Natural Building Project–Phase 2

Messy Feet and Happy Hearts

With our foundation, framing and stem-wall complete, we were ready for the good stuff to begin. Our local Natural Building guru Pete Poli found some clay for us from a water main dig in Mettawa. He was looking for clay with little to no organic matter. A common waste product from construction projects, we were able to obtain the clay for no cost, other than the transportation to and from to pick it up. The clay was dumped onsite and covered with tarps to keep it dry.

Sand was obtained from a nearby landscaping supply. We were looking for angular, course sand–not the refined play sand. We had 2 tons dumped onsite, in an area that would later become a permanent sand box. Having the materials as close to the work site as possible made for less work hauling materials around.

Straw was easy to find and needed to be kept dry. Lastly, Pete had me mix up a material called slip, a product that ceramic artists will be familiar with. It’s clay mixed with enough water to form a pudding-like consistency. This material works as a glue between layers that have dried from prior work and new layers. I obtained free barrels with waterproof lids from our nearby mulch facility–their mulch dye (ewww) comes in pretty useful barrels. I used a paddle mixer on a drill make the slip and remix it when needed.

Prior to our cob making class with Pete, I made test bricks of varying amounts of clay/sand/straw. Since clays can vary substantially, there isn’t a set ratio. I tried 1 part clay: 1 part sand, 1 part clay: 2 part sand, 2 parts clay: 3 parts sand. As well as some bricks with straw and some without to illustrate how the fibrous material adds strength.

We added nails pounded partially into all lumber surfaces that would be eventually surrounded with cob to keep the cob bonded to the normally too smooth surface of the lumber.

Even though I had watched youtube videos and read books on making clay, nothing beats in person instruction. So one evening in late August, Pete joined a group of eager learners and introduced us to the wonderfully therapeutic craft of making cob.

We evaluated the strength of the test bricks and determined that the 2:3 clay to sand ratio was the strongest. We translated that recipe to 5 gallon bucket loads of the materials. On a tarp adjacent to our worksite, we started with 2 bucket loads of clay, added water and got our barefoot into it. As the water loosened the clay, it started squishing between our toes, a sure sign that the clay was ready for sand. We added the sand slowly, working it in with our feet. As more and more sand was added, the mixture became less squishy but stickier.

At times we would fold the trap over the mixture and step on the tarp to mix the material. We would also use the tarp to flip the mixture over for better access to the material that was on the underside. When the material became bonded together enough to be formed into a ball without falling apart, we added straw. We sprinkled in on in layers and worked it in with our feet. Wetter mixtures were easier to work with, but getting the mixture too wet would require some drying time before it was ready for building. Sunny and windy days would dry the mixture quickly (sometimes too quickly and would require rewetting).

Once the mixture was easily formed into balls, we were ready to build!! The stem-wall was covered in slip to help establish a bond between the stem-wall and cob. Then the balls of cob were added to the walls and wooden rods (cut and carved from broken tool handles) were used to poke aggressively through the mixture to integrate the straw throughout the layer. We attempted to keep the wall height uniform as we built. And as we finished applying our first batch of cob, I realized shockingly how many batches of cob this was going to require! 😳

After a couple of batches with Pete, we were feeling like we were getting the idea of the feeling and consistency we were aiming for, as well as getting a rhythm between a team of cob makers and wall builders.

At the end of the evening, Pete showed us how to build a “key” which would help link and lock future layers with older layers that may dry in between build sessions.

The key was time consuming to build and not necessary if you could keep the top layers of the wall damp enough to integrate the new layers using the wooden rods. For the remainder of the build over the next two months, I would keep the wall from drying out by covering with tarps to protect from the wind and drying sun. If it was particularly dry weather, I would spray the top portion of the wall with a misting spray to keep it moist until the next build.

Numerous work groups joined our build! Homeschool groups, scouts, corporate work groups as well as having a few very dedicated volunteers who couldn’t get enough of the messy process. Thankfully so, because my goal was to finish the cob portion of the build by early October, to give the walls time to dry before freezing weather.

Adding windows

The live edge lumbar for the windows came from Pete’s stash. The larger sections of oak came were purchased from GH Woodworking in Mundelein. Mundelein resident and a very lovely man Earl Brown built the large window for us. When the time came to add the window frames, we laid them in the level cob and added some supports to hold them in place. We pounded nails to the sides of the windows that would be covered in cob to hold the window in place, and then continued to build the walls up the side of the window frame.

The arched windows required a bit more planning. We built a mold to create cob bricks to form the sides and the arch. Gravity would have likely caused a free form cob window to collapse. In addition to using bricks (held together with slip), we found a rigid pipe section to provide support until the arch dried.

Power tools have their place

I saw cooler fall weather approaching, and my compulsion to finish was strong. My cob making dance crews were back in school and dancing solo seemed, well, not as fun. I experimented with using the paddle mixer with an electric drill to mix clay and water in 5 gallon buckets until there were no clumps of clay. I slowly added sand, using the same ratio we had used on the tarps. My mixtures had more water than with the tarps–the mixture resembling a Wendy’s Frosty. With a bit of trial and error, I discovered that if I mixed the clay/sand mixture the day before I intended to use it for building, it would dry the right amount and be ready for incorporating the straw on the tarp right before building. I estimate that this sped up the cob mixing process 3-fold. It became a single person process if need be, or a very speedy process with a 2-3 person crew.

Cob-scaping

Cob walls are wild and hairy looking. I’m guessing that many leery onlookers were thinking that our creation was looking very amateurish and nothing at all like the example online pics I had proposed. But unlike many building surfaces that are purchased flat and perfect, a flat cob requires effort to create. Conversely, curved surfaces can also be formed with tools and hands to create organic curves, making cob a superbly creative material.

Contrast the trimmed lower walls with the untended upper walls

We wanted reasonably flat walls, so once the walls had partially dried, we used a wood saw to sometimes cut and other times shave off extra material. The trimmings were recycled into the next batch of cob. After this process our walls were less hairy looking, but still had plenty of roughness for the next layer of clay plaster to adhere to.

A coat of plaster

At least a month before freezing temps, we applied a coat of clay plaster to our cob walls. This material was mixed from our slip, some finer grained sand, and chopped straw. Some mixes use horse manure or horse hair for fiber, which, had I been collecting materials earlier in the year, I could have acquired plenty of horse hair from nearby barns. Unfortunately for me, this was the time of the year that horses needed that hair for winter warmth. I opted for chopped straw , which was my biggest challenge material-wise. I decided to run the straw through my garden chipper multiple times until the straw pieces were 1/2-3/4″ long. I needed tarps to contain the flyaway straw and every chipping session left a humongous mess to clean. But it did work. I used the paddle mixer and drill for this mixture. I discovered that this is not a mixture to make in advance! It will decompose anaerobically and smell like a pig farm within 24-48 hours. Gross. This mixture can be applied with plaster tools, but we mostly used our hands. The following year we would be adding additional layers of plaster, so we intentionally left this layer with a rough finish so subsequent layers would adhere. In fact all layers were left rough until the final layer of lime.