Morells Barn • Park Lane • Lower Bognor Road • Lagness • Chichester • West Sussex • PO20 1LR
Telephone: 01243 265845 • Fax: 01243 267582

MIXING
Weighed quantities should always be used. The bulk density of plaster is variable and so measurement by volume is not reliable. Records should be kept of the plaster water ratio used in each type of cast so that mixes can be adjusted accordingly.

Use clean water only. It should be as pure as possible. Usually if water is drinkable, it is suitable for mixing plaster. For consistent results, a constant water temperature should be maintained. Ideally, this should be in the range 16-20°C (61-68°F)

Ensure all containers and tools are clean and free from set plaster. Do not mix fresh material with plaster that has been mixed some time previously.
Hand Mixing

HAND MIXING
1. Weigh the plaster and water accurately for each mix.

2. Sift or strew the plaster into the water slowly and evenly. Do not drop handfuls of plaster into the water. Always add the plaster to the water, never the reverse.

3. Allow the plaster to soak for at least one or two minutes. This soaking period removes air and allows each gypsum particle to be saturated with water so that it is easily dispersed during mixing. While waiting, the sides of the container can be tapped to bring air bubbles to the surface.

4. Mix as required by stirring. This is best done by placing an outspread hand into the bottom of the container and stir. Keep the stirring action below the surface of the water so as not to entrap any additional air. Move the hand from the bottom to the top breaking up any lumps encountered. Skim off any air bubbles or scum that may have risen to the surface. Mixing will take 2 to 5 minutes, depending upon the type of plaster being used, the plaster water ration and the quantity being mixed.

5. Gradually, the mix will change consistency and begin to take on a creamy, uniform texture; at this point it is ready to pour.

6. Pour lifting the mix carefully over the mould and gently pour in a thin continuous stream in one spot, letting the plaster flow freely up and around the inside of the mould until it crests just above the lip of the mould. Tap the sides and shake the mould to release air bubbles that will rise to the top.

7. After the mould or moulds have been filled, pour the excess plaster remaining in the mixing container into a waste bin and clean the mixing bowl and any tools used. When the plaster has begun to set, remove the excess plaster on the mould by scraping the plaster flush with the lips of the case mould.

MIXING & POURING
Permeability, porosity, hardness, strength and durability of plaster moulds are directly related to the mixing and pouring procedure.

The plaster/water ratio (consistency) is the amount of plaster used with a measured amount of water. For example, a 70 consistency mix would be 100 parts of plaster to 70 parts of water. It is always specified by weight and for consistent results weighted quantities must always be used. The bulk density of plaster is variable and so measurement by volume is not reliable. Records should be kept of plaster/water ratio used in each undertaking and the final results that are achieved, so that mixes can be adjusted accordingly. The mix can be made more fluid by increasing the proportion of water. However, increasing the water content of the mix will result in reduced strength, hardness and expansion. Conversely, reducing the water content increases strength but reduces porosity and permeability.

As a guide, it has been found that weak mixes of pottery plaster below 100/800 can result in poor mould life. Thicker mixes of over 100/74 produce a more viscous mix that may entrap air and also reduce the quality of the cast pieces from the mould. Similar constraints apply when using Keramicast with a typical range of use between 100/77 and 100/67.

MECHANICAL MIXING
1. Weigh the plaster and water accurately for each mix.

2. Sift or strew the plaster into the water slowly and evenly. Do not drop handfuls of plaster into the water. Always add the plaster to the water, never the reverse.

3. Allow the plaster to soak for at least one or two minutes. This soaking period removes air and allows each gypsum particle to be saturated with water so that it is easily dispersed during mixing. While waiting, the sides of the container can be tapped to bring air bubbles to the surface.

4. Use a mechancial mixer and mixing bucket of an appropriate size and design. Mechanical mixing considerably reduces the required mixing time. The time taken will depend upon the type of plaster used, the plaster water ratio, the volume of the mix as well as the type of mixer used.

5. As a general guide, direct drive propeller mixers are the best. The propeller should clear the bottom of the contaienr by 25-50mm (1 to 2 inches) and the shaft should be about half way between the centre and side of the mixing container. Mixer speed should not exceed 450rpm; a variable speed control can be of assistance. The propeller should force the mix downwards, but not form a vortex, which will induce air into the mix. All equipment must be kept clean to avoid causing accelerated set of the plaster mix.

STORAGE
It is essential that plaster is stored under dry conditions and that bags should not be placed in contact with cold or damp surfaces such as walls or floors. Stored under dry conditions, plaster will have a minimum usable life of three months. Each bag is marked with the date of manufacture. The oldest plaster should be used first. Open or part used bags should be folded and closed after each mixing.

DECORATION
Before applying surface decration, the cast must usually be dry. A coat of an appropriate sealant may be necessary to ensure a uniform finish when painting.

LIMITATIONS
The information given for plasters is intended only as a general guide. The production of plaster casts is complex and relies on craft as well as technical skill. The successful production of plaster casts will depend upon the individual circumstances.

The plaster water ratios indicated relate to the water demand of each product under laboratory testing conditions. They are not necessarily recommended for practical usage. Practical proportions will depend upon individual requirements. The absorption of moisture by any gypsum plaster during storage may lengthen the setting time and reduce the strength of the set plaster.

The physical properties of plaster may change during its life. These can include a lengthening of set time and a reduction in set strength.

MOULD DRYING
Moulds should be dried before use. Better casting performance and longer mould life are achieved when moulds are dried slowly. The casts should be dried in a thermostatically controlled oven at a temperature not exceeding 40°C (104°F). Higher temperatures up to a maxiumum of 60°C (140°F) may be used, but it is essential to remove the casts as soon as they are dry, otherwise the surface will deteriorate.

The moulds should be dried so that the drying front moves away from the working surface. This will avoid the migration of soluble salts to the face of the mould. This can be achieved by protecting the face by single reverse drying cup moulds, drying with saddles or other suitable means.

When removing moulds from the drier, care must be taken to avoid cracking due to thermal shock arising from substantial difference between drier and ambient temperatures. This is particularly important in winter.

Moulds and patterns should be stored in a dry place at a temperature of about 16°C (61°F).