This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong. # Clay & earth plasters  --- > "The skills needed to create and apply natural finishes are easy to learn and the tools required are minimal, most people quickly picking up the skills as though something inside them has been rekindled from former lives!" - James Henderson ## What are clay plasters? Clay or earth plasters are blends of unfired clays mixed with fine sand to provide a natural, breathable finish for internal walls. They have been used all over the world for thousands of years, and still exist in lots of traditional vernacular buildings. Clay plasters were often used in old buildings in the UK as a base coat, with a top coat of [lime](/lime/lime), as clay is cheaper than lime. <WRAP center centeralign> <figure>  <caption> Clay plasters add an extra dimension to living areas. </caption> </figure> </WRAP> Clay is a better description than earth, as earth contains aggregates (stone, gravel) and also living material. Clay is formed from the chemical weathering of feldspar rock, and is found in every country. It's a clayey subsoil that's used, not topsoil. Clay plaster can be used as a base coat when mixed with coarse sand, with hemp or straw binder chopped into it, and can be used with finer sands to produce a top coat. There are many different kinds of clays that can be used in plasters. Some clays are hard but not very hygroscopic (see 'benefits'), and some are softer but absorb more moisture. The best plasters will be hard and hygroscopic, and often the best way of achieving that is by blending clays. <WRAP center centeralign> <figure>  <caption> Clay plasterers at work. </caption> </figure> </WRAP> Clay plasters fell out of favour in the 20th century with the move towards synthetic, non-breathable building materials, but we're seeing a revival now, with more interest in natural homes for environmental and health reasons. Clay plasters are for internal use only, but can be used externally as a base coat for a lime top coat. ## What are the benefits of clay plasters? **Environmental benefits** - If you make your own base plaster, material can be sourced locally, with lower or no transport costs / emissions. {{ https://lowimpactorg.onyx-sites.io/wp-content/uploads/clay-plasters-bar.jpeg }} - Unlike gypsum (and lime, although lime re-absorbs CO2 on setting), no firing required, so lower energy use / emissions. - Totally non-toxic and biodegradable. - Clay doesn't set chemically like gypsum does - it just dries; so at least for the base coat, if it dries, you can wet it and knock it back up again, which means less waste. <WRAP center centeralign> <figure> {{url>https://www.youtube.com/embed/MpGVMg_Wh1s 560,315}} <caption> Demonstration of how to make and apply your own earth plaster. </caption> </figure> </WRAP> **Benefits for buildings** - Breathability: clay plasters prevent excess moisture in vulnerable buildings. Clays are hygroscopic - they absorb moisture if humidity is high, hold it without being damaged, and when humidity falls, release moisture back to the air. So they maintain a consistent humidity and minimise damp, and moulds caused by condensation. - Gypsum, on the other hand, absorbs moisture, but it's not breathable, and it doesn't let moisture out again, so it gets damaged in the process - via moulds, and / or the plaster 'blowing' (losing adhesion), and falling off. - Clays can be used with / over other breathable materials without taking away their breathability. {{ https://lowimpactorg.onyx-sites.io/wp-content/uploads/clay-plasters-pattern.jpeg }} - Flexibility: clay is soft and pliable, and can move, like lime, without cracking; it can cope with situations where you get minor movements, and so is excellent in old houses. Gypsum isn't flexible, and cracks with any minor movements. - Protection of timbers: lime absorbs moisture through capillary action, but with clay, there's a chemical attraction that draws moisture in - and because clay absorbs moisture at a faster rate than lime or timber, it takes moisture away from the timbers of the building, protecting them from damp, mould, rot and insects. <WRAP center centeralign> <figure> {{url>https://www.youtube.com/embed/mql2YX714dc 560,315}} <caption> Making clay plaster from on-site clay. </caption> </figure> </WRAP> - As clay absorbs moisture, it seals and prevents water from passing right through into the structure of the building. Moisture is held and released when the air around it dries. This is why clay can be used to line a pond - clay allows so much moisture in and then