Earth plasters are hard to work with. That’s not something that most people expect to hear when talking earth plaster. Most people expect that the natural plaster will be easy to work with and more fun because it is basically just glorified mud. Unfortunately, this is not the case. Earth plaster is an art form and a learned skill rolled into one. With all other plasters, your materials are consistent; however, with earth plaster, the ingredients can change from one square foot of your land to another. You have to fully understand the materials you are working with and how they fit the bigger picture of the overall mix.
There will be lots of trial and error learning when creating an earth plaster. For this reason, I suggest you build a small bale wall on which to test your mixes. This can be as simple as a stack of bales that will accept plaster. Make sure that you can put some pressure on the bales as you apply the mud without them falling over. You want a realistic test of the material. If you find that the plaster is too crumbly, then you don’t have enough clay in the mix. If it is cracking like crazy, you have too much clay in the mix.
A Mason Jar Test: Fill a mason jar half way with soil you think would work for your plaster and the rest with water. Shake the jar vigorously for a few minutes to mix everything up. Set the jar down and wait for it to settle out. The sand will settle out in the first few minutes. The silt will be next and will settle out in a few hours, maybe as long as 10. The clay will take a couple days to fully settle. Measure the sand layer, the silt layer and the clay layer. You can determine the percentage of each based on the relationship to each other displayed in the test. You want a soil that is roughly 80% sand and 20% clay and silts.
Testing your soil is a must. Find a large area that you are willing to dig up and use for your plaster. One option is to use the soil removed from any excavation for the house or structure. Screen it through several grades of screening starting at about 1/2″, moving to 1/4″ and ending at 1/8″. This will give you a quality material to work with when plastering with no rocks or major lumps. With the material screened you can run mason jar tests on the final batching of material as before, this time with an actual finished material that is all mixed together. If your testing shows weakness by either too much or too little clay, make adjustments with bag sand or bag clay.
You will want to stabilize the plaster as well. The most common stabilizers are cement, lime, and asphalt emulsion. Each should be added to your test mix until you have enough to resist a hose spraying on a dry test patch. The hose should not erode the plaster at all. Once you meet this grade, you are ready to plaster. You can add horse hair or chopped straw to the mix as well to give it more tensile strength. This does not effect the ratios of the materials, but it will require that you add more water to the plaster.