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Loam plasters are breathable plasters made in different layers of loam enhanced with aditives filling- and reïnforcement materials.
Allthough they are refered to as Clay render, COB, dirt wall, earthen plaster, etc. I prefer to use the "Loam" as this seems to be scientifically the most correct term. |
Understanding loam
To understand loam and it's behavior, you have to know what ingredients are forming loam and what function they have.
When you dig loam from the soil you'll find 3 particles mixed together:
Clay
The binding agent of a loam plaster is clay.
It's the 'sticky' part of the mix.
Clay particles have the shape of plates. When clay is compressed and smoothened out, the plates are laying in the same direction and (almost) touch eachother. Depending on the type of clay and the pressure applied, this can results in a water resistant behavior. Water is necessary to make the clay particles stick together. It also means clay will absorp water and swell.
Adding more clay to a mix will make it more sticky, smoother, more water resistant and dustfree.
Sand
Sand is the filling particle in a loam mix.
It's hard, doesn't absorp water and has different shapes.
For a loam mix you need an irregular shaped grain as this type of grain will form a stronger structure.
Imagine having to pile up balls or bricks, whats's more stable?
Also a mixture of different grain sizes help enhancing the ridgidity of the mix. Ususally you use a grainsize up to 2mm, the smaller the size, the smoother the surface finish of your plaster.
Adding more sand to a mix makes it more stable, harder and prevents cracking.
Silt
Silt is a particle you find in almost all soils. This particle looks like a clay particle, but is slightly bigger (still smaller than sand).
When you hold silt between your fingers if feels slippery like clay (the plates glide on eachother), but it doesn't stick.
Silt is a particle without any use in a loamrender.
Finding the perfect balans between sand and clay is the art of mixing loam.
When the clay content is high, you have a "greasy loam" otherwise you would call it a "sandy loam".
It's always better to have a greasy loam from the start, because it's easy to add sand (and difficult to add clay) to the mix.
You can find more information on how to know the amount of clay and sand in your soil here
When you dig loam from the soil you'll find 3 particles mixed together:
Clay
The binding agent of a loam plaster is clay.
It's the 'sticky' part of the mix.
Clay particles have the shape of plates. When clay is compressed and smoothened out, the plates are laying in the same direction and (almost) touch eachother. Depending on the type of clay and the pressure applied, this can results in a water resistant behavior. Water is necessary to make the clay particles stick together. It also means clay will absorp water and swell.
Adding more clay to a mix will make it more sticky, smoother, more water resistant and dustfree.
Sand
Sand is the filling particle in a loam mix.
It's hard, doesn't absorp water and has different shapes.
For a loam mix you need an irregular shaped grain as this type of grain will form a stronger structure.
Imagine having to pile up balls or bricks, whats's more stable?
Also a mixture of different grain sizes help enhancing the ridgidity of the mix. Ususally you use a grainsize up to 2mm, the smaller the size, the smoother the surface finish of your plaster.
Adding more sand to a mix makes it more stable, harder and prevents cracking.
Silt
Silt is a particle you find in almost all soils. This particle looks like a clay particle, but is slightly bigger (still smaller than sand).
When you hold silt between your fingers if feels slippery like clay (the plates glide on eachother), but it doesn't stick.
Silt is a particle without any use in a loamrender.
Finding the perfect balans between sand and clay is the art of mixing loam.
When the clay content is high, you have a "greasy loam" otherwise you would call it a "sandy loam".
It's always better to have a greasy loam from the start, because it's easy to add sand (and difficult to add clay) to the mix.
You can find more information on how to know the amount of clay and sand in your soil here
Layers
A loam plaster is always applied in several layers. Every layer has it's logic function.
Before aplying a new layer you'll have to let the previous layer completely dry, then make it wet again several times (one time the day before, on time in the morning and one time just before applying the new layer).
This will prevent the previous layer from sucking the water out of the new layer and therefore it prevents cracking.
Water is also crusial to make the bonding between clay particles of different layers.
Generally there are three layers:
Before aplying a new layer you'll have to let the previous layer completely dry, then make it wet again several times (one time the day before, on time in the morning and one time just before applying the new layer).
This will prevent the previous layer from sucking the water out of the new layer and therefore it prevents cracking.
Water is also crusial to make the bonding between clay particles of different layers.
