Slip Casting and Plaster Mouldings

Simplest Methods of Producing Pottery and Ceramics

Having covered wheel-throwing techniques in building pottery and ceramics, its now time to look at making pottery and ceramic using slip casting and plaster mouldings. Slip casting and plaster moulding may be considered as more suitable to large scale production but there are still very many possibilities for the individual potter or ceramist. Slip casting and plaster molding are the simplest methods of producing pottery and ceramics as they allow you to make several exact replicas of an original model with very little effort.

Slip Casting and Plaster Mouldings

Slip casting allows you to make pieces with good finish that are hollow – and hollow pieces is what a ceramist is interested in because they hardly crack on firing due to expansion differential, they also need less energy to fire, and are light to handle. In slip casting, a Plaster of Paris mould is made using the original model (figure) and liquid clay casting slip is poured into the plaster of Paris mould. The plaster will draw water out of clay slip and a skin-like layer of clay will form on the inner surface of plaster mold reproducing the exact replica of your model. As more water is drawn by plaster the level of your slip sink and you will need to add more slip to top it. When you are satisfied by the thickness of your piece, the excess slip is poured out. The plaster will continue to absorb more water from the clay wall and the walls will shrink and harden parting way with the plaster. It is at this time that the clay piece is removed from plaster and left to dry to leather hand. At leather hand stage, the seams are cleaned up, the pieces is decorated and glazed before firing.

Plaster of Paris

Different brand names of plaster of Paris are manufactured for different uses and its necessary you read manufacturers’ literature on their plaster-water mixing ratio. Generally, a plaster water ratio of 3 to 2 is used. This means that 3 kilograms of plaster powder is mixed with 2 litres of water.  Plaster will set hard 20 minutes after mixing with water. The mixture of plaster and water is stirred by hand with wide arm motion in circles. Setting time for water-plaster mixture can be delayed for hours by addition of boric acid, acetic acid, or vinegar to the plaster mixture.

One Piece Mold

Choose your model – it can be made of anything from clay, wheel-turned clay, wooden figurines, found objects, plastic objects, metal objects, etc. The number of undercuts in your model will determine the number of pieces your mold will have. For a one piece mold, the model is a simple profile like a glass or a cup without handle. The mold should be able to lift off the model.

Slip Casting Steps

One Piece Mold
One Piece Mold

Casting Slip

To mix clay with water to make the right slip, we need to use a lot of water which would result in moulds with weak clay shell. To prevent using a lot of water, a deflocculant such as soda ash, sodium silicate is used in the ratio of 0.005% to clay body. By adding a deflocculant, you will then need only 40% of water weight to clay weight – 4 litres of water to mix10 kilograms of clay into casting slip. It is this casting slip that is poured in to the plaster mold. The plaster will draw water out of clay slip and a skin-like layer of clay will form on the inner surface of plaster mold reproducing the exact replica of your model.

Slip Casting Steps

  1. Build coddle to fit your model. You can use wood, metal, or carton to contain your plaster. I find it easier to cut cartons and to join them using masking tape to make a box.
  2. Apply Vaseline or soap on the surfaces of box to prevent plaster sticking on the box. Also apply Vaseline or soap on the outside walls of your model.
  3. Put your model (glass) upside down as shown in the figure above.
  4. Pour the mixture of water and plaster on the box until the model (glass) is fully covered. Let the plaster harden for 20 minutes.
  5. Remove your plaster mold from the carton box, and then remove your model from the plaster mold.
  6. Let the plaster mold dry for at least a day.
  7. To make a replica of your model, pour the clay casting slip in your plaster mold. Keep on adding more slip as water is absorbed by plaster mold until you get the right thickness of your figure. Pour out the excess slip and wait for 20 minutes to let the walls of your figure shrink and part way with the plaster mold.
  8. Remove your figure from the plaster mold and let it dry to leather hard before working it out with a knife/tool to add any finer details that you may want.

Two-Piece Mold

A mold of two pieces or three pieces or even four pieces is possible if one or more undercuts exists on your model. The fewer the pieces of molds the form of your model needs, the easier it is. Multi-piece molds are known for chipping easily. Below is a figure showing a form that will need two-piece mold because the wider part AB is at the middle. If that wider part was at the top-end or the bottom-end, we would use a one-piece mold.

A model for two-piece mold casting
A model for two-piece mold casting

Steps for Slip Casting Two-Piece Mold:

  1. Build coddle to fit your model. You can use wood, metal, or carton to contain your plaster. I find it easier to cut cartons and to join them using masking tape to make a box.
  2. Apply Vaseline or soap on the surfaces of box to prevent plaster sticking on the box. Also apply Vaseline or soap on the outside walls of your model.
  3. Cover your model with clay from AB downward and put it in the carton box such that when you pour plaster, the plaster will only cover the upper part of AB as shown in diagram below.

Making a plaster mold. The lower half is covered by clay and a mixture of water and plaster is poured on the upper half.
Making a plaster mold. The lower half is covered by clay and a mixture of water and plaster is poured on the upper half.
  1. As soon as the plaster has hardened, turn the box upside down and remove all the clay (below AB) attached to your model.
  2. On the plaster brim/walls where plaster touched the clay, cut a few hole as indentations marks for fit-locking for the second half of plaster mold. Apply soap or Vaseline on plaster walls where plaster was in contact with clay and also on the indentations marks that you have cut.
  3. Pour plaster on the second half (the part that was initially covered by clay) and let it harden for at least 20 minutes.
  4. Remove the two plaster molds from the box and separate them using a chisel and hammer at the same time remove your model.
  5. Before the molds harden too hard, lock them together and make a hole (2” in diameter) using a knife that will be used for pouring in the slip
  6. Allow the plaster molds to dry at least for a day
  7. After the molds are dry, lock them together and reinforce them with a string or rubber band before pouring in the slip.
  8. Keep on topping it with slip as water is absorbed by plaster until you get the right thickness of your figure. Then pour out excess slip and allow the cast to stiffen.
  9. After drying for 20 minutes, separate the molds with a chisel and hammer, and remove your figure to dry to leather hard before working on it to smoothen the edges that may not have formed well.

You can do the same with three-piece molds, with the 2nd and 3rd thirds first being covered with clay as you make the 1st plaster mold, and so on.

Tutorial Videos on Slip Casting Using Plaster Mold and Clay

Below are three lengthy tutorial videos that should take you through the entire process of slip casting using plaster and clay. These really are good videos and the producer ought to have charged us for them but luckily he is providing them free. Enjoy the video on slip casting whilst they remain free.

Tutorial Video Part 1 on Slip Casting

Tutorial Video Part II on Slip Casting

Tutorial Video Part III on Slip Casting

The next article is on how to glaze pottery and ceramics, and glaze recipes


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Comments 4 comments

saleheensblog profile image

saleheensblog 6 years ago from Dhaka,Bangladesh

great information, thnx for the useful tips


Hello, hello, profile image

Hello, hello, 6 years ago from London, UK

This is wonderful art and thank you for describing how to do it, so explicitely.


Jin 5 years ago

Cheers! This helps me a lot.


Arachnea profile image

Arachnea 2 years ago from Texas USA

I've been looking for an article like this for some of the beads I've been thinking to design. Great hub.

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