by Neil May

Clay plasters have been used extensively in buildings in the UK and indeed all over the world, for thousands of years. Although it is not widely known there are probably over a million buildings with clay materials in their structure in the UK, and a great many of these have clay plasters. Very often clay plasters are not recognised because they are painted or have a thin lime putty skim coat over them, both internally and externally. Many clay plasters are still performing well after many centuries, both in vernacular buildings and in higher status properties (including their use in mouldings, ornamental shields and so forth). The point of this article is to demonstrate not only that clay plasters are an important historic material, which need proper repair and preservation, but also that clay plasters are a viable and high quality material for use in standard renovation work, and even within modern building contexts

What are clay plasters and how do they compare with other plaster types?

Clay refers to the binder in the plaster, just as lime, cement or gypsum refer to the binder in their respective plasters. Clay is therefore a better description than "earth", as earth contains aggregates, and other materials which may be found in all types of plaster. Clay soils used in building are generally a graded mixture of particles in the clay and silt size ranges, which exhibit cohesion and plasticity. The clay sized particles can be either finely ground rock or clay minerals.

The presence of clay minerals exerts a considerable influence on the properties of the soil, usually out of all proportion to the percentage content. There are various types of clay mineral formed from combinations of stacked sheets of silica and alumina. The most common minerals are Kaolinite, Illite, and Montmorillonite. These have very different qualities due to the different arrangement of the stacked sheets and the different strength of the bond between the sheets. Basically Kaolinite has the strongest bonds (of Hydrogen) and forms the the most stable, least shrinkable clay, but has less good binding, and hygroscopic qualities (see below). In Montmorillonite the bond between the sheets is weaker and is provided by water molecules. These clays have better binding qualities, exhibit considerable shrinkage and swelling and can absorb more water both through capillary and hygroscopic actions. Illite clays are somewhere between.

It is important to note that the clay is unfired, and is not the same therefore as the fired clay particles added to some lime plasters as pozzolans. Fired clays have very different qualities.

Clay plasters (and mortars) also distinguish themselves from other plasters (and mortars) by the way that they harden or cure. For cement, gypsum and to some extent hydraulic limes, the process of curing is through a hydraulic reaction between the binders and water. For non-hydraulic limes, and to a lesser degree for hydraulic limes and cement, the process of curing is through carbonation, the conversion of Calcium Hydroxide to Calcium Carbonate by the absorption of Carbon Dioxide from the atmosphere.

Clay plasters however become solid as the water added during mixing is lost and the electrically charged surfaces of the clay particles move closer together. The relative strength of the forces of attraction and repulsion between clay particles is responsible for the amount of cohesion and binding force of clay in the plastic state and for the compressive and tensile strength in the solid state. The strength of the attraction between the clay particles can increase if the particles are brought closer together by external pressure (ie a trowel, float, or if thrown) or if the water surrounding the particles has a higher concentration of positive ions (for example by the addition of additives such as urine). In addition to these electrostatic charges there are also capillary, electromagnetic, frictional and cementitious forces at a molecular level.

What are the qualities of clay plasters compared to other plasters?

Clay plasters have very specific and in many ways unique qualities which are extremely well suited to historic building conditions for a number of reasons. These may be divided into "Breathability", flexibility, reversibility and their aesthetic qualities.

Breathability

In terms of "breathability" clay plasters not only have excellent vapour permeability (a m factor of around 8 ie only 8 times the equivalent thickness of air), but also extremely good hygroscopic qualities. What is significant here is not only the amount of moisture that can be absorbed from the air but also the rate of response. In most materials hygroscopic qualities relate to the capillary structure of the material, whereas in clay plasters, moisture can also be drawn in and held by ionic bonding with the clay particles themselves.

For this reason the type of clay is highly significant, as explained above. Kaolinite clays have less hygroscopic qualities, while Montmorillonite clays have a very rapid ability to take in moisture from the atmosphere when humidity rises. On the other hand burnt clay and expanded clay have very poor hygroscopic qualities.

Overall however clay plasters have a much more rapid uptake of moisture from the atmosphere than with other materials such as timber, which take in and release large quantities of moisture but over a much longer period. They can therefore act to protect vulnerable organic materials (and in particular timber) from high levels of relative humidity, when microbial and insect attack can be triggered. Particularly with modern building usage (showers, cooking, indoor living) this can be an important strategy in the control of excess moisture in vulnerable buildings.

In addition to the building benefits of clay, the hygroscopic qualities mean that moulds caused by condensation are minimised, and that a relative humidity of 50% - 60% is maintained. This is the ideal level for mucus membranes of the human body, and also for the control of dust mites and other organisms which affect human health.

Clay plasters also have very good capillary qualities. They actually have less capillary draw than materials like lightweight brick, and even certain cement products, but more capillary draw than most types of timber. This means that within an exposed traditional timber frame building they will draw water droplets away from the timbers, but not suck in water droplets and hold them against timber. (It should be noted that when clay plasters are used externally they do need a limewash or a fine lime plaster skim to protect them against driving rain, depending on levels of exposure. Rain resistance can also be substantially improved by controlling the proportion of clays and fibres in the mix, as well as by additives such as casein.

