[Cob] Rototiller Cob

[Henry Raduazo]

	The Washington DC Cobbers Association has begun a series of garden structures.  All will be fabricated using the fast and furious rototiller mixing system. The first to be completed is the Garden Wall at Maryland University. This is intended as a bee habitat for the wildflower garden scheduled to be built in the fall and spring this year and next. In this structure we used tiller mixing on asphalt pavement. The next structure to be completed will be a combination obelisk and a water harvesting trench in which we will be studying pit mixing with a rototiller. The third structure (also rototiller pit mixing) will be a round column with a Sun Goddess seated on the top. Anyone who lives in the Washington, DC area and who would like a free one day lesson on pit mixing cob please contact me. I expect to complete the obelisk in the next week and begin on the column. I am particularly looking for someone who would like to try cob sculpting the Sun Goddess. I suspect this will require a combination of wet forming and dry carving. The sculpture will be treated with boiled linseed oil for protection from the weather.

	The Bee Wall at Maryland University 
(This is the short version, there will be a longer version with pictures and details about attracting solitary bees for garden pollination and an article about the Bee Box built into the wall. People wanting the longer picture version should contact me separately raduazo at cox.net)

	It has been two weeks of hard work and a little bit of frustration but our bee wall habitat is done. Our goal was to build a meandering wall on Maryland University Campus suitable for earth nesting solitary bees and other critters, and a protected bee box with openable viewing panels and a microphone for viewing and listening to bees attracted to our bee box. 
	The wall has layers of materials with red, brown and even a black layer containing top soil. The contrasting colors are to simulate the cut that a tiny stream might make in passing through a meadow. 
	Construction started by spray painting the grass to show the wall footprint and scraping off the sod. We originally planned a wall that was to be 24 feet long, around 2 feet wide, and 4 to 8 feet high, with a cob roof at the higher end. That turned out to be too ambitious by half considering the working conditions and the difficulty we had obtaining materials when we needed them. We also planned to have a foundation, but the soil there was so hard and compact that a foundation was not needed even at the projected 8 foot height.
	Our second step was obtaining urbanite. There are several places on the campus where construction crews had torn up piles of rubble. Unfortunately all of it was reinforced with "pig wire," a heavy wire mesh with 4" x 4" openings. Most of the urbanite was between 4" and 6" thick, and I want to especially thank the two young men and one woman who helped me pick through the rubble to find and separate out pieces that were suitable. We ended up recycling an estimated 7,000 pounds of old concrete.
	The concrete had been poured onto a bluestone base, giving us a finished side and a gravely side. After some discussion we decided to place the gravely side facing out. The center of the wall is also urbanite standing on edge, with small chunks bridging spaces between larger chunks to produce a base that is honeycombed with passageways for drainage, and for snakes, toads, crickets and other things that like to live in such places. We also placed a section of drainage pipe in there but just ran it into a shallow French drain.
	Picking and placing the concrete took three full hard days and many cut fingers since the pig wire had to be cut or broken off where it stuck out on the outer face and the pieces we wanted were usually attached to pieces that we definitely did not want. We cobbed it in place on the interior. Our ideal piece of urbanite had to weigh 25 to 75 pounds, small enough to lift and put in place but big enough to stay where we put it. Preferably we looked for a flat lower surface, but nearly perfect pieces were reshaped with a 26 pound breaking hammer, a 4 pound sledge hammer and a wide range of chisels or, as a last resort, a diamond saw.
	We also made good use of an ugly stick. An ugly stick is just that, an ugly-looking 15 pound wooden club with a handle on it. If you try to make one out of a beautiful straight-grained piece of wood it will quickly split and become useless. If on the other hand you select a piece of wood with knots and lots of irregularities in the grain of the wood it should last for many jobs. We used the ugly stick for tamping down the soil in the footprint of our structure so that the soil will not settle. Also, we used it for tamping down and leveling the various pieces of urbanite so that slight irregularities in the bottom of the urbanite would be closely meshed with the underlying soil and so that the urbanite pieces would stand perpendicular as we built around them. When some of our pieces of urbanite started to tilt out we used the ugly stick to pound the pieces of urbanite back into vertical. We also used it to break up lumps of dry hard-packed clay in order to make a special plaster used for cob mixtures. At some points in the wall you can see streaks of bright red clay. This color came from  lumps of red clay that we separated out from ordinary reddish brown clay and shattered on the pavement with the ugly stick prior to hydrating them with a 1/2" drill and a mixer paddle usually used for mixing drywall mud.
	The foundation wall is often referred to as a knee wall because in many English dwellings it is about knee high. My goal here was to provide a 6" to 12” air space between the bottom of my cob material and the earth surface. Water from heavy rains should be able to flow around and below the urbanite and into our drainage pipe and French drain with out touching the cob. In places where foundation pieces did not provide at least 6” of clearance we placed additional pieces of urbanite on top of those contacting the earth. These additional pieces of urbanite look like they are cobbed in place but in fact they are supported in place by small stone wedges, and then sandy cob was forced into the cracks between upper and lower foundations elements to look like they are mortared in place.
	The faux mortar was made by rototiller mixing 4 wheelbarrows of mud and sand material on an asphalt road surface to make small very sandy batches with little straw.  This was force into cracks and crevices. The high sand content minimized shrinking and gave us fast drying times. When the entire first layer was finished we left our work for a weekend of undisturbed drying.
	Finally we where at the point to begin our cobbing runs. We had one clay supplier with reddish brown clay that was available for the remaining 7 days of cobbing, but sometimes, when there was a concrete pour going on at the building site, we were denied access to the site and had to go elsewhere for clay. There was another another supplier of dark bluish brown clay that appeared and disappeared in the middle or our work. This is normal. Usually, as in this case, the site manager had to remove a quantity of material, so he rented trucks and a backhoe for two days and then he was done. Finally, the garden manager at the university gave us a batch of nearly black clay with lots of topsoil in it. The lack of storage space forced us to stop work many times while pickup trucks roamed the campus looking for someone to provide us with dirt. Also, we did not have the space to stockpile materials because we had only a small bit of paved surface available to us, and in this space we had to park our cars, store the sod and gravely dirt removed from our wall footprint, store our sand tools and other materials, and do our mixing. This gave us only enough space to tiller mix one batch of clay, sand and straw at a time, and no space to stockpile the differently colored clays as they would have blocked our parking spaces. As a result of this our structure became less than half of what we originally planned, but we still mixed and moved around 20,000 pounds of cob with a crew that varied from 4 to 6 working adults and a few one-day volunteers and children.

