Plaster Testing:

     Ingredients

     Test batches

Applying Plaster:

    Techniques

    Case Study

Testing your plaster ingredients is the most essential step in the plaster process.  Every material at each building site will be different; therefore, one recipe will not work for multiple plaster projects.  Some generalities can be made about sand to clay ratios (generally, a 3:1ratio is a good place to start), but many combinations of your particular ingredients should be mixed in small portions and applied to a test surface.  In many situations, you may go ahead and apply the test samples directly to the final project surface, because they can be scrapped off after they have dried (see picture below).  Tests should be performed at least two weeks before applying the plaster in order to allow the samples to be tested in their driest form.  Also make sure that the test patches are large enough to get a good idea of their performance.  Once they are dry you should perform a few simple tests before choosing the appropriate ratios for your project:

Scratch Test- If the mixture has too much sand or fiber it will scratch away or flake off when scratched.

Check for Cracks- Plasters with too much clay content will crack when they are dry.

Spray at Close Range-  Spray the patches with a hose from 8in. away.  Check for plaster running off or flaking apart from the pressure of the hose.  Do not forget this step as you might be surprised how your plaster performs under its first rain.

Appearance and Smoothness- Observe the general appearance of each plaster.  Check if any samples sloughed down as they dried.  Feel the texture and smoothness of each plaster.

Test batches can be as informal as this sample on the side of a cob oven.  Many different samples will give you a good feel for how your materials work.  With this example you can see where some of the test patches crumbled away.