General mixing guide; Unless similar pieces have been made previously, it is good practice to carry out a trial with a small amount of material to establish the ideal consistency for the particular type of work and setting time: this is particularly important with impression work; Except for non-critical work always weigh the plaster and use a calibrated jug, etc., for measuring the water, using the water to powder ratios shown in brackets in the left hand column of the table below, e.g. Fine Casting Plus (70:100) requires 0.7 litres (i.e. 700g) of water for every kilogram of plaster powder used. Old plaster or plaster that has been stored in damp conditions sets more quickly, as does unset plaster brought into contact with freshly-set plaster. Setting times can be retarded by 10 - 15 minutes simply by adding culinary lemon juice at the rate 1 - 2 teaspoonfuls for each litre of water used. If you havenít used plaster-of-Paris before you will probably be surprised how runny the fresh mix appears to be.
Weigh out the required amount of plaster into a clean dry vessel; break up any clumps in the powder at this stage.
Measure out the correct amount of water; use tap water only. Use a clean container that will hold around 3x the volume of the loose plaster powder, to ensure plenty of space for mixing the paste.
Gently sprinkle the plaster into the water and allow the material to sink; allow a few minutes for the powder to properly soak.
Mix slowly and firmly using strokes of the stirring tool that limit any air entrainment, i.e. avoid any beating or dropping action; if using an electric drill with a stirring attachment, use a low speed setting.
The fluidity will vary with plaster type, but the mix should have an even consistency, with no bubbles or swellings under the surface, and no free water on top or at the sides of the mix; make certain that there has been no clumping in the corner of the bucket, etc. Gentle tapping will help to bring any air bubbles to the surface.
Allow the mixture to stand for a minute or so before using, and pour gently if making a solid piece to avoid splashing and air entrainment. Again, if feasible, gentle tapping of the mould/box will release air bubbles.
The mix can be become quite warm as setting progresses, depending on the normal set time for the particular plaster (see table above), the amount used and ambient temperature, etc. It is usually safe to remove the cast at about 2x set time, when the initial warmth has subsided.
Clean any non-disposable utensils and containers thoroughly in water immediately after use, but never down a drain or sink, etc. Alternatively, if using a flexible plastic bucket, etc, allow the excess plaster to completely set, and crack it off for disposal.
Never attempt to dry plaster in a microwave or pour molten material into moulds unless they are known to
be completely dry, as the plaster may explode! Plaster can become very warm during the setting process - never attempt to embed or
mould hands, feet or fingers, etc.
How Much Plaster? As an illustration, it takes 1.5kg of Fine Casting plaster to make a block the size of a brick - but 2.5kg if using Crystacal R which is much denser. First work out the volume of the finished piece. If this is difficult to measure directly as with a latex mould, for instance, use a calibrated jug to see what volume of water would fill the mould - or how much water it would displace - in litres. Multiply this figure by the relative density value in the bottom table to arrive at the weight of plaster powder needed in kilograms - then add a little for wastage, etc. So, for a mould that holds 0.4 litres, if using Crystacal R plaster - which has a relative density of 1.67 - you will need around 0.7kg for each casting (0.4 x 1.67 = 0.66kg).
Technical information and water-to-powder ratios. Density is a good indicator of impression capability and surface absorption, but the values are approximate and are dependent on the correct water:powder ratio and consistent mixing. Strength characteristics are typical 7-day values.
Plaster-of-Paris Type (Water:powder ratio, by weight).
2 Hour Comp. Strength
Set Time at 18°C
Fine Casting Plus (70:100)
under 4 N/mm²
under 4 N/mm²
Dental SP (50:100)
Herculite No.2 (42:100)
Herculite LX (40:100)
Crystacal R (35:100)
Herculite Stone (33:100)
Yield is the approximate volume of finished plaster-of-Paris work measured in litres for each kilogram of dry plaster powder used in the mix.
There is some latitude in the choice of plaster - both surface density and absorption can be adjusted with a small change to water content.
*Cassini's is water- and weather-resistant. Cassini's has very high flexural strength and is ideal where reinforcing is difficult.
Poor quality moulds are usually the result of incorrect curing and premature separation. Poor quality plaster casts are usually the result of using moulds which are not completely clean/dry, and premature demoulding - allow at least double the stated set-time for intricate or undercut work.