ABOUT PLASTER

Plaster is now generally based not on lime but on gypsum, of which there a large natural deposits in the UK. Gypsum is mined from geological deposits produced by the gradual evaporation of lakes containing the mineral. Historically, fibrous materials have been used to reinforce plaster and particularly to control shrinkage in lime plaster. Traditionally ox, horse and goat hair were the standard materials; however, straw, hemp and jute have also been used. The earliest lightweight support for plasters was interwoven hazel twigs, but by the fifteenth century split timber laths were common. The modern equivalent is the use of galvanised and stainless steel expanded metal.

To manufacture gypsum plaster, rock gypsum is mined, crushed and ground to a fine powder. The natural mineral may be white or discoloured pale pink, grey or brown due to small quantities of impurities which to not otherwise affect the product. On heating to temperatures in the range 130 to 160 degrees centigrade, water is driven off the hydrated gypsum; the type of plaster produced is largely dependent on the extent of this dehydration process.

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Plaster of Paris is produced by driving off three quarters of the water content from natural hydrated gypsum. It sets very quickly on the addition of water, and is therefore often used as a moulding material. The majority of plasters in current use within construciton are based on retarded hemi-hydrate gypsum. The addition of different quantities of a retarding agent, ususally keratin, is used to adjust the setting time for different products.

The main constituents of undercoat and one-coat plasters are retarded hemi-hyrdrate gypsum, with expanded perlite or exfoliated vermiculite for the lightweight products, together with small quantities of limestone, anhydrite, clay and sand. For finish coat plasters, like undercoat plasters, the main constituent is retarded hemi-hydrate gypsum, but with a small addition of lime to accelerate the set. The lightweight products contain exfoliate vermiculite. Finish coats on masonry substrates are usually 2mm in thickness, and board finish plaster is normally applied to 2 to 3mm thickness.

Plaster bonds to the background by a combination of mechanical key and adhesion. Backgrounds should be clean, dry and free from other contamination. Where possible, as in the case of brickwork, a good mechanical key should be obtained by raking out the joints.

LEADING UK PLASTER SUPPLIERS

British Gyspum is a leading supplier of plaster. Their Thistle plasters offer a wide choice of undercoats and finishes for applications from traditional hand applied two coat work to replastering after dpc installation, machine applied projection and Gyproc plasterboard finishing.

Decopierre, specialises in interior and exterior wall decoration. Its product is a made from a mixture of limestone and powdered marble, coloured with natural pigments. This produces a finish resembling natural stone. The coating is projected onto the wall, and is then carved by Decopierre plasterers. Examples of finishes include Devonian sandstone and Scottish potpourri.

Click here for an easy-to-use listing of leading UK suppliers of plaster, provided by Archbone.

Other useful links relevant to plaster and plaster suppliers:

Gypsum Products Development Association

DETAILED INFORMATION RESOURCES ON PLASTER

We recommend the following websites, with building product information including manufacturers' specs, standard specifications, and CAD details: Barbour Compendium, RIBA Product Selector Plus, ASC Bricsnet, Building Products Index, British Board of Agrément, British Standards Online.

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