Cob and earthen plaster recipes

There are a lot of projects underway simultaneously at the moment, but two of them have involved clay plastering. There is the cob bathroom, now well and truly dry and ready for plastering inside and out. And there is the interior of the upper room of the smallest of the two dry-stone schist animal houses on the quinta, the ‘wee house’.

Rather than include the recipes in posts about the buildings, I’m describing them here separately for ease of reference.

BIG caveat: clay and clay subsoils are highly variable, so there’s no such thing as a standard recipe for cob or earthen plaster. It takes a bit of experimentation to arrive at the right proportions and all the books and internet resources on the subject will tell you that. It isn’t a stage you can skip unless you’re using a standardised commercial product.

So why bother posting recipes? Well for one, it’s so I don’t forget them. And secondly, when I was trying to work out the best proportions to use with our particular clay, I kept wishing someone somewhere would describe what they had done with their clay, just to give me a rough idea and somewhere to start. Better still, if enough people did that then it would be easier to get a grasp on the extent of variability in proportions and techniques and pick and choose what felt right to experiment with. These recipes are offered in that spirit. Unless you’re using the same clay as I am, they likely won’t work in the same proportions for you.

The best books on cob building all have detailed instructions on how to assess your soil for clay content and make test bricks to arrive at the perfect mix – including Becky Bee’s The Cob Builder’s Handbook which is available free online – so I’m not going to go into that. There are also lots of different ways to plaster walls and lots of different ways to mix earthen plasters, so I’m just going to write about what we did because it worked for us.

I bought in clay from the local brickworks, not having enough subsoil on these steep slopes to spare for building, so we are working with pure clay, not clay subsoil. While many resources on earth building will state that the basic cob recipe is clay, sand and straw (or other equivalent natural fibre), they’re often referring to clay subsoil rather than pure clay. I found that without the addition of soil, the clay-sand mixture wasn’t ‘fat’ enough and didn’t have the right properties to build with. It just wasn’t sticky enough: it didn’t adhere together as well as it should and didn’t hold the straw well. Higher proportions of clay made no difference. It needed something else, and the silt and schist dust in the local soil proved to be the answer. (Another good reason to experiment with what you have before you start building with it.)

We worked with wet clay throughout, finding it much easier to deal with. The huge 12m³/425ft³ pile of clay the brickworks truck delivered was wet when it arrived but dried out through the summer into big solid clods. Trying to pulverise them into powder was too much like hard work and we found soaking them in big 50-litre/10 gallon buckets for 24 hours or more before use made that step redundant. With soaking, the clay became maleable/treadable and since the clay deposits our clay was dug from are so pure, no sieving was necessary to remove stones. All we sieved was the soil we used in the mix, which was a mixture of topsoil and subsoil. The topsoil element was poor quality and very low in organic matter.

Cob – for building or plaster base coat

1 measure soaked clay
1 measure sieved soil
3 measures sharp sand (good mixture of coarse and fine particles)
Water, if necessary
Straw

Tread and roll clay, soil and sand on tarpaulin until the mix is homogeneous and forms a single large sausage which comes away cleanly from the tarpaulin when rolled. Add water if necessary to get it to this consistency. When this stage is complete, add straw, treading it in as you go, until the mixture has as much as it can hold without starting to break up. We evolved the ‘jump test’ for the straw content. If someone launched themselves into the air to land with both feet in the middle of a freshly-rolled cob sausage, their feet shouldn’t make an impression much more than a third of the depth of the sausage.

We used this recipe to build the cob bathroom, and also to both point the surface voids in the dry-stone walls of the wee house and form the base coat of plaster. As a plaster, it’s a stiff mix best applied by hand to a thoroughly dampened (though not soaking wet) surface. We used a depth of around 5cm/2″ to even out the rough stone walls and provide an insulative layer with some thermal mass.

Cob bathroom under construction

Pointing the walls of the wee house

For the top two plaster coats, instead of mixing clay and soil, I fermented clay and fresh horse manure in water for at least 10 days before use, breaking up the manure and stirring the mixture occasionally through the fermentation period. I did some experimenting, adding some soil and finely chopped straw to earlier mixes to make up for a lesser quantity of horse manure, but settled finally on a 50/50 mixture of the two.

Fresh horse manure contains a high proportion of plant fibre, so adds both fibre and adhesiveness to the mix. The fermentation allows the dung and clay to bind and gives a very smooth, almost glutinous consistency to the final plaster. Yes, the place smells like a stable for a few days until it’s dry, but it’s very definitely worth it.

Second coat of plaster – the float coat

2 measures fermented clay and horse manure
3 measures fine sand
Water

Pass the fermented clay and horse manure through a coarse sieve to remove any stones and overly coarse fibres and to thoroughly mix in any remaining lumps of manure. Tread into sand on a tarpaulin as for cob. Put into large buckets and add water to mix to a sloppy consistency suitable for trowelling. Trowel onto walls.

Clay plaster ready for application

Trowelling on the final coat of plaster

For the final coat of plaster, I added some extra ingredients. Wheat paste was added to bind and prevent dusting. Casein (milk protein) in the form of powdered milk was added to harden the surface and give some resistance to abrasion. Linseed oil was added as a stabiliser, much less than might be added to an exterior plaster for water resistance as I still want the walls to be able to breathe freely. Whether such a small quantity makes any difference whatsoever, I have no idea, but it seemed like a good idea at the time so I added it.

Wheat paste

1 measure white flour
5 measures water

Heat 4 measures of water in a large enough pan to hold the lot. Mix the remaining 1 measure of water into the flour with a balloon whisk. Beat well to remove all lumps and then slowly pour the mixture into the water in the pan, whisking all the time. (Some recipes for wheat paste say add the flour when the water is already boiling, but I find the risk of lumps is much reduced if you add it before the water boils.) Continue to stir with the whisk until the mixture boils and thickens, then simmer for 5-10 minutes, stirring all the time. This should ensure lump-free paste. Cool before use, though it doesn’t have to be cold.

Finishing coat of plaster – the set coat

2 measures fermented clay and horse manure
3 measures fine sand
Water
Wheat paste
Powdered milk
Raw linseed oil

Make up plaster as for second coat, then pass through a fine sieve to remove all but the smallest fibres. Add wheat paste – I used about 1 litre per 50-litre bucket of plaster (2 pints/4 cups per 10-gallon bucket). Mix some powdered milk with enough water to make a creamy consistency and add. I used around 125g (4oz) powdered milk for each 50-litre bucket. Lastly, linseed oil. Just a small glassful – around 150ml/½ cup – for a 50-litre bucket.

For the bathroom internal plaster, I also added a borax solution to the final coat. With this being such a damp environment, I wanted to prevent any potential mould growth.

The consistency of this plaster has to be felt with both hands buried in it up to the elbows! Just gorgeous! A sumptuous, creamy loveliness. Plaster that feels this good surely can’t fail to look amazing on the walls. And so it does. The amount of cracking has been minimal and the finished surface is smooth and doesn’t dust.