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Casting from a Plaster Mold

After the mold is cleaned and left to dry for a few days to a week, it is time to make your first cast.

This mold is held together by inner tube rubber, cut into pieces
This mold is held together by inner tube rubber, cut into pieces
Pour the liquid slip carefully into the mold up to the top.
Pour the liquid slip carefully into the mold up to the top.

You should put on a timer for about 15 minutes at first, keeping a look out for the level of the slip. As the water is absorbed into the plaster, the level of slip will go down.

Top up the slip as the moisture is absorbed
Top up the slip as the moisture is absorbed

You will want to experiment on the timing, but for this casting, 30 minutes is about right to cast a piece.  However, this could change depending on how many pieces you cast and how damp the mold gets.

When you feel that the casting is thick enough, pour half of the slip back into the container and then swirl the remainder of the slip around the casting for a few minutes before pouring it out.

Place the mold onto an elevated prop to allow the remainder of the slip to drip out.

The mold will now take an additional time to dry enough to remove the piece from the mold.  In this case it is 30 minutes of drying time.

Clean the spout hole, being careful to not let the trimmed pieces fall into the sculpture.
Clean the spout hole, being careful to not let the trimmed pieces fall into the sculpture.

When dry and ready to remove the piece, clean the spout hole.

Take the bindings off from the cast and carefully pull the two pieces apart.

One side of the cast has been removed, if the piece is dry enough, carefully pull the sculpture out from the second part of the mold
One side of the cast has been removed, if the piece is dry enough, carefully pull the sculpture out from the second part of the mold

In this sculpture, the third piece can now be removed as well.

As the pour spout is located at the bottom of the sculpture, we now have to close that opening.

Pour a bit of liquid slip onto a flat plaster bat and place the sculpture on top.  This will quickly adhere to the sculpture and create a smooth bottom.

Trim the opening.
Trim the opening.

Your sculpture will have seams that can now be cleaned up and you can cut out the opening to the vase, again being careful not to allow the piece to fall into the sculpture.

bisque-fired
ten castings coming out of the bisque kiln.

And there you have it, the end result, bisque fired and ready to glaze.

Making a 3 Part Mould

Here are some images and basic instructions on how to tackle a 3 part plaster mold for doing slip cast pottery.

If you are interested in taking a course in mold making, you can sign up for one that Fusion is offering as part of the Fusion Conference in May.  Angelo DiPetta will be teaching it.  See Fusion’s website for more information.  This mold was done with the instruction of Angelo.

original
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.
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.
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. Use cm and the calculation is length x width by the height of the highest point divided by ?
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.

cottle-boards
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.

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.
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.

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.
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.
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.

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 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 you see in this image).

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 seem between the two pieces.
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.

If you have a rubber mallet, tap the mould 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.

Here are the three parts now opened up.
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.

Next: casting a piece, which you can see here.

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