			The Mix
	My experience indicates that digger bees, particularly Anthophora plumipes and Anthophora abrupta, our target species, is that they prefer a mixture of mason’s sand or mortar sand and rock-free clay. I know for certain that they do not like cob made with rock dust. I have several cob structures on my property and no bee has ever successfully nested in cob made with rock dust, probably because of the large numbers of small pebbles in the mix.  Concrete sand is just a little coarser than mason’s sand, and I have no idea if it is acceptable for bee habitat because I have never used it.
	We started with small sandy batches, 4 wheelbarrows in size, and tried to keep the mix fairly dry to limit splooging and to prevent large amounts of shrinkage around our urbanite foundation. These small batches were well mixed and moved to wheelbarrows using dirt forks and flat-bladed sand shovels, which can be slid along the paved surface to lift the heavy blobs of cob. Often we found that the mix in the wheelbarrow was too dry so we had bottles of water available all along the wall so that we could add water as needed to form cobs. Often the cobs were granular and would not hold together. We were able to improve their physical properties by slamming them on the sides of the wheelbarrow and punching out air spaces with the side of our fists.
	Later, when we began using more clay, the batches got stickier and we began forming giant cob pancakes by slamming clay on the wheelbarrow sides to improve their feel and pound out air pockets.
	The largest clay-rich batches consisted of 4 wheelbarrows of sand and 4 to 5 wheelbarrows of clay. These batches, when mixed with water and straw, were about 2,000 pounds each. The blades of the rototiller reach down only 3” to 4” inches, so we tried to work thin layers by piling up the mix and then pull down the deep spots by dragging the tiller backwards against the forward movement of the tiller blades.  Assistants with forks and sand shovels tossed dry material onto wet spots, and the tiller operator would again run the tiller up onto the toss area and again drag the cob out by pulling the tiller backwards against the movement of the tiller blades. 
	We mixed only sand and clay at first, adding water till we had a mix that was just a little wet and sloppy, and then added straw to dry it up to the perfect mix. This gave us a huge leeway to adjust the moisture just by adjusting the amount of straw. One big surprise is that we did not need to use chopped straw. I have always assumed that long straw would wind itself up on the tines of my tiller, and I came prepared with a mason’s hammer thinking I would need to cut and chop straw and mud off the rotors, but it turned out that after 7 days of cob mixing the tiller was not clogged enough to need cleaning.
	We are using a cobblestone finish in areas of the wall not protected by our roof structure. This is formed by applying a thick layer of cob, letting it dry for a few hours, and then pounding rocks into the surface. The theory behind the cobblestone surface is that when it rains water readily soaks in and saturates the surface of the cob between the rocks and then begins to run off. The rocks however provide a choke point. Water passes deeper into the wall slowly and when moisture passes the choke point  it is wicked quickly into the larger mass of cob without ever saturating more than an inch into the surface.
	Our Bee Box was imbedded in a cob column adjacent one of our deadman roof support columns. After only 5 days of building and a weekend of drying time the deadman posts seemed strong enough to support our small corrugated roof.

	That’s about it! For an illustrated version of this please send an email addressed to me, Ed Raduazo (raduazo at cox.net), and I will try to send a response as soon as possible.	

Footnote: Visiting this structure is a little problematical. Even though there is a nearly empty parking lot 50 yards away, parking costs twice minimum wage ($15 per day), and the fine for not having a valid parking sticker is 10 times minimum wage ($75). I got a ticket while looking for the script machine. I sent my ticket and my $15.00 script to the parking authority and I assume my ticket will be forgiven, but 15 days have passed and I have not heard from the parking company. This means that the ticket is now automatically advanced to $95. Theoretically they can take possession of my car if they find it on campus property. That would mean a $100 + towing fee and a storage fee in the impoundment lot starting the moment my truck is towed to the lot. Our Grad Student Lisa also got a ticket which she is hoping to have forgiven. Obviously I am not going to recommend that anyone who is not a student go Maryland University unless they use public transportation or a bicycle to get there.