Earthen Plasters - Part 1

There are as many varieties of earthen plaster as there are soils to make them with.  As with all methods of earthen building, every soil, and therefore every plaster is different.  Different types of clay have different strengths and weaknesses, different minerals create different colors and soils have different amounts of clay in them.  

The main ingredients that make up an earthen plaster are soil with some amount of clay in it, and either an aggregate or fiber (most often both) that keeps the clay from cracking.  There are many things one can add to an earthen plaster to make it stronger, or a different color, or any variety of qualities you may want to achieve with your plaster.  

Base coat plaster layer: 

Base Coat plaster recipe:

Our Recipe (for the soil we most commonly use) - 

1 part clay slip - screened through a ¼” screen

1 part sand - probably don’t need to screen, only if there are rocks in it

1 part chopped straw

Water to feel

Your mix will vary depending on your clay soil.  I can't stress enough the importance of doing samples!

The base coat plaster is the first and thickest layer of plaster you will add to your wall, it prepares the wall for your next coat of plaster by evening out the surface and filling any cracks or holes. It is also sometimes called the scratch coat or brown coat. The materials we use for the base coat plaster layer are very similar to the materials used for cob, but a little more refined.  If there are rocks or pebbles in your materials, you will want to screen them through a ½” or ¼” screen (.5 or 1 cm). Instead of long straw like we use for cob, you will want to use short straw, or chopped straw.  If you don’t have a way of chopping it, you can also just screen regular straw and use the short pieces that fall through the screen.  This is the layer that protects the structure, as well as the layer to use if you want to do anything sculptural on the surface.  

The way the plaster is applied is just as important as the plaster mixture itself.  The more the plaster is compressed, the stronger it will be.  The plaster can be applied with your hands or using a wooden float.  Using a wooden float increases the compression of the plaster and will result in a stronger plaster than applying with just your hands.  This takes some practice to get the hang of, but since it is just clay, sand and fiber (no chemical curing process) you can just keep working it, adding water with a mister if needed, until you get the shape and smoothness you desire.  This layer won’t result in a super smooth surface, but rather an even, durable outer layer.  

If you do choose to add sculptural elements into the layer, make sure that none of them have potential to catch rain.  Water needs to be able to run off the surface as easily as possible.

The goal for this layer is to even put the surface or sculpt the shape you want, but not to give it a super smooth surface (that comes with the final layer).  As with everything, there are also exceptions.  If you want to do a one coat plaster, you can spend time and get these rough materials fairly smooth.  

Generally there will be at least one other coat of plaster over the base coat.  The plaster on the interior has different functions vs plaster on the exterior.  On the exterior you can often get away with just one coat, but on the interior for both aesthetic and function, you need a second coat of finer, smoother plaster, that does not create dust in your interior space.  We generally want the interior plaster to be a lighter color than is often found in native clay soil. 

Sgraffito

sgraffito is method of carving down the layers of plaster to reveal the layers underneath.

Finish Interior Plaster Layer: 

The finish plaster layer is also a combination of clay soil, (or store bought bagged powdered clay), sand, and sometimes a fiber.  Once again there are myriad of additives that you can add to this mixture to create the finish you want.  One common additive is starch paste, which makes the final plaster more durable.  When using native soils and courser sands, you naturally get a gradation of aggregate sizes in your mix, but when you are using all bagged sands you will need to use different aggregate sizes.  One common mixture I use is a combination of 30#, 60# and 90# sands.  (the numbers relate to the size of the mesh, so the higher the number, the finer the sand).  Generally you can find bagged sands at local sand and gravel yards.  If I want a lighter color for my interior plaster, I will generally use a bagged clay that I purchase at a ceramic supply store.  Lincoln Fire clay is a good option, and fairly easily available.  Different clays have different qualities and some result in softer plasters than others.  Again, starch paste is often added to make the plaster harder and more durable.  This would also be the layer to add in any pigment if you want a colored plaster.  My favorite way to pigment a plaster is to use a small amount of a colored native clay, screened through a window screen.  

One of the beautiful aspects of earthen plaster is that it can wear slowly, and then simply be repaired whenever you desire.  I find the look of a weathered earthen wall to be just as beautiful, if not more, than a newly plastered wall.  As it weathers the straw becomes more visible, and the sand washes away.  Since there is nothing that has ‘cured’ in the plaster mix, you can simply dampen it down, and re-trowel it, or add another coat where needed.  This graceful wear is so contrary to our modern day structures, where we want them to never need any maintenance or tending, and then they are done.  What if we tended to our homes every year, and they lasted generations? 

Finish plaster recipe 

2 part 30 mesh sand

2 part 60 mesh sand

2 part 90 mesh sand 

3 parts Lincoln fire clay

1/2 part finely chopped straw

1/2 part wheat paste