Clay Plasters - Natural, Healthy, Desirable

By Steve Ruggier at Mike Wye and Associates

Earth, adobe or clay plasters are as old as construction itself. Clay pastes were used to create cave paintings in the earliest forms of decoration and the earliest roofed dwellings were made from rammed earth or a lattice of sticks plastered with a clay daub. Throughout the world clay has been used as either mass for building walls or as a plaster for wooden lattice or straw constructions in areas where it can be locally sourced.

The term clay refers to the binding agent and clay plasters are a mixture of binder, aggregate and straw or other fibres often with additional materials such as casein or corn starch added to increase the adherence of the plaster. Clay particles are cohesive and bind to the sand and straw, holding the plaster together, as well as securing the mixture to the wall. Clay is also plastic when wet, which makes the plaster mixture workable. Sand provides structure, strength, and bulk to clay plasters whilst fibres form a reinforcing meshwork which helps to hold the plaster together. Fibre also provides some flexibility to a dried plaster and reduces the risk of shrinkage and cracking.

There has been a resurgence in interest in clay plasters and most particularly thin layer coatings. For those concerned with indoor air quality, a clay plaster is a great choice. They are completely non-toxic and do not off-gas in the wet or dry state. They involve no petroleum products or other chemicals in their manufacture  and have been shown in German research to have some effectiveness at absorbing and transforming pollutants in the household air. The same research shows them to have excellent moisture-handling properties, helping to regulate humidity in the home.

In addition to the colourfast properties of clay plaster surfaces, the beneficial effect of adequately large clay surfaces on indoor room climate remains a key advantage of earth-based materials over conventional plaster products.

Plasters made with clay are beautiful, durable, and made from inexpensive and non-toxic ingredients. Clay creates a breathable finish material with a natural capacity to regulate moisture and temperature in the surrounding air. Clay naturally absorbs excess moisture in the air, helping air to feel more comfortable in summer, while its high thermal mass creates a battery heat, or cooling, storage that helps to maintain constant air temperature.

Clay plasters not only have excellent vapour permeability 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 absorption. Because the rate at which clay plasters absorb moisture is much higher than that of other materials, clay plasters can act to protect vulnerable organic materials, and in particular timber, from high levels of relative humidity, when microbial and insect attack can be triggered. This can be an important strategy in the control of excess moisture in vulnerable buildings, threatened by increases in moisture level caused by showers, general indoor living, draught-proofing measures and the switch from open fires to central heating in particular. Clay plasters 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.  

Clay plasters are reversible and can be easily reworked to effect repairs. Because of their 'breathable' qualities they can protect more vulnerable parts of structures, and absorb large amounts of moisture, salts and pollutants where these are present.

Unpainted clay plasters have a very particular aesthetic. Due to the shrinkage of the clay particles on drying, the plasters have an open texture even when polished. This means that light reflects and refracts on the surface in such a way that there is always variation and never a gloss sheen. This is particularly noticeable in the modern self-coloured thin layer plasters that are becoming increasingly popular.

However, the main reason for the demand in thin layer clay coatings is neither the physics of the material nor the building technique but the magical qualities of the materials colour and depth. In Europe, coloured clay and earth surfaces do not have a long tradition. Instead, the structure and construction has traditionally featured more strongly, for example in half-timbered construction, rammed earth walling or wall linings. In Asia, by contrast, there is a long tradition in clay plaster surfaces, and the technique has been taken to a degree of perfection.  

Plastering systems for use in combination with the thin-layer clay coatings are playing an increasingly important role. The development of thin-layer clay coating systems, where the plasters are applied at a thickness of up to 3 mm, are enabling the use of clay plasters on a wide range of substrates. These usually comprise fine-finish or reinforced plaster mortars for creating an even substrate often in combination with clay panels or wood fibreboards.  

Clay plasters can be applied directly over most existing wall surfaces and finishes, including plasterboard and panelling. Clay plaster relies on mechanical adhesion and requires a rough surface to allow the plaster to grip the wall. Where the substrate is smooth or has high or uneven suction a primer will be required. The plaster can be re-coated in the future, or can be painted over with conventional or clay-based paints.  

For many architects, the specific quality of plasters made of coloured clays offers new design possibilies. They choose them to achieve a particular colour or spatial effect. The colours of the clay from different geological formations have a special warmth and sense of depth that lasts without fading, peeling or ageing. Such interiors are both sustainable and colourfast.

The various different techniques for creating a suitable plaster base has made it possible to use clay building products on almost all kinds of internal wall surfaces, whether made of concrete or clad with plasterboard or composite-wood panels. For the end user, however, a key criteria is the quality of the visible (thin) surface. The effect of the material in the overall spatial concept or within a room now plays as important a role as its building biological and ecological credentials. The healthy and environmental qualities of earth and clay building products are now well known, and for many the ecological argument for earth building products - as for other natural building materials is taken as given. It is therefore increasingly the aesthetic qualities of earth and clay surfaces that set them apart from other materials.

Thin coat clay plasters are being used in hotels, art galleries, museums and retail spaces. It is in the nature of clay plasters and coatings that their beautiful but subtle surface qualities are seen and appreciated in passing but not always noticed as being clay plaster.  

A fine example of the use of clay plaster in hotels is the 4-star bora HotSpa Resort in Radolfzell am Bodensee, Germany. The timber structure of the hotel was designed by the Milanese architect Matteo Thun, while the Munich based architecture office Bruno Franchi designed the interiors. The foyer and wellness areas were plastered with Claytec YOSIMA coloured clay plaster. In the spa area, Bruno Franchi specified typhaboard, a comparatively new product made of compressed typha reed, as a plaster base. In combination with a suitable primer, YOSIMA clay plaster, applied here as a 2 mm thick coat of white plaster, proves to be an ideal partner for this unusual plaster base.

The consistent colour and quality of the clay plaster surfaces throughout the interior of the hotel has proved to be a winning formula for both the designers and the clients.

Munich has many well-known museums but the Lenbachhaus is a gallery of world renown thanks to its unique collection of works by the group of artists known as the Blaue Reiter. After extensive conversion and renovation works designed by the British architects Norman Foster Associates, the Lenbachhaus was reopened in 2013.

As part of the conversion works, two rooms of the collection were given an application of brush-applied plaster. The plaster build-up of undercoat clay plaster, topcoat clay plaster and coloured clay fine-finish plaster as well as providing an aesthetic counterpoint to the artworks on display also helps to regulate airborne moisture, contributing to the long-term protection of the priceless artworks.

This project illustrates the clear advantages that clay plasters offer for such situations, and the end result is a unique colour concept within the overall concept of the exhibition spaces.

The Apple Watch store in Selfridges, London has the shop window areas clad with a special white tone from the YOSIMA palette of clay plasters providing a signature backdrop for its striking window displays.

Thin-layer clay plasters and coatings made of natural, geologically-occurring minerals are durable, easy to repair and exhibit lasting colour. The broad range of colours, 140 in the YOSIMA pallette, now available for clay plasters has made clay plaster attractive for projects with a stronger design focus.  

The breathability, hygroscopic characteristics and aesthetic qualities of clay plasters offer a healthy indoor room climate and a material that compliments and benefits the fabric of the property. Whether in a commercial or domestic setting clay plasters have a lot to offer and deserve consideration for inclusion in any project.


[1] M. Lemke Modern thin layer applications of clay plasters for sustainable coloured interiors (2016)