This is a post I have been meaning to write for a while: a review of the experience of clay plastering. The first point to make is that I don’t think I arrived at the absolute definitive method, finding myself constantly experimenting with different mixes and application techniques in order to find the optimum balance of the various considerations involved: these being adhesion, cracking, gap filling and time.
It was much harder to get clay to stick to straw than the Limecote I used on the outside which could be sprayed-on and built-out to possibly even 40mm in a single application. Using clay, it is necessary to put-on an adhesion coat. This should be mostly pure clay, to make it sticky possibly with a little sand added to give it a bit more strength. This layer can’t be too thick, because a clay rich mix, while sticky, will crack a lot and also will not be strong enough to allow it to be built-out. It s also worth noting that clay is more expensive than sand, so you don’t want to use more than is necessary. The issues with this adhesion coat are how sloppy to make it, how thick to apply it and how to apply it. My solution was to make a pretty sloppy mix (consistency of double cream) and apply it with a gun to a thickness where the straw was coated and stray sticky-out bits could be encouraged to lie flat but not to the point where the coat starts to build-out.
I did find that I got some areas where, after applying subsequent build coats, the clay was not strongly bonded to the straw and therefore was inclined to crack, but this mostly occurred where there was weakness with the underlying straw surface, either because the straw was rather soft and wispy or because there was a gap which wasn’t adequately packed-out. Where these were an issue I hacked off the clay and started again, making sure the clay was very well worked into the surface.
I probably wasn’t as diligent as I should have been on gap filling. This was partly because the Limecote was so good at bridging across gaps or soft areas and also because of the time required to do a thorough job packing out gaps with a straw mix. I therefore tended to rely on my first build coat to fill-in any gaps but this could lead to cracking and weakness if I didn’t sufficiently force this coat into the gaps. For this reason it would probably be best to spend some time really packing-out any gaps with a long straw dubbing mix.
Cracking can happen for two reasons: either your mix doesn’t have enough sand and/or fibre added to it, or if the clay is insufficiently bonded to the straw underneath – because the straw surface is wispy and unstable or there is a gap beneath the clay that isn’t well packed-out.
I think I got the fibre issue cracked (as it were) by using concrete screeding fibre. This is much easier than using chopped straw because you don’t have to chop it. I did use chopped straw initially and found the best way of prepping this was to put a load into a dustbin and then plunge the strimmer into it, preferably with someone holding the dustbin lid over the top to prevent too much flying out. The only issue with fibre, especially screeding fibre, is that if you use a sponge to finish-off your top coat, this will drag bits of the fibre out, resulting in a hairy finish – but this can be dealt with by using a blow torch to burn these off.
As mentioned earlier, I got some minimal cracking of my first build coat, but this was mostly over gaps. I dealt with this by tapping the cracks with a hammer, and if the area around the gap sounded hollow, I would knock the gap out and ram clay (or sometimes dubbing straw) into it. If the gap seemed ‘natural’ and reasonably firm either side I just left it and filled it out with the second (and final) coat.
I didn’t get any cracking of my final coat – which I figured must have meant that the mix was right and the fibre was doing its job.
I did initially experiment using a fibreglass mesh which I stuck on top of my first build coat. This did stop any cracking but I was deterred from using it extensively because the supplier of it said it should be sandwiched into the final coat which effectively meant making the final coat two coats (as in two bits of bread if the mesh is the butter in the middle) and this was too time consuming. That said using it as a layer between the first and second coat didn’t seem to create any problems, such as creating a weakness or potential for ‘shearing’ between the two coats – although I guess only time will tell if this might develop as a problem.
I underestimated the length of time it took to do the clay plastering, possibly because the experience of the external rendering was easier than expected (Limecote being such a magical material to work with) or perhaps because the internal walls meant there was a considerably greater area to deal with. Using the gun didn’t seem to speed things up, other than when using it to spray the adhesion coat. When I used the gun for the first build coat, I found that I could go faster by hand, especially if I kept this layer relatively thin and when it came to the final layer pushing it on with a trowel was also faster than standing there with the gun waiting for it to build-up to a sufficiently thick layer. This was probably because I was using a basic gun and compressor. If I had more powerful kit I probably could have sprayed thick coats on quite quickly.
Because of the length of time things were taking I also tended to miss-out the gap filling stage, as mentioned above. This was probably not ideal, but I don’t think it has caused any long-term problems (i.e. clay dropping off at some point in the future), but I guess only time will tell on this one.
If time (or labour) wasn’t an issue the absolute best process would probably be the following.
1 Adhesion coat
Apply a slip coat of 3 to 1 clay to (plastering) sand with a gun, mixed to a consistency of double cream. Apply this to a thickness such that the straw is well coated, but not starting to fill-out.
