On the last day that I was at the adobe workshop, we learned about plaster, a fascinating subject. Jesusita Jimenez had told us that she considered plaster to be the most important, make-or-break aspect of an adobe house.
We made several batches of plaster to check which one would work the best on the particular wall we were working on. It was the wall of one of the vaulted outbuildings at the Swan house. Stevan told us that with any adobe project, it is necessary to do this trial and error testing to get the mix of clay, sand, and straw right for the particular conditions of the site, and the particular qualities of the ingredients you are working with. The objective is to get a plaster that won't crack as it dries. The clay is the element that causes it to stick like glue to the side of the building; the sand and straw keep it from cracking.
We started out with a mix that was one part sand, one part clay or dirt, and one part chopped straw. The sand and clay had been sifted through a 1/4" screen, and the straw had been chopped in a leaf eater machine (essentially a weed eater in a plastic funnel). Plaster can be mixed in a mortar mixer or with your hands.
We plastered this on the wall using hands and trowels, working upwards rather than downwards, and feathering the plaster at the sides as it met the walls.
We also tried three other mixtures, with a bit more sand and straw. Test number two for example had one part clay, one part straw and one and a half parts sand.
When the coarse plaster had dried a little, we put a finer finish later over it. That was when it really got fun. The finer finish layer just had finely sifted clay and sand. You can also add color at this point: we added some yellow color.
I thought this was really beautiful, and I got excited about the decorative possibilities: I began inscribing the wet plaster with spirals. At that point Stevan told me that there are whole workshops devoted to plaster artistry at the Canelo workshops in Arizona. I looked at their website and got very excited. Maybe I will make an earth structure darkroom at my farm just so I can decorate the walls with earth plaster. An adobe vault wouldn't work in Tennessee, but earthen walls of cob or adobe or stucco over straw bale with an adequate roof and foundation should work.