Generally there are three layers:
The "sticky" layer
The first layer of loam is determined by the construction and material of your existing wall.
When you have a rough wall that sucks up water easily (rough brick wall, COB wall, ...) you might not need a sticky layer. But when your wall is smooth and "water repellent" (polished concrete, smooth hydraulic lime, ...) you'll need to apply a layer that will form a bridge between the existing wall and your actual plaster.
On smooth surfaces and surfaces that are hard to cover with loam you can make a sticky layer with Wheat paste. Scroll down to the 'Additives and coatings' section on this page for the recipe.
When you are working on a strawbale wall, clay hemp or light straw-clay wall you don't need to use a wheat paste. Just make a very wet mix of loam, with a very low content of sand and no fibres. It should have the consitancy of oat meal porridge.
Apply the first layer of loam (< 1cm) . If necessary you can use long fibres of straw to reinforce the mix at certain points (eg. holes in the wall).
Push and massage the loam on top of the wall. When applied on strawbales it is really important to push it hard inside the strawbales to get a maximum adhesion.
Don't worry about bumps yet! You'll level them out later. This layer doesn't need to have a certain thickness.
When the first applied layer is very smooth you can roughen it up by adding scratches, this will guarantee a better (physical) bonding with the next layer.
This layer follows the exisiting wall and doesn't have any other function than bonding the next layer of loam to the wall.
The "fill" layer
As the name imposes, this layer will fill up all irregularities of the wall to get the bumps out.
You'll need a high content on straw.
Use long fibres of straw to fill out deep recessed (more than 2cm) and short straw on all the remaining wall.
Try to add as much fibres as possible, but take in mind if you add too much, the loam will not attach to the layer underneath.
Because this layer will also hold the finishing layer you don't want cracks.
The sand/loam ratio is difficult to determine theoretically as it depends on the clay-content in your soil and also depends on how much fibres you are adding to the mix.
The best way to determine your mix is to make test samples:
- Apply different samples in a 2cm thick layer to a wall and let it dry.
- When you apply it, it should stick to your metal trowel held vertically, yet it should easily be flicked away.
- If the plaster sample comes off in one piece when fully dried (2-4 days) or you have cracks you should add sand
- If the plaster sample comes off in portions you should remove sand from the mix
The finish
In this layer you add less straw and a bit more sand (fine, not coarse!). By reducing the straw, you are able to get the surface smooth with a trowel. By adding fine sand you'll get a harder surface finish. Always test-drive your mix on different samples to see what clay / sand ratio is working out best for you.
When "leather dry" use a humid sponge to wipe the surface.
After sponging you can use a brush with poneyhair to generously brush the surface. Both actions will prevent loos grains (usually sand) on the surface and make it easier to maintain.
When the plaster is fully dried you can add a colour layer (paint, lime, ...)
The first layer of loam is determined by the construction and material of your existing wall.
When you have a rough wall that sucks up water easily (rough brick wall, COB wall, ...) you might not need a sticky layer. But when your wall is smooth and "water repellent" (polished concrete, smooth hydraulic lime, ...) you'll need to apply a layer that will form a bridge between the existing wall and your actual plaster.
On smooth surfaces and surfaces that are hard to cover with loam you can make a sticky layer with Wheat paste. Scroll down to the 'Additives and coatings' section on this page for the recipe.
When you are working on a strawbale wall, clay hemp or light straw-clay wall you don't need to use a wheat paste. Just make a very wet mix of loam, with a very low content of sand and no fibres. It should have the consitancy of oat meal porridge.
Apply the first layer of loam (< 1cm) . If necessary you can use long fibres of straw to reinforce the mix at certain points (eg. holes in the wall).
Push and massage the loam on top of the wall. When applied on strawbales it is really important to push it hard inside the strawbales to get a maximum adhesion.
Don't worry about bumps yet! You'll level them out later. This layer doesn't need to have a certain thickness.
When the first applied layer is very smooth you can roughen it up by adding scratches, this will guarantee a better (physical) bonding with the next layer.
This layer follows the exisiting wall and doesn't have any other function than bonding the next layer of loam to the wall.
The "fill" layer
As the name imposes, this layer will fill up all irregularities of the wall to get the bumps out.
You'll need a high content on straw.
