This is a guest post by Clare Whitney, Clayworks 

clay plasters

Beautiful clay plaster in neutral tone. Image credit: Nicholas Worley, via Clayworks

The humble clay plaster has evolved into a thing of beauty.

Clay is one of the world’s most ancient building materials – and one of the most naturally abundant. It is a natural mineral, deriving predominantly from Feldspar, one of the most common minerals on the earth’s surface. Clay Plasters are basically a mixture of clayey subsoil, aggregates and a natural form of fibre. The special qualities that led us to focus on them are the abundance of easily accessed raw materials and its ability to become plastic and malleable when wet and hard and water repellent when dry.

To this day, around the world, it is dug up on sites and applied directly to walls.

New techniques and geochemical understanding are making it a viable alternative to modern, high-energy materials, as it is now not only readily available in a ready mixed form but also reinvented as a contemporary and highly fashionable material of breathtakingly beauty.

Clay Plasters require minimal processing, contain no synthetic ingredients and only natural mineral pigments, but there are different subsets of clay, with different mineral groups, that behave differently when mixed with water: if the mixture of different clays in a plaster is wrong then it becomes unstable and will ultimately crack and fail. Contemporary clay plasters have undergone extensive research, development and testing and have been proven to be not only robust, but also easy to apply.

The Top 3 Benefits of Clay Plasters

  1. Natural beauty. The unique, earthy patina of clay plaster is breathtakingly beautiful with its sumptuous finishes and intricate detailing: it is capturing the hearts and minds of homeowners, designers and architects around the world. Clay plasters evoke a unique, emotional connection to a building.
  2. Design freedom. Because clay does not go through a chemical reaction when it ‘sets’ it will hold textures and sculptural shapes. It can also re-create the look of many fashionable interior wall finishes such as concrete, without harm to the environment or the building occupiers.
  3. Healthy air. There is increasing evidence suggesting links between indoor dampness, fungi and human health. Allergy UK recently emphasised the ‘importance of dry environments to limit the risk of asthma and other immune system related diseases’.  They added that the majority of adverse health effects are kept at bay by maintaining a relative humidity indoors between 40-60%. Clay plasters’ thirst for moisture means they can safely absorb and hold moisture vapour within their molecular structure when relative humidity levels are high and then release it back into the atmosphere when relative humidity levels drop.
clay plasters health and wellbeing

Other benefits to using clay plasters for your interior wall finish. Image credit: Clayworks

Healthy Buildings

Clay’s most prized characteristic is its ability to readily attract and take  moisture from the atmosphere and retain this moisture within its pore structure. Earth plasters are therefore hydrophilic (water loving) and hygroscopic (can safely hold onto this moisture). Furthermore, as it incorporates this moisture into its structure, it causes the clay molecules to expand, blocking the further passage of moisture through its structure. This forms a water resistant barrier. This self-sealing ability can prevent moisture from wicking into the wall substrate beneath, while still allowing moisture to move back out. This function will, however, be overridden if a constant stream of liquid water hits the surface.

clay plasters

Clay plasters offer incredible design freedom in terms of textures, patterns and colours

clay plasters low carbon

Using clay plasters is a low carbon option. Image credit: Clayworks

Ozone is a pollutant with known health effects such as respiratory illness, asthma etc. The by-products of the reactions that ozone has with many indoor products, including furniture, carpets, cleaning fluids and electromagnetic equipment can also be irritating or harmful. A recent study by Dr Richard Corsi and E Dowling of the University of Texas concluded that:‘The addition of clay plaster when carpet and ozone were present result in significant improved Indoor Air Quality and lower formaldehyde concentrations’. 

Formaldehyde is a Category 1B carcinogen, that can also cause skin, eye and respiratory irritation and while the authors of the study caution that further research is required, these early indications that clay plaster is a passive removal material for indoor pollutants is encouraging.

The moisture absorbing property of clay not only helps to prevent moisture from reaching the structural materials in the fabric of a building but also promotes humidity control within the building protecting the objects within it. Hence, museums, art galleries, libraries and archiving spaces are specifying clay plasters to help protect the items within.

Healthy Planet

The diagram to the right shows the amount of carbon saved when using clay plasters, compared to more typical wall and finish build up. The manufacturing process for clay plasters is very simple – just mixing – and there is relatively little energy consumed and no landfill waste as any unused materials can be returned to the ground. Clay plasters contain no synthetic ingredients and produce less carbon emissions during their extraction, production and construction than other building materials.

Clay Plasters are where health meets beauty, where science meets nature: they are super natural materials that are truly problem solving and now have the potential to reframe the interior wall finishes sector.

For more on the other uses and benefits of clay building, see our Super Natural Materials post on clay.

Have you been persuaded to use clay plaster in an upcoming project? Let us know on Twitter at @KoruArchitects.