Earth plaster

Earth plasters (also known as clay plasters) are currently enjoying a revival, for both internal and external use, after a long history in traditional buildings around the world.

The earth-sheltered building by Glenn Howells Architects at Gloucester Services on the M5 has reinvented what motorway services can be, and how they interact with the local economy, community and food suppliers. It was the national winner of the Civic Voice Design Awards in 2015. The walls are finished with pigmented clay plaster by Clayworks, which helps to regulate humidity.

The resurgence in the use of earth plasters is partly in the form of using home-made, locally resourced materials, and partly due to an emergence of ready to use, pre-manufactured earth plasters from both the USA and Europe. In this article we use the term ‘plaster’ to denote both internal and external work.

Earth plasters, prepared at their most basic level, from clayey subsoil, aggregates and a natural form of fibre, would have been the first method of weatherproofing for primordial structures.

This would have been due to the abundance of the necessary materials and the special qualities of the clay, specifically its ability to become plastic and malleable when wet, and hard and water-repellent when dry. Earth plasters have been used all over the world, in all climates, since shelter building began.

In the UK, earth plasters were routinely used on internal wall surfaces alongside lime, up until the end of the 19th century. Their regular and common use was due to the local abundance of the materials, and the fact that building lime and high-quality aggregates were comparably expensive and sometimes difficult to obtain. Often they were used as an undercoat, to even out the wall surface before receiving a top-coat of lime plaster/ render or limewash.

Throughout the rest of the world the use of earth in plasters also has a rich history. In both Africa and the Americas the application of earth plasters on to earth structures was traditionally women’s work. The term enjarradora describes women in the south-west states of America who carried out these earth plastering techniques. Their methods of application and the materials used were highly specific from region to region, and these traditions are still very much alive. In Africa, special relief work and painting carried out in earth plaster on to individual homes (known as litema) remains a culturally significant way for people to decorate their homes, defining their identity within the wider community.

There are many benefits to using earth plasters. They are porous, so they allow a building structure to breathe, acting as a third skin around the building and its inhabitants. Earth plasters have a thirst for moisture. This means that they can regulate levels of relative humidity in the atmosphere. They can safely absorb and hold moisture vapour within their molecular structure when relative humidity levels are high, and release it back into the atmosphere when relative humidity levels drop. As well as providing high-quality internal air for the inhabitants, this mechanism also serves to protect the building fabric from moisture.

Due to clay’s thirst for moisture, and its ability to hold on to it, it will help to prevent moisture from reaching materials that provide the structure of the building. It will actively draw moisture away from these materials if they do get wet. For this reason, earth finishes are the perfect option for structures made of the softer, breathable materials, such as earth, straw-bale and wood. Earth plasters are now also being used in museums and galleries because of these characteristics, to regulate humidity and hence help to protect old and new works of art. An example of this is the Museum Kuppersmühle in Germany.

Earth plasters, if made without synthetic additives, are entirely non-toxic, making them ideal for those with chemical sensitivities. They even have the ability to absorb toxins from other materials and bind odours, such as cigarette smoke, decreasing levels of indoor pollution. Research is currently being carried out in Germany into their ability to screen electromagnetic radiation, such as from computers and mobile phones.

On a more subjective and unquantifiable level, earth plasters generally feel good. They are soft to the eye, and can moderate temperature swings, making walls feel warm in winter and cool in summer. They work acoustically to soften and round off sound, creating peaceful, calm spaces.

Like most materials made from natural ingredients, they demand to be used respectfully and appropriately. They are often not generally conducive for external use, unless positioned in a very sheltered area, used in a predominantly dry climate, or used in conjunction with special building design details, such as an external wrap-around porch.

If making an earth plaster from materials sourced on site and/or locally, the user must be prepared to become his or her own detective, testing and feeling their way to a good mix through trial and error. If prepared this way, they can be inexpensive, but labour- and time-rich. The alternative is to purchase a ready-made, pre-manufactured earth plaster, which requires only the addition of water. These will provide consistent and excellent results (as long as all other due considerations are carried out effectively), but can be more costly.

There are wide variations in clay over the world. For this reason, different craftspeople working with earth plasters will sometimes hold different opinions on recipes and methods of application. There is therefore no single formula that can be prescribed for success in all situations. This is perhaps one of the most exciting reasons to embark on using earth plasters.

