Plaster is paradoxically both a simple and a complex material. At it’s simplest, it’s a mix of sand, a binder (lime in traditional plasters, gypsum in modern plasters) and a range of possible additives to improve both properties and lifespan. It’s a versatile, practical building material that can be applied to brick, stone, earth and timber. It’s strong, durable and breathable, fire resistant, and also reduces sound transmission. It can be finished in a variety of ways including stencilling, decorative painting, wallpaper, whitewash and with ornamental plaster fixtures. And while this all sounds quite wonderful, owners of period properties are often faced with damaged and decaying plasterwork requiring restoration.
When faced with restoration, the first question should always be “Why is this happening??”. The most essential prerequisite to restoring plasterwork is to identify the cause of the problem, and to address this. Plaster is a rigid, durable and long-lasting material, and wholescale failure is uncommon unless the house has been abandoned and/or roofless for some time. The most common problems are staining, sagging, cracking and failed sections of plasterwork. Another common problem is salts erupting on or just under the surface, disrupting the painted or papered plaster finish, and progressively worsening over time. Faced with a combination of these, total replacement may appear at first glance to be the only alternative. However, traditional plasterwork may not be as badly damaged as first appears, and three things should be borne in mind before embarking on restoration:
1. Replacing plaster is expensive, correctly identifying and addressing the problem may reduce the amount of repairs necessary, and ultimately reduce costs.
2. Plaster has a number of values, not the least of which is the contribution it makes to the historic character of your property, and should be conserved wherever possible.
3. Problems tend to be small-scale, and can originate from structural problems, poor workmanship, poor materials, or moisture ingress. Total plaster replacement is rarely necessary.
The characteristics of the problem point you in the direction of the cause, which indicates how you should go about repairing it. For example, if you have diagonal stress cracks in a wall at doors or windows or across the ceiling – there’s no point repairing the crack until you sort out the structural problems causing it – otherwise it may simply re-appear. Some failures, such as the separation of plaster from the wall may have more than one possible cause. Sagging indicates that the plaster has lost its bond with the wall behind. In timber and lath construction, there may be rotting laths, rusted nails or poor quality plaster leading to this. If the plaster has been applied directly onto a masonry wall, you may have a damp problem. Also, sadly, if there was a bad mix, or was applied at the wrong time of year, the backing and top coats of the plaster may simply be incompatible, and can just separate from the wall.
When considering the repair of historic plasterwork, the first key concept to keep in mind is that there is no “right” way to plaster a wall. The modern standards of 1 part lime to three parts sharp washed sand for the base coat, and equal parts of lime and silver sand for the top coat have very little to do with the historic plaster on your walls. Traditional plaster has much more in common with the lime mortars and exterior renders found at medieval castles and churches than plaster to modern building standards. Each plasterer had his own particular mix. The sand can be sharp or soft and rounded. The plaster can contain varying quantities of hair, wood fibres, burnt fuel, oils, fats, glues and traces of blood, or it can show no additives at all. Some plasters may be so hard they feel like a stone finish, yet the flexibility of other softer plasters may have allowed them to survive by accommodating structural movement in the building.
The second key concept is that the plasterwork we see today are the best performers, and other poorer quality plasters have long since disappeared. By replicating a good plaster from your building, we’re learning from a material that’s been field-tested for a couple of centuries in the particular environmental conditions of your property. Plaster and mortar analyses are carried out for a number of reasons, for example it is possible to radiocarbon date wood fibres and burnt fuel to assist archaeologists and art historians in uncovering the history of a building.
For restoration, there are two main reasons to analyse: to find out why the plaster is failing, and to design a new plaster for repair work. There are a range of chemical and instrumental techniques which can be used including electron microscopy with x-ray analysis (SEM/EDX); and x-ray diffraction (XRD) which give high resolution images of the surface of the plaster with a magnification of up to 100,000x. Petrography is one of the most useful techniques, giving information on the type of sand and additions used, and the binder-to-sand ratio. The type of analysis carried out depends on the answers you want, and depending on the nature of the problem, a plaster can be replicated or improved upon to compensate for the particular conditions of the building.
If faced with restoration and repair of historic plasterwork, get a few opinions before starting work. Lime plaster is durable, and to a certain extent self-repairing. Failures tend to be from outside agents such as water penetrating the walls, or the decay of timber lathes or floor and ceiling structures. Make sure you know why the plaster is failing, and that the repairs will address the root cause before you start work.