By Andrew Morrison
There are a lot of conversations out there about plaster and what options are best. What is often overlooked, or at very least not considered as heavily as it should be, is the choice of sand. This is especially true when using natural plasters such as lime and clay. Cement based plasters rely less on the mechanical bond of the sand particles than natural plasters; however, the choice of sand is still very important. The reason sand is so important is because it is responsible for several jobs in the process of creating a tight, strong, and durable plaster for your straw bale home.
Firstly, the sand acts as the bulk of the material in the plaster. Keep in mind that a scratch coat (the first coat) is typically a 1.5:1 ratio of sand to lime (using a simple lime plaster formula). This means that there is literally one and a half times as much sand in the mixture as there is lime. In the brown coat (the second coat) that ratio goes up to 2:1 sand to lime. How could “just any sand” possibly be an acceptable approach when deciding the majority of the material?
Secondly, the sand provides the structural strength for the plaster by creating a strong mechanical bond.