Page 188 - ASOP ROT Study Guide

which may be double woven to prevent it fraying (known as leno cloth). The starch which sticks the
plaster of Paris to the cloth adds nothing to the strength of the cast but it does influence the
speed of the setting by speeding up the chemical reaction - it is an accelerator. Manufacturers
can vary the setting time by altering the amount of starch. Other materials can be added to slow
down setting. These are called retarders and common examples of these are alum and borax.
The temperature of the water used to wet the bandage also influences the setting time of
plaster of Paris. Professionals in different countries prefer different setting times.
The plaster of Paris itself is made up of two types of crystals. Some are long and sharp and others
are shorter. The long crystals occur naturally as alabaster. They give a finished cast a hard
quality. The smaller crystals give the cast a softer feel. Experiments have shown that
commercially available casting materials, which differ in their proportions of the two crystal types,
have similar strengths. However, professionals can tell the difference in the wet and finished
qualities of the differing casting materials.
The properties of the material making up the final splint are largely dependent on the physical
interlocking of the two types of crystal.

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This interlocking will be influenced by how wet the plaster material is at the time of application.
Ideally just enough water should be present to completely crystallize the calcium sulphate
hemihydrate. However this would be insufficient to make the material workable, as it would only be
vaguely damp! The amount of water required to make a plaster bandage workable means that
excess water must evaporate before the cast is strong enough to bear weight. This may take up to
two days in a typical cast. This inevitable weakening of a plaster cast is unfortunately necessary to
give plaster that versatility of application which makes it such a useful material.