2 Gap filling
Make up a batch of long straw dubbing mix. This is straw with a clay / sand slurry poured into it. You could make this slurry by taking the slip coat and adding some sand to it to firm it up a little. This mix should have the characteristics of straw bound together with clay, rather than clay with straw added to it. Use this to pack out any gaps or soft areas, ramming it in as firmly as possible. Unlike the lime (a chalk fibre lime such as Limecote), clay is not very good at bridging across gaps and soft areas and even if it appears that you have covered a soft areas well, the likelihood is that it will be weak and crack easily as it dries. You could (probably should) do this while the slip coat is still wet. This should get you to the point where the surface is relatively flat and firm, but still basically strawy.
First build coat
Make up a batch of clay that is 3:1 sand to clay with fibre added. I used concrete screening fibre (a couple of handfuls per 60kg batch – ie one 15kg bag of clay powder plus 45kg of sand). I mixed this in a regular cement mixer, tumbling the dry mix first and then adding sufficient water to take it beyond the golf ball clumps stage, past the big lumps stage to the point where it starts to slump. Something like the consistency of cream cheese. Then apply this by hand, really ‘grinding’ the mix into the straw, using the heel of your palm, having first dampened down the straw with either a mister on a hose or by using a pump-action sprayer. This layer need not be that thick, but you should get to the point where the wall no longer looks strawy, but more clay like, with odd bits of straw poking out.
Let this coat dry out so that any cracks can fully develop. Then test the cracks by tapping them with a hammer to see if they sound hollow around then. Those that are hollow, knock-out and fill with a clay mix or possibly by ramming a long straw mix into the gap. Alternatively you could wait until you start to see cracks develop as the coat dries and then push the still damp clay into the straw along the crack line with your finger tips.
Second build / final coat
Get the wall good and wet with a mister. It is amazing how much water the dry clay wall will suck-up. I tended to give it a good soak, to the point where water is just about to run-off the surface, and then wait five minutes or so and give it another go. Apply a 3:1 sand to clay mix with fibre using a trowel to a sufficient thickness to give a decent covering to any straw poking out. I find descriptions of coats by thickness rather unhelpful with straw, since its irregularity will mean that a coat which in one place may just cover the straw, will in others be 30mm thick. I used the trowel to get the surface reasonably flat and smooth, but tried not to work it too much, and then used a wet sponge trowel to further smooth over the surface. This produced what I guess you would call a rustic finish, which I was happy with. If you wanted a better finish you would probably need to do a third skim coat which you could spend any length of time working-up to whatever finish you want, rubbing it with a wooden float or a stone or whatever. The great thing about clay is that it doesn’t set, it simply dries-out so you can rework or revisit the surface as much as you like.
Finally, once this coat is dry, get a blow torch and ‘wash’ the surface with flame to burn-off any bits of fibre or straw.
I am now close to finishing the second coat – having been at it for nearly a month. Fortunately I have been able to borrow a cement mixer, which makes the mixing much easier and has probably come just in time to preserve the life of my hand-held electric mixer which has been showing the strain.
The basic procedure that seems to work is as follows.
First put a dry mix of one bag (15kg) of clay plus ten shovels of plastering sand into the mixer together with a couple of good handfuls of concrete screeding fibre. I have opted to use this instead of creating lots of chopped straw, simply because it is easier (and seems to work).
Then leave this to turn-over for a good 15 minutes to get it well mixed and also to tease-out the fibres (they come out of the bag as short strands, but are in fact pieces of mesh and by stretching the out you greatly increase their volume/area).
Then add about half a bucket of water to get a consistency of mix that slumps well in the mixer, but doesn’t stick to the sides. It is also important to ensure it is wet enough to avoid turning into small pellets because if this happens it can be hard to get these to blend-out. Leave this for a good 10 or 15 minutes to get smooth and well mixed. Then add the remaining water to produce a consistency of mix that is close to that of butter icing. This will tend to stick to the sides – so may need a little prompting with a spade to ensure the water works its way through. While the clay is mixing give the wall a good soaking. It is astonishing how much water a dry clay wall can suck-up and if you don’t take the edge of this it will pull all the moisture out of your final coat. I put the mister on the hose and wet the wall down just to the point where water is about to start running down. Then I leave it for a minute and do it again and perhaps even give it a final mist just before starting to plaster
I then apply the clay in quite a rough-and ready way using a plastic trowel and once I have done a decent-sized area I use a sponge trowel to smooth things out. The advantage of a sponge trowel is that it can move the top 2mm or so around without dragging the whole coat.
