Machine Mixing Cob for Barefoot BuilderPosted: July 7, 2012
Christina Ott is a natural building consultant and owner/operator of Barefoot Builder, she also happens to be a very good friend of mine.
During the nice sun shining months of summer, Christina offers a number of natural building and permaculture related workshops. Christina hosts workshops at her home base outside Woodbury, Tennessee but she has also held workshops at various locations across the country.
During the summer of 2011, I had the unique opportunity of working with Christina on her “off grid” cob building workshop in Middle Tennessee. The workshop helped our friend Mati Karol, of the Daffodil Meadow Contemplation Center, grow his presence at the meadow.
This summer, Christina invited me back to work with her again as her tractor operator for her 2012 “building with cob” workshop at a different site in Middle Tennessee. As the tractor operator, I was responsible for machine mixing the ingredients for cob (sand, clay, water and straw) in large quantities and delivering it to the building site as opposed to the labor intensive, but educational, foot mixing method.
For Christina’s 2011 “off grid” cob building workshop Chris McClellan performed the duties of tractor operator using a small Bobcat skid-steer loader. For this particular instance, we used a Kubota M5040 tractor with a LA1153 front end loader. The Kubota is rather large for the task but convenient as it is the site’s resident tractor and as such I was already well acquainted with the machine. If renting a machine, the Bobcat skid-steer might be a more appropriate sized machine for most job sites.
The type of sand sourced for this kind of project is rather important. Christina uses a limestone manufactured sand produced from a local quarry down the road. This sand is slightly finer and certainly more rough than what I would describe as gravel. Another important attribute of this sand, in addition to it being rough, is that it is also relatively flat, like stones in a stone wall (this is the ideal behavior). In fact, cob can often be figuratively related to a stone wall, where the traditional mortar is supplemented with clay. The straw in cob behaves much like glass fibers in a fiber reinforced plastic structure (fibreglass).
For this particular building workshop I mixed four batches of tractor cob. Each batch contained six tractor buckets of sand, three of clay, two bales of dry stored straw and approximately one-hundred-gallons of water. The ingredients were mixed primarily with the large front and rear tires of the tractor. The tractor bucket was used minimally and primarily to keep the mixing pile confined to a relatively small area and to occasionally turn the batch to insure thorough mixing. The sand, clay and water are well integrated before adding the straw. It is important to add the straw last to ease mixing of the other ingredients and to prevent excessive segmentation of the straw.
By my third batch, I had a routine that I believe could produce cob that exceeded normal expectations for machine mixed cob. Christina explained, that all too often, machine mixed cob is of inferior quality as compared to hand/foot mixed cob. This is because machine mixed cob often consists of large veins of under mixed material usually resulting in large chunks of unmixed clay. Unfortunately, my second batch of “tractor cob” was typical of machine mixed cob and was certainly inferior to the other three batches I produced. However, by my third batch, I believe I had figured it out.
After I mixed the sand, clay and water for my third batch I covered it for the night so that I could add the straw and deliver the batch to the building site early in the morning. I heard that after I had covered the batch for the evening, at least one workshop participant checked the material under the tarp to insure it was of better quality than that of my second batch. Of course, I took additional measures to ensure that all batches following my second batch were up to snuff.
The workshops participants worked long and hard for approximately ten days in Middle Tennessee while a high pressure system parked above them producing record breaking temperatures for the region. It was a fun group and I’m thankful that I was able to work with them and Christina.
The building features a stone wall foundation that integrates large boulders already present on the site, a beautiful and creative mural, colored glass, framed door and windows, and a large built in bookcase with a window.