Clay plasters, renders and alises

To combat climate change we need to draw carbon out of the atmosphere. The way to do this in building is to use carbon-sequestering materials. Plants and trees take up carbon dioxide as they grow via photosynthesis. If we use them in our buildings, this carbon is then stored. Low density natural building methods such as light earth, hempcrete, and straw bale are good at locking up carbon, as is any timber that is used. To finish these walls (at least on the inside), a natural clay plaster can be used. This is a beautiful, soft and breathable finish that has many benefits..

Benefits include

● Clay introduces thermal mass into buildings and if designed according to passive solar principles this can aid in thermal regulation i.e. free night time heat in winter and lunch time ‘coolth’ in summer
● Clay is hygroscopic. That means it can absorb high humidity from the air, and release it when conditions reverse. The result is that internal humidity is moderated leading to a more uniform and comfortable internal environment. Clay also moderates the moisture content of adjacent cellulose materials such as timber, or plant fibres, which reduces their likelihood of decomposition
● Clay is highly vapour permeable or ‘breathable’. This means it allows the ready transmission of water vapour, and so prevents the build up of moisture within the walls. For this reason surface coatings with low water vapour permeance may not be used on earthen surfaces
● Clay is non toxic and can even absorb toxins and neutralise odours from the environment
● Clay is EMF shielding. This means it can dampen the electromagnetic fields that are ever present in our urban environments
● Clay is a natural negative ion air purifier. That means it creates healthy negatively charged air like that find at beaches, lakes, rivers, mountains and forests
● Clay is abundant on our planet, and if it's obtained locally it has very low embodied energy from the minimal digging, transporting and processing. It can also simply and safely return to the earth at the end of the building's life, without causing any pollution.

Clay plasters can be applied over many surfaces, and clay renders can be used eternally on walls well protected from the weather. Clay paints or alises can also be used for decorative effect on protected walls.

Clay plasters are covered in the Earth Building Standards.

Points to consider

● Clay plasters can be applied over cob, adobe, rammed earth, light earth, straw bale, hempcrete, timber lath (use heavy straw clay mix), reed matting lath, plastic geogrid mesh lath, or even plasterboard (with the correct key)
● Usually applied in two or three coats. Scratch and moisten between each coat. Scratch coat applied first up to 20mm thick, and finish coats applied 3-6mm thick
● Composed of clay, sand (well graded and clean), plant or animal fibre and water
● Use more clay in base coats to get plaster to stick, and more sand or straw in finish coats to avoid cracking
● Can add horse or cow manure, casein-borax glue, paper pulp cellulose (watch shrinkage), or wheat paste (wallpaper size) to increase durability and reduce dusting
● Natural mineral pigments, or mica can be added to the final coat for aesthetic effect
● Mesh on slip layer required over dissimilar materials, such as timber framing
● Let each coat dry before the application of the next one to avoid cracks, stains or mould
● Trowel on or apply with heel of hand in an upward motion for better waterproofing
● Splash backs are required to areas subject to water splash. Clay plasters are not appropriate to wet areas such as showers
● Plastering is messy work so tape up and cover glass, woodwork, floors and furniture
● Burnish plaster with a damp sponge for a smoother finish, and to expose the straw and aggregate as it starts to set hard
● Don’t apply plaster in direct sunlight
● Clay is available by the bag in powder form, as clay putty from potter supply stores, or often directly from the ground beneath the topsoil. To process damp clay dug fresh from the ground or clay putty, slake with water and whisk with a paddle drill to create thick creamy mousse. If the clay is in big chunks but 100% bone dry, it will break down when added to water. This removes the need to break it up into little pieces. You will want to remove the larger aggregates before plastering. Powered clay is the easiest to process. Simply add water and whisk to create thick cream.
● Always soak clay for at least 24 hours (perhaps some time longer for green clay) before use, to get it nice and sticky

Further reading

● Japan’s Clay Walls by Emily Reynolds, 2009.
● Essential Natural Plasters by Michael Henry & Tina Therrien, 2018.
● The Natural Plaster Book by Cedar Rose Guelberth & Dan Chiras, 2003.
● Using Natural Finishes by Adam Weismann & Katy Bryce, 2008.

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