Once the coat has firmed-up a bit I then rub it down with a regular sponge. The use of the two types of sponges can pull some of the fibres out of the mix a little, leaving a slghtly ‘hairy’ finish, but these can either be brushed-off when the coat is nearly dry or I am hoping they can be encouraged to ‘lay down’ when the wall is painted. Even if some still poke-out I don’t mind the fact that the fibres are visible because, to come-over a little bit Kevin, it “tells the story of the material” (we can see the horse hairs in the 16th century plaster on the inside of our house for example). It gives a slightly rough finish, but I think this looks fine. It would be possible to give a smoother finish if you worked the surface with a plastic or wooden trowel, bit this would take ages and probably look a bit odd, given the natural irregularity of the walls. You could try and use a flexible rubber-edged trowel, but this would probably only work if you were able to get a much flatter finish, which would involve a great deal more clay.
Results from the walls which have dried out are encouraging – very little cracking, such that it will be easy to hack-out the one of two that have developed and patch them over, possibly with a strip of hessian of fibre-glass mesh.
It is, however, very slow-going, especially doing the detailing around the windows. Renderng and plastering will be by far the two biggest jobs involved in the build.
I am now getting round to the fiddly detail bits. One of these is fitting notched bales between the top the main interior wall and the roof (the bales have to be notched to allow for a purlin) and also putting bales in place above the doorways. It was good to return, one last time to ‘straw work’ athough cutting the notch with a chainsaw meant becoming swamped in loose straw. It also reinforced that by far and away the best tool for cutting or shaping straw is an electric chainsaw (not a petrol one because the straw gets in the air intake and it overheats).
I have also been finishing off the clay around the windows and door frames – using a ‘cobby’ mixture of straw and clay. I want to plaster right up to the window frames, rather than put decorative wooden frames and window ledges in place. I think this sets-off the window frames to best effect, but it does mean plastering over the window sub-frames as well as sticking clay ‘upside down’ to the sub-frame above the windows. To do this I covered the wood in a flour, water and clay paste to give it maximum adhesion. This appears to have worked – although for the final coat I will also stretch hessian or fibre-glass mesh around curve, both to stop cracks developing over the join and also to hold the upside down plaster in place should it feel like falling-off. The job feels a little like being a housemartin, sticking clay and straw to the underside of eaves – and the resilience of their nests gives you some confidence that the clay, with added flour paste, will be sticky enough.
We have now had a chance to see what happens as the base coat dries-out. There are some cracks developing – but not many. I have probed any large crack to see if this indicates any fundamental weakness in the straw or adhesion beneath, and for those where there is significant softness I have pushed my fingers in more strongly and forced the clay back into the straw / gap – and then filled this out with more base coat.
I have also changed the approach to the first coat – moving away from a very liquid pure clay slip, to a mix that is 1:2 clay to plastering sand (no fibre). This gets over the probem of pebbles blocking up the gun because the sand is finer and better sieved (albeit more expensive). I get the mix to the point where it is a cross-over between liquid to solid but where if you dip a piece of straw into it you still get a good amount clinging to it. I have then sprayed this on just to the point where you stop just covering each bit of straw and start getting ‘fill’ between the bits. This we will then leave to dry-out a bit before putting the 1:3 base coat (plus fibre) on.
I have also noticed there is a marked difference between the bales whch present a cut face and than those with a folded straw face. The cut face is already pretty rigid and stable and therefore doesn’t require much to cover it, whereas the folded face is more wispy and requires more material to really get into the straw and stabilise it.
I have abandoned putting the first coat on with a trowel, relying instead on putting it on by hand and then really grinding it into the straw with the heel of the palm. This way you can put some real force behind it and actually feel when you have a really firm contact. I then trowel on a 10mm or so layer to build the coat up a bit – as well as mashing in some long-straw to dub-out the uneven places. We will now wait until this layer dries before making a decision on whether to use a mesh in the final coat.
I have also worked out a way to get good chopped straw for fibre. This involves chain-sawing the face of a bale and then putting the resultant chopped straw into a dustbin and then using a stimmer to shred it further. I got this idea from this video (https://www.youtube.com/watch?v=lEfmPKnzvZ0). I am also using a slip coat mix about the same consistency as the one in this video – which is more liquid than the type Barbara Jones uses in her plastering course video.
I have also slightly adjusted the mix – moving from a 1:3 clay to sand mix to one which has a little more clay in it. It came down to feeling the final mix. There is a fine line between a mix which is grainier than it is sticky, to one which is stickier than it is grainy. The sticky mix is what I am after – and there is not much in it in terms of how much clay to add. The technique is to start with about 3/4 bucket of water plus one bag of clay plus a handful of chopped straw, mix – then add a bucket of sand and mix again, then add two more buckets with a handful of straw – mixing between each one.