Earth plasters are both dynamic and timeless. They can be used to create the most exquisite, contemporary, fine plaster finishes, but they are equally at home in rustic, organic and traditional environments. Clay subsoil, the main binder for creating these finishes, far from being just the earth beneath our feet, is a chemically complex material. Yet it produces a finish that is easy to handle, and safe and simple to use. The use of earth in buildings could make a significant contribution to creating structures that are healthy, long-lasting, and which tread gently on the planet.

Clay is but one element that makes up the soil covering the earth’s surface. The science of soil formation is complex. It brings into play many different elements that come together to form different types of soil in different parts of the world. Soil is formed over time through the disintegration of the rocks making up the earth’s crust. They are weathered both mechanically and chemically. Mechanical weathering involves the action of wind, agriculture and extreme weather conditions, such as heating and freezing. These shatter the rocks into smaller components. Chemical weathering involves the action of rainwater, charged with acids, which seeps into the soil and works a transformative magic to break up the parent rock and turn it into earth.

Clay is a mineral component of the soil. It predominantly originates from the weathering of feldspar, one of the most common minerals in the earth’s crust. Clay is found in a band of soil known as the subsoil, so named because of its position underneath the topsoil. The topsoil is formed from organic material, derived mostly from decaying vegetable matter. It is the fine, dark, organic layer in which gardens are cultivated. The topsoil is not suitable for making an earth plaster with.

Along with clay, the subsoil also consists of particles of sand and silt. Sand, silt and clay are classified according to their particle size. These can be defined as (although authorities differ slightly): gravel 2mm to 75mm; sand 0.05mm to 2mm; silt 0.002mm to 0.05mm; and clay less than 0.002mm. Soils vary with regard to their proportions of sand, silt and clay, and this will have implications for the suitability of the soil for use in an earth plaster. For the purposes of making a sound earth plaster, the soil must consist of at least 10–20 per cent clay minerals, and silt should be present in quantities of no more than 25 per cent of this clay element.

Clay minerals are present in a wide variety of forms. They belong to a large family and can be characterised by their layered, crystalline structure. There are three main members within the clay family, known as kaolinite, illite, and montmorillonite, with other transitional forms occurring within these: examples of these include kaolin, mica, and smectite respectively. These different clay mineral groups vary with regard to their chemical make-up, and hence all behave slightly differently when they come into contact with water.

This behaviour is an important determining factor when selecting suitable clay for making up an earth plaster. This is because it has direct implications for how much the clay will expand and contract with water, which has a direct correlation with how much an earth plaster will shrink as it dries, how much cracking will occur in its dry state, and how it will interact with water once applied to the wall as a protective finish.

To understand which of these clays are most suitable for use in an earth plaster, and to understand how clays behave when they come into contact with moisture, it is advantageous to become aware of some of the more basic chemistry involved in their functioning.

Clay minerals derived from feldspar consist predominantly of microscopic particles of aluminium and silicon dioxides. These aluminium and silica molecules are shaped like plates, which are alternately stacked on top of each other. The alternate stacking between the aluminium and silica platelets creates an electrostatic charge between them. This electrostatic charge chemically attracts water into the spaces between the platelets. This is why clay is considered to be hydrophilic (water-loving).

The water acts as a bridge between the aluminium and silica platelets, bonding them tightly together to form a cohesive structure. When the spaces between the platelets are filled with these fine films of water, the platelets have the ability to slide over one another. This is why clay feels so smooth to the touch and is easily worked and moulded when wet. Clay expands when the spaces between the platelets are filled, and contracts when the water exits and the clay dries.

The presence and organisation of the aluminium and silica platelets determines a clay’s tendency to attract and absorb water into its structure. These differences can be exhibited by the distinct behaviours between the main clay groups mentioned above. This is why some clays are more stable, and hence more suitable for clay plasters, than others. Bentonite, for example, is a very unstable clay. It will absorb large amounts of water into its structure, and expand and contract excessively, causing it to crack when it dries.

Clay has the ability to relinquish any absorbed moisture as quickly as it was taken on. This is through the process of evaporation. When the water evaporates from the body of the clay, the platelets are pulled closely to one another, hence the characteristic nature of clays to shrink when they dry. But the fact that the platelets will remain in the same shape that they have been moulded into, even when they dry, is beneficial to anyone working and sculpting with clay.

