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 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.