provides a barrier. - Aesthetic: clay plasters are beautiful in their texture / the way they reflect light, and they have an organic, natural feel. {{ https://lowimpactorg.onyx-sites.io/wp-content/uploads/clay-plasters-cottage.jpg }} **Health benefits** - Clay plasters maintain a consistent relative humidity of around 50-60%, which is beneficial for people with asthma or other respiratory problems, but detrimental to dust mites - which reduces problems associated with allergies. - Clays contain no nasty chemicals, and are able to absorb toxins and thus improve indoor air quality. Even if you avoid toxic [cleaning](/ntcl/natural_cleaning) / househould products, in this day and age, it's difficult to avoid toxins completely, and so clay plasters can help. - You have to be in a room with a clay plaster to understand how beautiful, sensuous, natural, earthy (add your own epithet here) they are. They just make you feel good. <WRAP center centeralign> <figure>  <caption> Clayworks pigmented top coat plaster with colour chart. </caption> </figure> </WRAP> **Other benefits** - Clay plaster is a product that can be produced locally, without benefitting the extractive corporate sector. ## What can I do? Learn as much as you can first. See our [further info section](https://www.lowimpact.org/categories/clay-earth-plasters/further-info) for more details on application and maintenance of clay plasters. You can buy base coat and top coat plasters, or you can have a go at making a base coat (bought plaster will be much finer, so it may be better to buy the top coat). First test whether you have clay. The field test is that clay is sticky and can be rolled into a sausage shape. See _Using Natural Finishes_ and other books for more detailed tests, plus lots more information on making and applying clay plasters. {{ https://lowimpactorg.onyx-sites.io/wp-content/uploads/clay-plasters-room.jpg }} The clayey earth is dug out of the ground and then soaked and sieved to remove aggregates. What's left is clay. Let that stand and keep taking water off the top until it dries. Clays are not uniform, so they will differ in composition depending on locality. This may take a little experience. Mix the clay with coarse sharp sand for a base coat - roughly 3 parts sand: 1 clay (and chop in straw or [hemp](/hemp/hemp_building) for binder, which helps with shrinkage and stops it cracking when it dries). Then add water until you have the right consistency for a plaster. For a top coat, use the same ratio of sand and clay, but use a finer grade sand, like silver sand. <WRAP center centeralign> <figure>  <caption> Clay plasters can deal with humidity and add beauty to kitchens. </caption> </figure> </WRAP> Apply 10-15mm of base coat, and 3mm of top coat. Apply the top coat in thin layers, and don't over-trowel, as it brings the moisture to the surface, with makes it liable to crack as it dries. Lay the base coat on with a steel float, then scour with a wooden / plastic float, which compresses the surface and gives a good hard surface for the top coat. Apply the top coat with a metal float, then burnish with a plastic trowel - steel might mark the pigment, but plastic doesn't (and it doesn't bring water to the surface as much either). If you're plastering over 2 different surfaces, e.g. brick and [cob](/cobb/cob_building), to prevent cracking, it may be a good idea to use hessian or glass fibre mesh. Apply the base coat, then push the mesh in, and trowel over it. Clay plasters can be painted with [breathable paints](/ntpa/natural_paints), but nowadays you can also buy more refined top coats that will provide a finer finish and can be bought already containing coloured natural pigments, saving time and money. <WRAP center centeralign> <figure>  <caption> Clays, pigments, tools & binder. </caption> </figure> </WRAP> Clay plasters have good impact resistance - but not to constant banging / rubbing, so they may not be ideal for areas of high traffic, or for hospitals or schools. But they can be used if there's a handrail to stop too much contact. They're certainly be a good idea in public buildings, because of their health benefits. ## Further resources - [Tips & tutorials](https://www.lowimpact.org/categories/shelter/clay-earth-plasters/further-info/) - including preparation, application and recipes - [Clay plaster pros & cons, and where to use it](https://www.homebuilding.co.uk/advice/clay-plaster) - [How to make earthen plaster](https://www.themudhome.com/earth-plaster.html) ## Specialists clay/clay_plasters.txt Last modified: 2025/12/10 18:06by 127.0.0.1