Unlike other binders used for plasters and renders, such as lime and cement, clay does not undergo a chemical transformation as it dries and cures. This means that it can be indefinitely re-wetted and reworked, and ensures that it can be infinitely recycled as a building material. But it also means that, although good for areas of high humidity (water vapour), it cannot be used on walls that come into direct and prolonged contact with liquid water.

The mechanisms with which clay minerals react with water can have beneficial implications for their use within an earth plaster. This is specifically the case when building with breathing materials, which rely on finishes that will avoid the build-up of moisture, through their ability to breathe similarly. As has been established above, clay will absorb moisture into and out of its open-pore structure. Unlike lime, however, which is also a breathing material, clay will self-seal itself on contact with moisture. This is because of the tendency of the water molecules, coming into contact with the clay, to bind themselves to the surface of the clay platelets and cause them to expand. This swelling closes up the spaces between the platelets, preventing the further passage of moisture through the full thickness of the earth plaster. The moisture is held here until conditions are dry enough for it to evaporate out safely.

This prevents the moisture from being wicked into the underlying structure, as would be the case with cement, and can be the case with lime. The traditional method of using clay to line a pond provides a clear demonstration of this process at work.

This mechanism makes earth plasters resistant to water, meaning that they can to a certain extent resist the passage of moisture through them (unless exposed to a constant flow of liquid water). This must not be confused with the concept of being ‘waterproof’, which would imply that moisture was unable to penetrate into its structure at all, like a waterproof paint.

The self-sealing ability of earth plasters is highly beneficial for their interaction with moisture vapour (small molecules of moisture suspended in the air) where there is a gradual and gentle feed of moisture into its structure. This is why they are excellent in humid conditions, such as in bathrooms and kitchens. When an earth plaster is exposed to consistent driving rains (moisture in the form of liquid water), however, the clay in the earth plaster will become saturated, and hence its water-storing function will be overridden. In this circumstance, the earth plaster will inevitably begin to deteriorate, as it moves from a dry state into a plastic state and ultimately into a liquid state. At this point the clay molecules will be forced apart and the plaster unable to hold its form.

An earth plaster can be used externally, as long as there is enough time between rain showers for the plaster to dry out fully, and as long as other building design details are in place. There are also certain ingredients that can be added that will go some way to improving durability. In the damper climate experienced in many parts of the UK, it is not generally recommended that they be used except in special circumstances. In all cases, an earth plaster should not be used as the final protective coat for a weather-facing wall (facing south-west in the northern hemisphere), as repeated maintenance will be needed to ensure that it continues to function as a protective coating to the wall substrate below it. A top-coat of lime render can be applied on to base coats of earth plaster to improve weathering. Numerous coats of limewash, or another type of breathable paint, applied directly to the earth plaster will also add protection, but these will similarly need regular maintenance.

When an earth plaster is used internally in areas where there is direct contact with liquid water, it must also be protected. An example of this would be covering the area around baths, sinks or showers with ceramic tiling, or oiling and waxing the plaster.

The hydrophilic (water-loving) nature of clay also means that it will absorb moisture from building materials, such as wood and straw, and allow it to evaporate safely into the atmosphere. This will help to retard premature decay. It can also act as a protective barrier, preventing excessive amounts of water from accessing these materials.

Clay is not the only ingredient of an earth plaster. If it were, the material would be inherently unstable. It would be in a constant state of taking on water and expanding, and then relinquishing this water and contracting. In such a case an excessive amount of cracking would occur in the dry material. This would create a weak plaster, unable to fulfil its protective function. The additions of aggregates and fibrous matter are therefore vital for stabilising the clay in order to provide a durable and well-functioning covering. With the addition of these ingredients, it is possible to create a plaster that can benefit from the positive attributes of the clay without being dominated by them.

This article originally appeared as ‘Earth plasters and how to use them’ in Context 143, published by the Institute of Historic Building Conservation (IHBC) in March 2016, it was written by Adam Weismann and Katy Bryce, who run the Cornwall-based company Clayworks, manufacturing natural clay plaster for the architectural and interior design industries.

--Institute of Historic Building Conservation