Flexibility

Clay plasters are flexible in relation to their fibre content. In this sense they are similar to fat lime plasters. Their inherent soft and pliable qualities mean that fibres such as straw, flax, and hair, are able to hold together the plaster without cracking in situations of minor or gradual movement, provided there is sufficient quantity of fibre. This is a significant quality in old buildings.

Reversibility

Clay plasters to an even greater extent than fat lime plasters, are reversible. Unlike lime, cement and gypsum they are also re-workable, provided they are not contaminated (particularly by salts). They are not only easy to remove, but they do not have the staining or caustic qualities of lime. Because of their "breathing" qualities they can protect more vulnerable parts of structures, and absorb large amounts of moisture, salts and pollutants where these are a danger - ie they can be used sacrificially.

Aesthetic

Unpainted clay plasters have a very particular aesthetic. Due to the shrinkage of the clay (micelles ) on drying, the plasters always have an open texture even when polished. This means that light reflects and refracts on the surface in way in which there is always variation and never a gloss sheen. This is particularly noticable in the modern self coloured plasters that are becoming more commonly available.

Clay plasters in practice

Earth materials were used extensively in historic buildings of all sorts, up until the 19th century. Their disappearance was partly an effect of the separation of agriculture from construction, as a result of the industrial revolution. The disappearance was not because the material was primitive or had been failed, but because of the changing economic conditions of the time. This is an important fact to remember when looking at clay plasters in practice. These are not primitive materials, even if they are not synthetic or a result of high energy processes. Similarly the raw ingredients are not simple, even if they are commonly available.

Clay plasters have many complex qualities and require a proper understanding. Given that understanding they can perform to a very high level, and are durable and attractive.

Clay plaster preparation:

Clay plasters can either be made from local soil, or can be bought in a proprietary form. If in a proprietary form, they come dry bagged and are mixed with water to a plastering consistency, prior to application. If the plasters are being made on site or from local material there are a number of issues that must be addressed, such as the type of clay, the size of aggregate, the proportion of clay, the even dispersion of clay (a problem if the clay is wet), the addition of fibres etc. If this approach is to be taken, then soil analysis, good crushing and grinding machinery, possibly drying machinery and a lot of patience and knowledge are required.

All clay plasters can be stored dry indefinitely. If already mixed up with water the only material that will go off will be the fibre, and this will take several months.

Clay plaster application:

Clay plasters may be applied like any other plaster, by hand or by spray application. As with other plasters background suction must be controlled and for top coat application the suction should be even. A very important point to note is that clay plasters should not be overwetted as part of the plastering process. Too much water will increase the shrinkage on drying and then the plaster will crack or powder when it dries.

For tight plaster finishes a fine slightly damp sponge is used to consolidate the clay. For polished finishes specialist plastic trowels are used. If a steel trowel is used at this stage, then the clay can be marked.

If a mesh is required then hessian, or glass fibre is best. Metal beading (galvanised or stainless) can also be used, but the heavy metal beading common for gypsum plastering is not suitable. On angle beads in particular a thin wire bead should be used and the clay plaster should cover the corner bead entirely.

Clay plasters cure by the evaporation of water. If there is not sufficient background suction, heat or air movement then they will not dry, and will stay soft. After some time moulds may also develop particularly if there are natural fibres in the plasters. This is natural and can be reversed by improving the drying conditions.

Conversely if there is too much suction, heat or air movement the plasters can dry too quickly and this can cause a separation from substrate, and powdering of the surface. If this occurs then the product can we reworked as appropriate, either on the wall, or by being removed, knocked up again with water and reapplied.

Clay plaster protection:

Clay plasters can be used in all locations internally and externally without protection except where they are subject to direct spray. So for instance they could not be used in showers or just above sinks. Here tiling or some other impermeable material should be used in the vulnerable areas. Where clay plasters are subject to occasional water, then a coat of limewash silicate paint, or casein paint is usually sufficient.

Clay plasters can also be coated with matt emulsions, and other more conventional paints, so long as these paints are reasonably vapour permeable (a m factor of no more than 300). Non-vapour permeable coatings and harder coatings will cause problems in the long term for the plasters, and obviously effect the breathing qualities.

Where a wall is subject to hard use or frequent impact (such as in an entrance lobby, at floor level etc) skirtings, dado rails, or panelling should be used. Clay plasters have good impact resistance, equivalent to many lime plasters, but they are not robust for all situations.

It is also suggested that around cold windows, where condensation occurs frequently on the frames, that timber reveals are preferable to the use of clay plasters.

Author's Biography

Neil May studied Modern History at Oxford University and then an M.Phil. in Sociology at Delhi University, India. During this time he co-directed an award winning documentary film about the coalfields in Bihar. At the end of this period he returned to England where he worked for 4 years as a general building labourer before becoming involved with ecological and traditional building. He founded Neil May Builders in 1994, an award winning ecological and conservation building company. He founded Natural Building Technologies (NBT) in November 1999, a company that develops, and sells ecological building materials for both historic and mainstream construction markets.

With help from Philip Allen BSc CMISE

Recommended Reading

Conservation of Clay and Chalk Buildings by Gordon T Pearson: Donhead Publishing

Earth Construction Handbook by Gernot Minke: WIT Press

Earth Construction by Hugo Houben and Hubert Guillard: Intermediate Technology Publications