Use long fibres of straw to fill out deep recessed (more than 2cm) and short straw on all the remaining wall.
Try to add as much fibres as possible, but take in mind if you add too much, the loam will not attach to the layer underneath.
Because this layer will also hold the finishing layer you don't want cracks.
The sand/loam ratio is difficult to determine theoretically as it depends on the clay-content in your soil and also depends on how much fibres you are adding to the mix.
The best way to determine your mix is to make test samples:
- Apply different samples in a 2cm thick layer to a wall and let it dry.
- When you apply it, it should stick to your metal trowel held vertically, yet it should easily be flicked away.
- If the plaster sample comes off in one piece when fully dried (2-4 days) or you have cracks you should add sand
- If the plaster sample comes off in portions you should remove sand from the mix
The finish
In this layer you add less straw and a bit more sand (fine, not coarse!). By reducing the straw, you are able to get the surface smooth with a trowel. By adding fine sand you'll get a harder surface finish. Always test-drive your mix on different samples to see what clay / sand ratio is working out best for you.
When "leather dry" use a humid sponge to wipe the surface.
After sponging you can use a brush with poneyhair to generously brush the surface. Both actions will prevent loos grains (usually sand) on the surface and make it easier to maintain.
When the plaster is fully dried you can add a colour layer (paint, lime, ...)
Preventing cracks
There are several ways to avoid cracks in all layers:
- Add sand (less shrinkage)
- Add fibres for more reïnforcement (animal or human hair, sisal fibres, straw, when inside sawdust can be used)
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- With adequate movement (massage the loam!) you will develop a higher binding force. Just pushing two pieces of loam together will make them stick, but not bond !!
- Add a reïnforcement net just before the finishing layer
- Try to keep the layer as thin as possible (IF possible)
While sand will make your render harder, pressure resistant and prevents shrinkage when drying, fibres will give it elasticity and resistance to movement. So you need to find a balance between both and adjust your mix according to the circumstances.
Adding a netting makes it possible to use very little fibres as it will make your render elastic in a very efficient way.
Repairing cracks
Cracks are very difficult to solve once they are there, many times it leads to a lot of frustration.
To my personal experience the best way to repair cracks is to make the area VERY wet and work a wet clay alis inside the crack.
I found it very difficult to completely remove the marks of a previous existing crack. The only remedie seems to be hard work and repetition.
Wheat paste
Wheat paste mixed with a little clay will make an excellent clay slip that heals cracks.
Vinegar
Since clay is slightly alkaline, the acidity of vinegar makes it act like a glue for clay. Apply it to both surfaces and press the pieces directly together without slipping or scoring.
Magic Water
This recipe is used to repair cracks in pottery when it's not baked yet. In theory it should work for loam aswell, but I haven't tested it yet.
Mixed together, the solution creates a stronger bond that helps prevent cracking during the drying phase, and fires into a stronger bond also.
To my personal experience the best way to repair cracks is to make the area VERY wet and work a wet clay alis inside the crack.
I found it very difficult to completely remove the marks of a previous existing crack. The only remedie seems to be hard work and repetition.
Wheat paste
Wheat paste mixed with a little clay will make an excellent clay slip that heals cracks.
Vinegar
Since clay is slightly alkaline, the acidity of vinegar makes it act like a glue for clay. Apply it to both surfaces and press the pieces directly together without slipping or scoring.
Magic Water
This recipe is used to repair cracks in pottery when it's not baked yet. In theory it should work for loam aswell, but I haven't tested it yet.
- 1 gallon water
- 3 tablespoons or 9.5 grams liquid sodium silicate
- 1 1/2 teaspoons or 5 grams soda ash
Mixed together, the solution creates a stronger bond that helps prevent cracking during the drying phase, and fires into a stronger bond also.
Additives and Coatings
Wheat paste or Flour Paste
Wheat paste is a natural glue by nature it binds particles together. When fully dried it also hardens the render.
You can use it as a coating to make a smooth surface more sticky before aplying a loam render or you can mix it into the loam render to have a greater binding and prevent cracks.
Recipe for wheat paste:
Wheat paste is a natural glue by nature it binds particles together. When fully dried it also hardens the render.
You can use it as a coating to make a smooth surface more sticky before aplying a loam render or you can mix it into the loam render to have a greater binding and prevent cracks.
