Close-up of slurry coat
Hessian on wood
The test patch
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).