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Gypsum Plastering

 

 

The most common gypsum plasters are browning, bonding, and metal lathing plaster. Modern plasters come already mixed with perlite, vermiculite and several other additives. These additives provide a greater degree of insulation and fire resistance. Other ingredients slow down the setting time and generally make the mixture more workable.

Today the only additive we need to add is good clean water. When plastering materials with differing absorption, it is a good idea to key the surfaces to be plastered with a PVA bonding sealer. This prevents the plaster 'going off' (setting) at different speeds.

Surface Guide

Browning plaster (perlite) - Brickwork, Coke breeze, Clay tile partitions, Thermalite blocks, Concrete bricks.

Bonding plaster (vermiculite) - Concrete, Stonework, Cork slabs, Surfaces treated with PVA.

Metal lathing plaster (perlite with vermiculite & rust inhibitor) - Expanded metal lathing, wood wool slabs.

One Coat Plaster - Used for any surface as undercoat and finish.

Lime Plastering

 

The earliest structures to which plasters were applied took the form of panels of woven hazel or willow spars supported by timber. When first applied, some of the plaster would protrude through the spars, creating interlocking 'nibs' in the void behind. The nibs help to secure the plaster to the lattice, reinforcing the key or bond between plaster and wood. For centuries hair and other fibres have been added to lime and gypsum plasters to give greater strength to these nibs and stop them breaking off.

 

For the reinforcement of lime plasters and renders, hair should be strong, long and free from grease or other impurities. Ox hair is the preferred choice, but horse, goat, donkey, and a variety of other hair, including reindeer, are suitable. Human hair, being relatively fine and of poor strength, should not be used.

 

Traditional alternatives to hair include chopped straw, reed, manilla hemp, jute, sisal, and even sawdust. Modern synthetic fibres such as glass and polypropylene which have been designed for use with Portland cement mortars have also been used successfully in pure lime mortars, despite their smooth and almost shiny appearance when viewed under a microscope. Natural animal hairs, by comparison, have a much rougher texture, and are generally more appropriate for historic buildings.

 

While woven hazel or willow spars work well and are often found in surviving wattle and daub, the practice of splitting oak and chestnut lath to produce riven laths became popular early in the 15th century. Oak and chestnut make particularly good riven lath as they both contain natural oils, thus ensuring long life.

 

Lath & Plaster

 

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