How To Make Clay Plaster | COB HOUSE Part 6

In this tutorial series we're learning how to build a cob house from the foundation all the way through the reciprocal living roof and so far in this series we've learned how to build a rubble trench foundation and earthen floor, how to make a cob mix, and how to build cob walls. In this episode we'll be learning about the sculpting and plastering processes which really bring the building to a whole new life. We left off last episode on day 15 when we had just finished building the cob walls and on day 16 we prepared to start adding sculptural details to the walls. We started the day by meeting with the owners of the home who showed us some of the design details they wanted to include in the sculpting process. We then had a brief demonstration on how to work with a sculptural cob mix. For this niche we wanted to add a sculpted shelf to the flat wall and in order to add support for a larger mass like this it can be helpful to embed structural support into the cob wall. In this case we're using strips of bamboo hammered in at a diagonal angle to support the weight and applying clay slip again helps to bond the bamboo to the cob. Similarly to building a cob wall you also want to make sure that the wall is moist as you begin sculpting. There are lots of different ways to make a sculptural cob mix but in this case we're just using the same mix from our cob walls with a little bit of extra moisture to make the more shapeable. In the same way that we build the walls to be a single monolithic mass we try to incorporate the sculptures into the wall system as much as possible by integrating the mix and sewing through it all the way into the wall. When building with structural support like this it can be helpful to wrap the cob around the top and bottom of the supports and sandwich the bamboo as you build out the sculpture. As you add more mass be sure to continue integrating the cob into itself while also making sure to integrate fully into the wall system as much as possible. Sewing through the mix like this ensures that the sculptural cob is fully incorporated into the cob wall. For sculptures with less mass you repeat a similar process just without adding the structural support. The sculpting process is more about building out the general shape and you'll want to sew through the mix into the wall which leaves it full of holes as it dries. You can then come back in the plastering process to add finer details to your sculptures. After the demonstration we spent the rest of the day building out our sculptures. We brought the goats over to be our live models as we sculpted them into the walls. So beautiful. It was really amazing to see everyone's creative abilities come to life as we co-created this beautiful space together. On day 17 we started the plastering process which would take four days to complete. Using the plaster samples that we had already applied we took note of which plasters had the best performance for their durability and finishes. We decided to use one part clay soil to one part gray sand, one and a half parts white sand and half a part donkey manure for the interior plaster, and one part clay soil two parts gray sand half a part white sand and half a part manure in the exterior plaster. This would give us a finer finish on the interior but a more durable finish on the exterior. These ratios can be used as references but you should make various test plasters with the materials you have available and determine your own ratios based on how your samples perform. The clay soil will again act as a binder in the plaster mix and we pass it through a quarter inch screen to remove any larger rocks. Both of the sands create the structural parts of the plaster mix and we screen the coarse sand through a one quarter inch screen as well. Our white masonry sand is already screened and lastly the donkey manure adds fiber to the mix which increases the tensile strength and we soak it and then blend it with a paddle bit on a drill. Now that we've prepared all of our materials we can begin dry mixing the clay and the sands for our exterior mix in the wheelbarrow. Once the clay and sand is well dry mixed you can add the manure and begin mixing again. You can then gradually add water and mix until the entire mix is homogenous. This is roughly the consistency that you're going for with the moisture level. It's also possible to make a plaster mix on a tarp like this and we all split into teams and made a bunch of plaster before adding it to one bin and mixing it all together. To prepare the wall for plaster you can trim any long strands of straw that are sticking out and then apply some water to the wall to moisten it back up. For flat portions of the wall an easy method to apply plaster efficiently is using a hawk and trowel. The hawk is the board that holds the plaster and using the trowel you can press the plaster off of the hawk and spread it upwards on the wall. During the plastering process you want to make sure that the thickness of the plaster is roughly even along the entire wall so that way it can dry evenly. Once you're able to get some plaster to stick to the wall you can then go back and even out the finish. You can achieve similar results by just throwing the plaster on the wall and then going back and troweling it until it's even. For more intricate designs such as the vine on this window you can use yogurt lids with the edges trimmed off to help you achieve a smooth finish since they're flexible and can be bent to the shape that you need. Yogurt lids can be helpful for rounded corners as well. After the demonstration we gathered our plaster in five gallon buckets and began applying it to the exterior of the building. We start at the top of the wall and work our way down. Another useful tool for fine detail work is a metal spoon. We'll be adding one last run of cob and plaster once the roof is installed so we put scratch marks in the plaster at the top of the wall to be able to integrate the final coat at the top. After applying some more plaster and focusing on detail work we were a little over halfway done with the exterior of the wall at the end of day 17. On day 18 we continued plastering the exterior of the building. After the plaster had set overnight and dried slightly we went back and burnished it with a yogurt lid to achieve a smooth finish. On day 19 we made our new interior batch of plaster and we moved to the interior of the building. We again wet the walls down and continued plastering the interior. Now that the exterior plaster had set and dried a little bit we also went back and burnished some more of the exterior. Sculpting the detail work at the edge of the wall into this flowing organic shape really adds a lot to the finish aesthetic of the building. Day 20 was our final plaster day and we continued plastering the interior as we also focused on more of the detail work. Using spoons we added a texture to this vine that went around the window. You can also embed tiles into the plaster layer by cutting around the edge of the tile, adding some plaster to the back of the tile, and pressing it into the wall. Embedding stones and marbles into the wall is possible with a similar process. This is how the finished interior plaster looked when we were done at the end of day 20. We finished the day with some demonstration on the reciprocal roof design and a beautiful rainbow. In the next episode we'll be learning about building a reciprocal living roof which will be the final episode of this tutorial series.