Recipe for wheat paste:
- Add 1 part of flour to 2 parts of cold water (mix!)
- Boil 4 parts of water
- Add the flour-water mix to the boiling water, keep sturing untill it turns a bit more transparant and thickens, turn of the heat and keep sturing.
Recipe for a sticky layer
Recipe for a wheat paste render
- 5 parts wheat paste (see above)
- 1 Parts of dry loam (50% clay + 50% Sand mix)
Mix together & paint on wall. Allow to dry completely. Do not re-wet wall surface prior to plaster application
Recipe for a wheat paste render
- 1 Part of wheat paste (see above)
- 2,5 Parts of water
- Add loam and mix untill the intended consistency is reached
Recipe for wheat paste paint
Cow Dung and horse dung
Manure acts as a binding agent, adds elasticity (tensile strength) to the render, reduces cracks and reduces water erosion.
As cows have several stomaches the cow dung contains more hardening enzymes and less fibres (broken down in the stomaches) compared to horse dung.
This means cow dung is prefered when you want a harder, more weather resistant render, while horse dung has the advantage of adding more tensile strength to the mix.
The cow dung is also an antifungal insecticide.
And did you know working with cow dung makes you happy?
The bacterium Mycobacterium Vaccae , which is found in the cow dung contributes to anti-depressant properties of cow dung. When these are inhaled it enhances the growth of the neurons in our body, which in turn stimulates the growth of serotonin and norepinephrine in the brain, basically the happy drug is released.
Recipe for manure render
- 1 part white wheat paste
- 5 parts fine clay/ potters clay
- Fine silica sand (optional)
Add water and mix untill the intended consistency is reached
Cow Dung and horse dung
Manure acts as a binding agent, adds elasticity (tensile strength) to the render, reduces cracks and reduces water erosion.
As cows have several stomaches the cow dung contains more hardening enzymes and less fibres (broken down in the stomaches) compared to horse dung.
This means cow dung is prefered when you want a harder, more weather resistant render, while horse dung has the advantage of adding more tensile strength to the mix.
The cow dung is also an antifungal insecticide.
And did you know working with cow dung makes you happy?
The bacterium Mycobacterium Vaccae , which is found in the cow dung contributes to anti-depressant properties of cow dung. When these are inhaled it enhances the growth of the neurons in our body, which in turn stimulates the growth of serotonin and norepinephrine in the brain, basically the happy drug is released.
Recipe for manure render
- 1 part of dry loam
- (less than) 1 part of manure
Put the manure in a closed container on a warm spot (in the sun) and let it ferment for a couple of days. Mix the loam and manure and apply the render
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Lime
Lime is often proposed as a final layer on top of a loam layer. I would not recommend this! There have been many cases where this application has failed. Reasons why they have failed are unclear, but to our opinion it's because loam swells and contracts more than lime when taking or releasing water. This causes the loam layer to push open the lime layer, making it crack and/or it causes the loam render to separate from the lime layer, leaving a gap in between. Some state the addhesion between lime and loam is very low (possibly because of a difference in PH level). This reuslts in a bad bond between layers and a failure of the combination. At Hamadryad we have applied a loam render on top of a lime render. If it works, we can be pretty sure the failure is due to the different expansion of the materials. If it fails, we can definately state the two materials have a bad bond. (More about this when we have finished the rendering!) |
Casseïne and Borax
Both ingredients can help to seal a render, harden it, make it wipe resistant and dust free while keeping it's breathable properties. It also prevents molding.
Recipe for casseïne and borax wash
Soak the casein powder over night in about 5lt of cold water (make sure the powder dissolves properly without forming lumps).
Next day, dissolve borax in 1lt of boiling water, let cool and stir into casein. Mix well (paint stirrer on electric drill) and top up with 7-10lt of cold water. Apply with soft brush or roller onto dry earthen surface, 2 coats for best results.
Both ingredients can help to seal a render, harden it, make it wipe resistant and dust free while keeping it's breathable properties. It also prevents molding.
Recipe for casseïne and borax wash
- 150g casein powder
- 50g borax powder
- water
Soak the casein powder over night in about 5lt of cold water (make sure the powder dissolves properly without forming lumps).
