The skin of the structure is as important as the structure itself. It can allow or prevent the flow of air and moisture between inside and outside (pretty much like our skin), making huge perceptible and imperceptible differences in the interiors.
The space between the horizontal wood sticks in the varchee frames were filled with a mix of mud, cow dung and eucalyptus bark fibre. This was an exciting discovery. We had used the eucalyptus bark until now only for mulch and because of its abundance from the trees that were cut and cleaned, we were looking for more ways to put it to use. The eucalyptus bark fibre was separated from the bark simply by hand. This was added to the mud mix to give it tensile strength and prevent shrinkage/ cracking without decreasing the bond strength much; as opposed to sand which reduces shrinkage but also reduces bond strength. (Cement/Lime will increase bond strength and somewhat reduce the shrinkage.)
The plaster on the wall was done with mud alone applied by hand. Once the first layer of the plaster dried and shrank, the cracks were filled with finely sieved mud. This was repeated several times, filling smaller and smaller cracks every time. Finally when the cracks were negligible, the wall was rubbed with a wet cloth. For finishing, cowdung mixed water was applied with a cloth to the wall. This helped by filling the few minute cracks in the plaster and creating a hydrophobic layer on it, protecting it from moisture.
When working with mud, empirical formulations are not as important as what feels right. Mud gets under your fingernails, into your bones and deep into your heart.
It is important to understand that a good balance of sand and clay is needed in building. The clay binds really well, but expands and contracts in presence or absence of water. Sand alone is really strong but it doesn’t stick together. For a structure to be solid, it should be able to resist both compressive and tensile forces. Sand provides compressive strength while the clay is responsible for tensile strength.