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.