Shellac is a material that is used to cover the surface of plaster moulds to make sure nothing can get in there. Plaster that has set has lots of little holes in and when covered with shellac, it stops things passing through. You must make sure that there is no suction before applying the shellac so that the plaster will come out of the mould easily once it has set. Shellac that is used in workshops is made by using methylated spirits to dissolve flakes of shellac. You must make sure that you use the right amount of spirit for the amount of flakes you are dissolving. Usually, 3 coats are used but more might be needed for moulds being used in cement casting etc.
Thick shellac is used for dry plaster mouldings and is made by using minimal amounts of spirit to dissolve the shellac flakes. By doing this, you do not need to make a key and the plaster surface does not need to be soaked.
Oils and greases make a thin divide between the model and mould, and the mould and cast. Plasterers grease can easily be made by heating tallow on a hot plate in a container until it has completely melted and then adding some machine oil. The amount of oil you add will depend on the temperature you want to use the grease at.
Oils that are used without being mixed with anything such as colza and rapeseed are used on special types of moulds.
Size - most glues slow down the time it takes for plaster to set when they are dissolved in hot water, if they are put in the gauging water.
Once the glue has been melted with hot water, lime putty should be added to make sure that it doesn't go jelly-like once it has cooled down.
Gelatine is a better type of glue that is made from collagen that comes from the bones, skin or cartilage of cattle.
Cake gelatine is brittle and when it is being used for moulds that are flexible, the cake gelatine needs to be flexible so it is soaked in water. Any extra water can then be removed and the gelatine needs to be heated. Once it has completely melted, carbolic acid should be added to get rid of any bacteria/living organisms. If you do not get rid of the bacteria, it can grow and cause mould to grow on the surface of the cooled gelatine which would cause problems.
Alum is used to make plaster set faster. It can also be used to harden the surface of gelatine moulds. Alum crystals which are aluminium sulphate need to be put in hot water so they can dissolve. When this is done, it should be cooled and a little bit can be added to the gauging water which is where it will be mixed with the plaster to make it set quicker. The quick set is helpful with gelatine moulds when the weather is hot because if it is left in the mould too long, it can be damaged. Crushed alum will dissolve if it placed straight into the gauging water but it will take longer.
PVC (polyvinyl chloride) can be used instead of gelatine and it has similar properties to rubber. PVC is flexible and is thermoplastic.
PVC is better than gelatine because it is stronger, it does not need oil or grease to be added to it. PVC can be used for a long time and you do not need to worry about it shrinking. Also, PVC does not get affected by heat, damp etc.
PVC does have a few disadvantages when compared to gelatine. It takes a lot of heat for the PVC to melt so special heating equipment is needed. These high temperatures would ruin shellac models so special treatment is needed to stop this from happening. Because of the temperature that is needed to make the PVC melt, you need to be more careful and too much of the fumes can be dangerous.
Moulding wax is a mixture of beeswax and resin. A container must be melted on a hot plate making sure that the wax does not boil. The resin makes the wax stiff when it is cool. These moulds are used when there is lots of sharp detail.
Scrim
Sheet iron and zinc are used to make the outline of running moulds. Zinc is easy to cut and file because it is a metal that is quite soft. Plasterers who need to keep running moulds for quite a long time prefer to use this because it does not rust. Sheet iron is cheaper than zinc so is used more often.
French chalk is a powder similar to talc that is used when moulding and casting as a dry lubricant.
Clay can be used in model making and casting bench work. It is stored in air tight containers because it must stay flexible. Sometimes, small amounts of clay are placed in a polythene bag that can close once it has been wrapped inside damp cloths.
Fibreglass reinforced plastics