Moulding and Casting
Body Part Life Mould and Twice Size Replica Modelled in Clay
In pairs, we alternated to take a detailed impression of a section of the head using our partner as a model. From these moulds, we then cast them in plaster to study during the twice life size clay replica modelling. The process is as follows:
Equipment List
For the moulding and casting process, we used the following equipment, with reference to the Tiranti brand pricing list (excluding VAT):
- Alginate Normal Set, 500g Bag, £7.41
- Calibrated Beaker 50ml x2, £0.27
- Calibrated Beaker 225ml, £0.36
- Mixing Stick (Box of 100), £2.30
- Red Poly Mixing Bowl, £0.74
- Plaster Bandage 15cm x 2.7m, £2.13
- Crystacal Fast Setting Dental Plaster
- Laminating Brush, £0.46
- Cotton Scrim (Roll), £7
- Grey Clay 25kg, £9
- Wooden Boards
- Netting Staples
- Cling Film
- Petroleum Jelly
Moulding Negative
I chose to cast an ear, and as such, the model needs preparing appropriately. Make sure hair is tied out of the way- cling film is used to ensure that hair is protected from the Alginate, Petroleum jelly can be used to coat any hair that cannot be covered by the cling film. Make sure to plug the ear hole using cotton wool to protect the delicate inner ear.
Normal Set Alginate sets within 3 minutes, so bare that in mind when mixing- take 1 minute mixing and use the remaining 2 minutes to apply the alginate. Using the 25ml calibrated beakers, one for the dry alginate, and one for the water (to prevent contamination), use a ratio of 1:1, and mix in the 225ml calibrated beaker with a mixing stick. Apply the Alginate mixture over the desired area with the mixing stick, making sure to fill all crevices- be careful not to poke your models eye out with over-zealous application, should you be taking an impression of the eye.
Take the opportunity, whilst the Alginate is setting, to clean equipment and prepare a bowl of water and small strips of plaster bandage. Once the Alginate has set sufficiently, you will need to create a plaster bandage support for your mould, so that it keeps it's shape and does not distort. Run the Plaster bandage through the water and apply to the Alginate mould (still applied to the model), smoothing out the plaster to cover the holes in the mesh. Two layers should suffice. The plaster will heat up as the chemical process it undergoes is exothermic, when the plaster bandage is dry and cool, it has fully set, ready to remove both the Alginate and the plaster cast together.
Here is what the ear mould looks like in negative:
We will next fill the Alginate mould with a "paint in" coat using fast setting dental plaster, mixed to the consistency of double cram in the red poly mixing bowl. This is applied with a laminating brush. Again, the reaction is exothermic, so using warm water will speed up the reaction, making the plaster set quicker. Wash out brush thoroughly before the plaster sets onto it, and avoid getting plaster in the sink.
The pain in layer should cover the Alginate mould completely. Use the brush to spread the plaster evenly from collecting in concaves of the structure to coat the Alginate in an even layer.
Whilst this layer sets, mix up more of the plaster as required, and cut the cotton scrim into strips. You may also measure a length of wire to cut and shape into a bracket for the positive cast. Dip the scrip into the plaster mix and place over the dry pain in layer.
Cover the surface in at least two layer, taking care to cover the metal bracket with a layer of scrim to hold it in place once dry. 24 hours is a preferable time to leave this to dry before removing the cast from the mould. Wrap it in cling film to cover the Alginate (as it will shrink and may distort the cast), leaving the plaster uncovered to let it "breath".
Next, carefully tease the Alginate mould away from the plaster cast- don't worry about damaging the Alginate (unless you intend to use it again). When you have removed the cast, you might have to do some scratching out of blemishes caused by air bubbles, take care not to damage your cast.
You will need a small modelling board for your cast one twice that size for the clay replica. Grid up the boards, 4cm squares for the small, 8cm squares for the large. Cover both in cling film. The small board will require netting staples hammering into the centre (2-3). We will mount the plaster cast onto the small board using casting plaster mixed to a slightly thicker consistency (resembling whipped cream) to fill the space between the cast and the board. Dampen the plaster cast so the newly applied plaster will fix to it better. The staples are used so the plaster has something to set around to stay secured.
When this is dry, faintly trace the grid over the contours of the cast with a sharp pencil, and using a felt tip marker, trace the outline of the shape of the cast onto the larger board as a starting point for modelling. You will also need to take note of the highest point of the cast using callipers (not with a Verneir Calliper).
Twice life size clay replica will be worked on today.
