that never comes apart. Use mold soap concentrate.
I’ve used vaseline or Murphy’s oil soap to create a resist between the two pieces, but it is best to get mold soap from a sculpture or ceramic supplier. You paint a few coats on and let it dry and then clean it off so the surface is nice and smooth. You know you’ve done enough when it buffs up shiny and water beads on it when dropped. Otherwise you will have a lovely solid piece of plaster, but won’t be able to get it apart.
Be careful not to get any into the inside of the cast, as you need the clay to be absorbed in that part.
You can now carefully pour the plaster for the second half of the mold. You should always pour the plaster carefully away from the piece you are casting, not directly on the piece, as it looks like I’m doing here. Do as I say, not as I do.
There is no image of making the final piece of the mold, but keep the two pieces you have already cast together, and remove the block of clay that was blocking off the last section. Clean as you did for the first casting and make a deep score between the two pieces to create another register. Coat the plaster again with the mold soap and when ready, pour your plaster into the last section. Let it set.
Now you can separate the pieces. In this case plaster covered the first piece again, so some cleaning was needed to see the seam between the two pieces.
Clean up the edges of the mold. Clean the sides as well, as it will often help you see the seam between the parts you have cast.
If you have a rubber mallet, tap the mold all over – this helps loosen the pieces. Then tap the stiff blade of a knife into the seem, and pry – do this all around the seem line. Sometimes the suction of the clay makes the pulling apart a bit harder. Be careful though, you don’t want to ruin the cast.
Here are the three parts now opened up.
What you should do now is round the corners of the mold and clean it up. This way you don’t have jagged edges that can chip off, and it keeps it neater. Put the mold in a warm, dry area for a few days until it is no longer damp and cold. This mold took 4 days of sitting near a register.