What a treat to have Bobby Wilcox, cob builder extraordinaire, come from Portland, OR. to lead the 1st ever Cob Building Workshop here on the island of Martha’s Vineyard. Having worked for several years with natural building groups in the Northwest U.S., including City Repair (http://cityrepair.org/), Bobby arrived with inspiring skills and much needed enthusiasm to bring this alternative building method to the island. This fantastic course not only satisfied the natural building craving that has been spreading across the island, but went far and beyond what we expected.
We began on a cold, misty Saturday morning, clutching our hot teas and listening to Bobby describe the history of cob, and his experience building with this mixture of clay, sand and straw (or other types of fiber) in one of the most humid environments in the country. “Good hat, and good boots,” he replied when asked how cob builders in Oregon manage the rain with such a vulnerable material. “Just like any other type of building. You always need a good roof, and a good foundation. Cob is no different.” With cob houses in the U.K. that are standing after 500 years, it is clear that this building method is much more than a hippie hobby.
Due to its formation as a glacial deposit, the island is replete with clay of all different colors. The purity of the red, charcoal, and beige clays that we collected is vastly different from the silty subsoil that cob builders are used to working with, and as such we spent some time experimenting with various proportions of sand to clay. For fiber, we had purchased two bales of straw, but were also given three trash barrels full of sawdust from some fellow carpenters – a bit more accessible, a bit more local – to see how it would work as an alternative fiber material.
Following Bobby’s lead, we leveled the ground, and built a foundation using cinder blocks and rocks we found on the property. We discussed the different layers of the oven, and their functions, and began mixing an insulating medium with clay, sand, and good amount of sawdust. This would keep the heat from escaping through the foundation.
The first layer, the “thermal mass” of the oven, was made out of a clay/sand mixture, without any fiber. We piled this around our sand mold (separated by wet newspaper) on Sunday, and after letting it dry throughout the week, emptied out the sand on the following weekend. A nerve-wracking step…but the oven held together!
Around this, we piled a mixture (with sawdust) as a final insulation to keep the heat inside the oven. We also began laying large, flat rocks around the base to begin the aesthetic phase of the oven…as well as the bench!
On our last two days, we got into our mixing groove – laying out tarps of clay and sand, and dancing with our feet to mix it as thoroughly as we could. By slowly adding straw and pounding it into the mix, we created a true cob mixture to pile on the final layers. This went around the cinder blocks, around the oven, as well as on a foundation of large rocks that became a beautiful bench.