Polished plaster

[edit] Introduction

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.

A wide range of decorative finishes may be grouped under the term ‘polished plaster’. They are characterised by mimicking decorative stone finishes such as polished marble or deeply-textured effects such as limestone or travertine. As polished plaster appears to be something it is not, it can be described as an illusion, giving a highly-realistic impression of texture and depth that has been prized and applied to great effect since ancient times.

[edit] Composition and application

Polished plaster is made from slaked lime, marble dust and /or marble chips – the exact ratio depending on the final effect required. Up to 40% of the final mix may be marble powder. Natural earth or synthetic pigments may also be added to provide special colour effects.

Polished plaster is typically applied to internal walls, ceilings and columns. A primer is first applied to the substrate followed by up to four layers of basecoat. A steel trowel is used to burnish (finish) the plaster to a glass-smooth effect; a protective wax may be applied to seal the finish.

Typical applications include hotels, public buildings, galleries, bars and restaurants. It has also become popular for luxury bathrooms and other wet areas due to its smooth, waterproof and anti-bacterial properties.

The final appearance depends on the material composition, the effects used and particularly the skill of the plasterer, who may be able to create effects ranging from plain, light polished surfaces to more highly burred and marble-type effects.

However, damage to polished paster can be difficult to repair.

[edit] Historical usage

The use of polished plaster goes back to the ancient Egyptians. It was also used by the Greeks and the Romans. The Roman military architect and engineer Vitruvius (c.90BC – c.20BC) extolled the virtues of polished plasters in his Ten Books on Architecture.

The next period of popularity came during the Italian Renaissance in Northern Italy when it was called ’marmorino’, short for marmorino veneziano (venetian polished plaster). It was used extensively both internally and externally, particularly as a background for ornate frescoes. Colour came from natural pigments but, due to their inconsistencies and variable strengths, required the expert manipulation of professional colourists. This was one reason it became a finish that was the preserve of the rich.

Another Venetian variant, stucco lustro, comprised a base of 2-3 layers of semi-dry marmorino followed by up to 10-15 fine coats of stucco. It was finished to a mirror-like effect using hot irons.

In 17th and 18th century Britain, the use of polished plaster on walls and columns for reasons of aesthetics, economy and practicality gave a highly-realistic impression of marble or travertine, creating an illusion of depth and texture, without requiring the stonemason’s craft – and at a fraction of the cost.

Today, not unlike the historical precedents, polished plaster finishes are undergoing a revival of interest and tend to lie at the upper end of the price range due to the labour input that is required.

[edit] Related articles on Designing Buildings

• Ceramic tiles.
• Finishes.
• Marble.
• Paints and coatings.
• Plaster.
• Rendering.
• Splashback.
• Stucco.
• Tiles.
• Travertine.
• Trompe l'oeil (part 2).
• Types of stone.
• Types of wall.