Plaster is a building material used for coating, protecting and decorating internal walls and ceilings. It can also be used to create architectural mouldings such as ceiling roses, cornices, corbels and so on.
The most common types of plaster are a composition of gypsum, lime or cement with water and sand. The plaster is typically manufactured as a dry powder and then worked to form a stiff paste by mixing in water before application.
 History of plastering
A form of plastering was used by primitive civilisations, creating durable and weather-resistant structures using mud. The Egyptian pyramids contain plasterwork comparable to that used today that remains hard and durable some 4,000 years later.
Greek artisans used plaster, mainly to cover the exterior of temples but sometimes also interiors. Through history, plaster ceilings became increasingly ornamental, with those during the Tudor period being particularly extravagant.
However, the use of plaster as a means of demonstrating artistic skill and expression had waned by the 19th century, when imitation and mechanical reproduction displaced it as a creative medium. However, plaster is still very commonly used as a surface finish for interior walls, ceilings, and still sometimes for exterior walls.
There are a number of different types of plaster, depending on the binder that is used.
 Gypsum plaster
Gypsum plaster, or ‘plaster of Paris’ (POP), is the most common form of plaster for interior walls. It is produced by heating gypsum to around 150°C (300 °F). When mixed with water, the dry plaster powder re-forms into gypsum. Unmodified plaster starts to set about 10 minutes after mixing, but it will not be fully set until 72 hours have elapsed. Gypsum plaster has good fire-resistant qualities.
 Lime plaster
Lime plaster is a composite of calcium hydroxide (lime) and sand (or other inert fillers). It may sometimes be strengthened with animal hair to preventing cracking and reduce shrinkage. The plaster sets through contact with carbon dioxide in the atmosphere which transforms the calcium hydroxide into calcium carbonate (limestone). It is typically more flexible and breathable than gypsum and cement plasters, and is most commonly used on older properties.
Cement plaster is a mixture of sand, cement and water. It is normally applied to masonry interiors and exteriors. While it is capable of achieving a smooth surface, interiors will sometimes require an additional finishing layer of gypsum plaster. Cement plaster offers greater moisture resistance than gypsum plaster.
 Clay plaster
Clay plaster is considered to be a more sustainable alternative to modern plasters, with a lower embodied energy than gypsum, cement or lime based plasters. It is available with fibre additives to increase its strength, and in a range of ‘natural’ colours. It is breathable and does not need to be painted.
 Types of plaster by application
Plasters can also be categorised by application:
- Board finish plaster is used on plasterboard.
- Bonding plaster is used as an undercoat, applied to new walls.
- Browning plaster is used as an undercoat for particularly absorbent surfaces.
- Dri-coat plaster offers moisture resistance for walls that have had a damp proof course (DPC) inserted or injected.
- Hardwall plaster is similar to browning plaster, but is faster drying and has better impact resistance.
- Heat resistant plaster is used for walls and chimney breasts where temperatures may exceed 50°C.
- Multi-finish plaster is a top coat plaster.
- One-coat plaster is typically used as a patching plaster.
- Tough coat plaster has exceptional impact resistance.
 Plastering techniques
Plaster is typically built up in layers, with the number of layers depending on the roughness of the surface being plastered. Rough, bare walls could require three coats of plaster, while plasterboard might just require a finishing skim of 2 to 3mm.
The plaster compound, which is typically supplied dry, must first be mixed with water, with care being taken to achieve the correct creamy consistency. Mixing is best achieved by adding the plaster to the water, not the other way round.
The plaster can then be applied to the wall. The first coat is applied, scratched and left to dry. This is followed by the second coat and finally a third finishing layer.
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