TYPES OF PLASTER
Fred Aldous Ltd. supply two types of plaster, Plaster of Paris and Stone Powder. PLASTER OF PARIS - is a standard plaster of medium strength and a fast setting time. STONE POWDER - produces tougher castings with hard surfaces and a slower setting time.
Both plasters can be used for casting figurines and plaques using rubber moulds and very fine detail will be produced.
PREPARING THE MOULD Make a support for the mould using strong cardboard. (A cardboard box will do) To cut out the exact shape of the base of the mould fold back the bottom and dip into water and then stand on the cardboard. The wet outline will give you a perfect cutting line. For larger moulds which need more support, make a papier mache or fibre glass casement to hold the body of the mould rigid when filled with plaster. To help eliminate air bubbles rinse the inside of the mould with water and a few drops of washing up liquid. Suspend the mould in the prepared cardboard support.
MIXING Ensure that all utensils are clean and free from any old plaster. The methods for mixing each plaster vary as follows:- STONE POWDER - The ratio of Stone Powder to water used should be100: 33 by weight. e.g. 3lb l3oz to 1 pint of water. Pour the water into a bowl and add about ¾ of the Stone Powder. Wait for about two minutes to allow the water to soak in. Sprinkle the rest of the Stone Powder on top and mix well. The consistency should be like thick cream but still pourable. The setting time is 20 - 30 minutes.
PLASTER OF PARIS -The proportions of plaster to water used should be 100: 60 by weight. e.g. 21b loz to 1 pint of water. Add the plaster to the water and mix well. The consistency will be thick and creamy. The initial setting time is 6 - 10 minutes. Always mix both plasters by hand; never use an electric mixer or any other mechanical method, as this will introduce air bubbles.
The ratios of plaster to water given here are recommended for maximum strength but can be altered as long as it is understood that the hardness of the finished product may be affected.
CASTING Pour the mixture slowly into the prepared mould allowing it to flow over the surface and bring up to the required level. Gently squeeze or vibrate the mould to bring any air bubbles to the surface. Leave to harden. As it is setting the plaster will heat up. When it is cold (about 1 hour) the mould can be removed. To ease removal, coat the outside of the mould with a little washing up liquid and peel it off. Be very careful with slender necks, fingers etc. Now is the time to level off the bottom whilst the plaster is still reasonably soft. Use a coarse file or surform and rub the base of the model back and forth until it is smooth and level. Finish off with emery paper.
FINISHING Many different finishes can be used on plaster casts. Paint with water colour, acrylic or enamel paint and finish with gloss or matt varnish. Metallic finishes are possible using sintered metals. Apply a coat of varnish followed by a coat of sintered metal/varnish mix. Allow to dry and then burnish with fine wire wool until the metal shines through. An antique finish can be achieved by staining this metallic finish with black stain. Rub down when dry with fine wire wool to emphasise the shiny high spots. A coat of wax polish will help to prevent the metal from tarnishing. To obtain a pottery finish paint the model with very dilute water or poster paint, rinse lightly and then rub gently with a damp cloth so that the white plaster shows through. It is possible to pigment the plaster from the outset by adding cement colouring agents available from a builder's merchant.
These are just a few of the ways to decorate your model. The range of finishes are too numerous to mention here but experimentation is the best way to find a finish that suits your purposes. The only limit is your imagination.
NOTES The setting times of the plasters can be adjusted as follows:- TO RETARD THE SETTING TIME - Using Sodium Citrate make up a 1% solution in water and add to the mixture to a maximum of 10% based on the weight of the plaster. This should not affect the physical properties of the plaster. TO ACCELERATE THE SETTING TIME - Potassium Sulphate powder added directly at the rate of 1% based on the weight of plaster.
This advice is given entirely in good faith and is based on the results of our own tests.
Available from Fred Aldous Ltd, 37, Lever Street, Manchester. Ml lLW
Tel: 0161 236 2477 Fax: 0161 236 6075
Email us at [email protected]
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