Earthen Plasters/Relief Sculpture:
There is a general mix for plasters I will display here. This can be put on any earthen surface whether that be adobe, cob, earthbag with an initial layer, and wattle-and-cob.
There are various looks that can be achieved with plasters from allowing natural undulations in the walls to smooth trowel look to fibrous materials as well. You can also do varying degrees of layers of plaster for the look you desire. Some plasters will be covered with natural paints and some may not or just sealed leaving the look and color of the plaster present.
Depending on the amount of undulations in your wall and the look you want will determine how many layers of plaster you use.
We use a sandier mix for plasters than you would for adobe, wattle-and-cob or cob. This will help to reduce cracking as it is not as thick of a layer as the materials are when you are using them to build with.
If you have a wall with many undulations you want to fill you’ll want to start out with a first plaster to just make the walls level. This should be a sandier mix with fiber (rice husk or short straw) in it as well. It needs fiber to stop major cracking because it will be thicker in certain spots where you are filling in undulations. This layer can be hand-plastered or troweled on depending on what look you prefer.
If you want a fibrous look add more short straw and when the plaster has dried a bit, use a wet sponge to wash off and expose straw fibers.
After this layer dries you can do a second finer layer. You will probably still get cracks in your first layer (how much depends on what kind of mud you used). It is good to let all cracking happen before applying the second coat. This second layer will work to fill in all of those cracks and do final smoothing where necessary.
To make this mix sift some sandy mud. If you do not have sandy mud you can add sand to a mud you have been using. One easy way to do it is to fill a wheelbarrow up with water. Dump the sandy mud onto a plastic flexible screen and dip in the water, moving back and forth until all is through except bigger pieces. Your mix should be so sandy that it can go through the screen with no need to squeeze and kneed clay through. Let this mixture sit overnight or for a few hrs. The mixture will settle to the bottom and the water will sit on top. You can then pour most of the water off the top leaving the mix quite wet. It will be quite similar to a slip mixture.
Apply this very wet to the walls rubbing it into the cracks and unsmooth places on the wall.
You can also use various materials to wet and smooth the walls when the plaster is still pliable but stuck and quite dry onto the wall such as foam sponges, yogurt lids, or feed bags. These all will give a bit of a different finish.
If you are going to apply a finish or earthen paint to the wall, allow the wall to dry completely before applying. If the plaster is not completely dry it can create discoloring in the final paint.
For plastering an adobe brick look:
If you want to be able to see the adobe bricks of the wall but want them to look more finished, start by cleaning them up a bit. You can do this by chipping off any extra pieces of mortar and redefining the inset lines between bricks. Then apply the second coat of sifted plaster to smooth the bricks.
For plastering on straw-bale:
We used three coats on the straw-bales for a final plaster. The first layer was just dug up clay-rich dirt in the area (we had to dig up dirt for the foundation so used this for making adobe interior walls and interior and exterior straw-bale plaster). The mud was full of rocks and such but that didn’t bother us because the purpose of this layer was just to get something sticking to the straw-bale, not to make a smooth finish. We applied it quite wet but not as wet as a slip. After that dried we applied a second layer which was the same mixture but also mixed with short straw. This layer was troweled on and the intention was to smooth undulations. We let this layer dry and then added the third layer which is described as the second layer above. This was the only sifted mixture and was sandy mud with no fiber.
The plastering stage is the time to add in any relief sculpture you may want to include. This is also a good time to add niches or other aspects that will indent on the walls.
For indenting spaces simply chip away at it with a machete. You can then use a plaster t o smooth over parts that are rough and shape more with that.
To add relief you use a very similar mixture to the wattle-and-cob mix. Dip pieces of long straw in a clay-rich mud mix until they are fully coated. Then wet the wall where you want to add them onto and push the clay-straw onto the wall with a bit extra mud to help to lock it onto the wall. Add piece by piece to the thickness you desire. If your piece will come out quite far the clay-straw may become a bit too heavy to hold itself. If you have time you can let it dry in between courses and keep adding on more pieces a bit at a time. If you want it to go quicker you can insert small pieces of sticks or bamboo into the wall and braced into the clay-straw. This will provide the structure it needs until it dries when the clay-straw will provide structural strength.