Next day, dissolve borax in 1lt of boiling water, let cool and stir into casein. Mix well (paint stirrer on electric drill) and top up with 7-10lt of cold water. Apply with soft brush or roller onto dry earthen surface, 2 coats for best results.
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Beeswax
Gives a high gloss finish and protects it against abrasion (water and mechanical). Take into account that it reduces breathability of your wall! Recipe for eart floor polish coating
Apply to earth floors after been hardened with boiled linseed oil. Melt the wax in a fireproof container in the oven or on low heat. When it is completely dissolved add the turps and stir well. Let cool down and stir from time to time. Store in an airtight container that stands up to the solvent! You can also mix beeswax into the final layer of linseed oil Beeswax is also used in the Mirror Wall of Sri Lanka. The wall has been there for more than 1600 years and still is smooth as a mirror. It is believed that this mirror-like sheen was achieved by using a special plaster made of fine lime, egg whites, and honey. The surface of the wall was then buffed to a brilliant luster with beeswax |
Urine
Usually cow or horse urine, enhances the property of loam and enables good curing of the soil. Urine added to mortars have also been proven to increase its plasticity. You can easily drive a nail through a render mixed with cow urine without cracking it.
Cow urine is also used as an additive for plastering owing to its antifungal property; it prevents growth of harmful fungi within the walls and floors, which is a cause of harmful diseases.
It is an extremely good sealant for earthen floors. Using cow urine for sealing the top most coat of floor avoids cracking of the finish.
Dilution of 1:10 (cow urine: water) shows effective fungicidal property.
Panchgavya mix
Panchgavya is a preparation made using the 5 ingredients that come from the cow directly and indirectly. It consists of milk, cow dung, cow urine and a more processed ingredients, ghee and curd. The other ingredients are jaggery, tender coconut, banana, and water. These are mixed in an order and kept covered in a shade for 27 days while being stirred periodically.
It is being used in the foundations for better protection.
Linseed oil
Lindseed oil is probably the best product to protect loam from water. It not only makes it water repellent, it also makes the loam finish very hard! There is one major downside: just like beeswax it will diminish the breathability of your loam drastically and so it should only be used on floors, ceilings or walls with sufficient venting on either side.
Adding linseed oil is done in several layers. There is an old expression that helps you remember when to apply a new layer:
Once a day for a week, once a week for a month, once a month for a year and once a year for a lifetime.
So you'll have a total of 21 coats in the first year (this is probably .
The first week you gradually dillute the linseed oil with boiled terpentine or citrus solvent (smells better).
Usually cow or horse urine, enhances the property of loam and enables good curing of the soil. Urine added to mortars have also been proven to increase its plasticity. You can easily drive a nail through a render mixed with cow urine without cracking it.
Cow urine is also used as an additive for plastering owing to its antifungal property; it prevents growth of harmful fungi within the walls and floors, which is a cause of harmful diseases.
It is an extremely good sealant for earthen floors. Using cow urine for sealing the top most coat of floor avoids cracking of the finish.
Dilution of 1:10 (cow urine: water) shows effective fungicidal property.
Panchgavya mix
Panchgavya is a preparation made using the 5 ingredients that come from the cow directly and indirectly. It consists of milk, cow dung, cow urine and a more processed ingredients, ghee and curd. The other ingredients are jaggery, tender coconut, banana, and water. These are mixed in an order and kept covered in a shade for 27 days while being stirred periodically.
It is being used in the foundations for better protection.
Linseed oil
Lindseed oil is probably the best product to protect loam from water. It not only makes it water repellent, it also makes the loam finish very hard! There is one major downside: just like beeswax it will diminish the breathability of your loam drastically and so it should only be used on floors, ceilings or walls with sufficient venting on either side.
Adding linseed oil is done in several layers. There is an old expression that helps you remember when to apply a new layer:
Once a day for a week, once a week for a month, once a month for a year and once a year for a lifetime.
So you'll have a total of 21 coats in the first year (this is probably .
The first week you gradually dillute the linseed oil with boiled terpentine or citrus solvent (smells better).
- First layer: 100% Linseed oil
- 2nd layer: 80% Linseed oil, 20% Thinner
- 2nd layer: 60% Linseed oil, 40% Thinner
- 2nd layer: 40% Linseed oil, 60% Thinner
- 2nd layer: 20% Linseed oil, 80% Thinner