Lime or clay plaster
Historically, many building interiors were plastered with non-hydraulic lime, sand and hair (sometimes gauged with gypsum) or, alternatively, clay, a lime binder and reinforcement such as straw, concealed under a lime skim. Such plasters are applied directly to solid backings, such as masonry or cob, or flexible supports, including timber laths or reed.
What does lime or clay plasterwork comprise?
Historically, many building interiors were plastered with non-hydraulic lime, sand and hair (sometimes gauged with gypsum) or, alternatively, clay, a lime binder and reinforcement such as straw, concealed under a lime skim. Such plasters are applied directly to solid backings, such as masonry or cob, or flexible supports, including timber laths or reed. Lime or clay plaster can have a more pleasing character as well as offer better internal comfort and sound insulation than substitutes widespread by the 20th century. Early wallpaintings occasionally survive, too. Old plasterwork is worth conserving wherever possible but is often indiscriminately replaced using gypsum-based products. The focus here is on plain internal plasterwork, excluding ceilings.
What can cause plaster to deteriorate, can it be repaired without removing areas?
Plasterwork should last indefinitely but when deterioration occurs it typically takes the form of:
• Cracking (for example, due to variations in the backing).
• Loose plaster or delaminating coats (perhaps because of poor keying).
• Staining, crumbling or flaking (caused, for instance, by rainwater penetration).
• Missing areas (frequently following deliberate removal).
Deterioration frequently looks worse than it is and repair rather than replacement is commonly possible. It is important, of course, to deal with the cause of any defects first, including underlying structural or dampness concerns.
How might cracked, loose or delaminating plaster be repaired?
Cracks may be filled using matching plaster or, alternatively, if narrow, proprietary interior filler gauged with whiting, followed by redecoration. They might require widening first to provide a good key and, where continuing movement is anticipated, bridging using a suitable mesh. The plaster should also be reattached beforehand if loose, although this is unnecessary where it is simply springy.
Conservators use various methods to reattach loose plaster or delaminating layers, especially where of important historic value or deterioration is not severe. These include inserting stainless steel screws and washers or injecting grout (liquid mortar).
How can plaster with surface damage be treated?
Small pockmarks etc can be filled with the same plaster as cracks. Lime plaster, however, is preferable where repairs will be limewashed. Another method is to conceal blemishes under new lining paper providing no important historic finishes survive. Occasionally, a new coat might be skimmed on instead, especially over small areas, but skill is needed. A good key is essential and frequently formed by scoring the existing surface.
Where areas of plaster, particularly those of historic importance, lose cohesion and become powdery, specialist consolidation techniques can be used involving the use of materials such as nanolime (fine particles of calcium hydroxide suspended in alcohol).
What does renewing plaster entail?
Retain as much as possible of the existing lime or clay plasterwork. Any replacement, where justified, should normally comprise the same material (perhaps reconstituted) and number of coats (unless inappropriate work has taken place previously). Adequate hair or other reinforcement is crucial on flexible backgrounds. In most cases, existing plaster can be adequately assessed from visual inspection coupled with local knowledge.
Where patch-repairing, slightly treat the exposed edges of the existing plaster with water or diluted PVA (1 part to 10 of water) to control suction and prevent cracking. The addition of gypsum to a lime plaster mix may also sometimes be justified for the same purpose. Adequate aftercare is also important to minimise shrinkage cracking.
What should I use to paint or finish the plaster?
It is normally advisable to select a paint that is vapour permeable, compatible with any existing finishes and can be easily reapplied. Limewash, soft distemper and, from the 19th century especially, wallpaper were amongst the interior finishes used widely historically. These frequently remain appropriate for redecoration, although modern alternatives, including clay-based formulations and, sometimes, proprietary renovation paint, may be suitable. Exceptionally, non-breathable oil-based paint might be considered where used historically. Paint or wallpaper analysis can help inform decisions, as well as highlight surviving historic finishes worth protecting.
English Heritage (2011) Mortars, Plasters and Renders, Practical Building Conservation, Farnham: Ashgate Publishing Ltd
Ratcliffe, T (2006) ‘Internal Lime-Plastering’ in Building Conservation Directory, Tisbury: Cathedral Communications Ltd