For the insulation layer I used 4 x 12.5kg clay and 4 x 25kg of sharp sand. To this was added as much sawdust as possible. We used a bale of it obtained from a petshop.
To faciliate the mixing this layer is made more wet. Again the wall thickness is 7cm.
For speed, I think this layer could be quite easily be made using large builder trug buckets and an electric mixer. The wetter mix would be quite easy to work with.
Again, this layer wants to be 7 cm thick and will protrude forwards towards the chimney. We depth tested it frequently using a wooden dowel to ensure uniform 7cm depth. Because it is a wet layer, it is prone to sagging a little.
Using straw would be the traditional building material. We did try some of this but it’s very messy and bits stick out..
The insulation layer should contain as much sawdust as possible. You are basically trying to trap as much air as possible in order to provide better insulation. Any small cracks on the inside can allow heat to enter layer 2 and you may then notice some smouldering in this layer especially if there are cracks through the third layer of clay. We did experience thsi and it is easily repaired with a bit of spare mix.