The Cob Mix

click on image to enlarge

"Cob" as a natural building material is a term that most find unfamiliar outside the sustainable building community.  Cob is essentially a mix of straw and clay soil. When I use the word 'cob' in this blog I normally provide a link to wikipedia so the reader can get a good definition. Click here to learn more.
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In this entry I'm going to focus on how I make cob and some background on cob as a building material.

I use an 8-horsepower concrete mixer with a 1-cubic yard capacity, instead of hand-mixing.  My walls are up to 2-feet thick and I estimate that when completed, I will have applied between 40-50 tons of cob to make the walls of my studio.  In order to finish the building in my lifetime, I find the mixer an essential tool to make the 2,000 pounds of cob I can apply to the walls on a productive day of 'cobbing'.

When using a mixer, the batch of cob needs to be wetter than if mixed by hand.  This is because if the straw-clay is too dry it will 'ball-up' in the rotating drum and not mix throughly.  I usually start with about 15-20 gallons of water, then start the mixer.  I throw in about a 14-inch section of straw from a 3-strand straw bale. As the mixer rotates, the straw thouroghly mixes with the water.  At that point I begin to shovel in the clay soil.  The clay is first sifted through a 1/2" screen to remove rocks and other inclusions.

Note that my soil is almost pure clay and some books recommend adding sand to the mix.  This helps keep the clay from developing cracks when drying.  I decided that I didn't want to have to haul in sand for this project, so what I did to compensate for the lack of sand was add more straw to the mix.  I'm finding that this works fine with very little cracking in my walls to date.

I continue to shovel in clay soil into the mixer until the batch looks like this composition (consistency) as shown in the above photo.

I call 'cob' natures fiberglass.  The interlocking fibers of straw in a clay emulsion are similiar to glass fibers in an epoxy polymer (fiberglass).  Two feet thick of cob provides a high-strength monolithic wall, that is both a structural (weight supporting) and a thermal mass system to mitigate large swings in outside air temperature, thus keeping a structure comfortable and/or requiring very little heating in the winter and cooling in the summer.