We have now refined the process. First up, I have decided to abandon using the render gun to apply a 10mm coat of 1:3 clay to sand mix in the first pass. The reason for this is that, as good as the gun is, the sand I am using has too many rogue 10mm stones in it, which block up the gun and the time I was saving was used up by frequently having to unblock it. If you have a better-graded (but therefore more expensive) mix this would not be an issue. Instead I am using the gun to put a clay slip-coat on – reverting to the technique recommended in the holy book. Putting this coat on with a gun is very fast.
We have also experimented with the mesh, working this into the top of the first / main, 20mm coat. Once this is on it is possible to ‘move’ the render around underneath it and flatten things out because the mesh stablises the surface. However, having discussed this with Chris from Back to Earth he recommends not putting the mesh onto the first coat, but rather allowing this to dry somewhat and then put the mesh into a 10 – 15mm top coat. We will try this from now on. I have also decided to add straw into the first coat, despite not necessarily needing this with the mesh, on a belt-and-braces approach.
I have also worked out a good way to get hessian to stick onto the wooden window sub-frames – namely to make a glue from flour, clay and water.
Finally I have made up a batch of ‘sticky straw’ to build-out around the windows. Since this involves creating a layer at least 50mm thick, this is too much for a render layer – even with lots of straw in it. Instead this is more like a very straw-rich cob. I used a mix of around 1 part flour, 2 parts clay and 2 parts sand – plus quite a lot of water and sufficient straw to get a mix to a point where the straw readily sticks together and forms a sticky, maleable material.
A couple of days ago we took delivery of ninety 15kg bags of powdered clay from Back to Earth. We spread the bags out in several piles within the building so as not to put a single 1.5 tonne load in one place, since I am not sure how happy the floor joists would have been with this.
I then started experimenting with mixes and methods which I will leave a few days to see how they dry out. The most important thing was to assess the suitability of using the render gun for getting the first coat on and also to determine whether to go for an initial pure clay slurry coat (as the holy book recommends), or whether using a gun would mean you could go straight to a thicker 3:1 sand to clay mix. Chris, from Back to Earth had reckoned that the sand mix should stick pretty well, if applied with a gun – and so it proved. I made quite a sloppy mix and it stuck very easily to the straw (see close-up picture). However, if you are using your hands, as the book suggests, I could see that it would be pretty difficult getting a sand mix on – once again emphasising that spray application, for both lime and clay, is definitely the way to go.
Having sprayed a thin coating onto a test area about 1m square, I then played around with building up different thicknesses – both to see how well this sticks and also to see how it performs as a base coat once dried-out. The answer was that it stuck pretty well – but not quite as well as the fibre lime I used for the outside, which was quite magical stuff. The main difference in terms of stickiness was that it was not possible to get the straw to ‘lie-down’ quite as well when smoothing the first layer off with a trowel.
I ended up with three types of thickness – a slurry-type coating, then a depth which was similar to my first lime coat where all the straw was covered but with plenty of gaps due to the unevenness of the surface (probably equivalent to a 10mm layer) and finally a small area (top left of big picture) where the straw was covered creating a reasonably flat surface with no gaps, such that only a few tufts of straw poked through (probably equivalent to a 20mm coat). I will now see how these develop as the dry out.
I will then experiment with a final finish coat on top of the thickest of the test patches once it is firm to see what sort of finish I can get. I have a suspicion that the finish I got for the outside using a sponge trowel may not work with clay because a sponge trowel produces a slighty gritty finish which is fine when you have a lime set which will hold the gritty pieces into the render, but with clay I suspect the set will not be firm enough to hold the sand grains and I will end up with a loose (rather than a firm), gritty surface – albeit then rubbing this down with a stiff brush might solve this problem.
I have also decided to go against the advice of the holy book and not put fibre (such as chopped straw) into the mix, relying instead on putting a fibreglass mesh into whatever turns out to be the main ‘building’ coat, such that the fibreglass sits within the last third of final depth. This is following the recommendation of Chris from Back to Earth who is a cob building specialist and said this was the best way to guarantee no cracking. However, I will use a straw mix to build out the plaster around the windows because I have decided that I want to plaster close-up to the frames, rather than have a boarded-out window recess. This will involve plastering over the existing window sub-frames so to this end I also experimented with sticking hessian soaked in plaster onto a piece of frame to see how well this works to provide a key for the clay over wood. I didn’t use an adhesive for the test piece, but if it looks like a good solution I will soak the hessian in either a solution of PVA or possibly flour and water paste to make sure it sticks well. I may also try this against the fireboards and for use on the studwork so that I can get a clay finish through-out the building.
One other thing that I might try, just to see what happens, is to get some artificial fibre and add this into the mix, to see if these helps both with the adhesion and stickiness as well as allowing for the application of thicker layers (and as a preventative to cracking).