On Friday we had an Oregon cob demonstration. Oregon cob is another form of earth architecture, developed in the Pacific Northwest, but based on ancient building techniques that were used in Devon and Cornwall in England for centuries, and also in the southwest of Ireland. In cob construction, you don't make bricks and then stack them, as in adobe construction; it's a "monolith," in the sense that the wall is like one big adobe brick that has been formed gradually. You start forming a "course" of cob on top of a stone foundation and you work all the way around the foundation; then when that dries, you trim and shape it a bit and start the second course.
The first thing to do is mix the clay and sand, just as in adobe brick making. The clay and sand are shoveled onto a tarp, and then mixed together by rolling the tarp. (Both were sifted first through a 1/4" screen.) The clay makes the mixture stick together, and the sand gives it compressive strength and prevents cracking.
Then you add water and mix with your feet.
Straw is added to give tensile strength to the mix. (Sometimes manure is used also because it too has fibers in it. The fibers in manure have been chopped up a bit more, obviously, by the animal's teeth and digestive system.)
Finally the cob is shaped into a loaf-like shape on top of the foundation. The cobber uses his thumbs to push down into the cob. This causes the cob mass to cohere, and it also creates a surface for the next layer of cob to stick to. It's a bit like a wet mushy lego.
It's interesting that walls can be formed without bricks or even forms .
Cob walls are often very thick, as much as 24" thick. The first course is often wider than subsequent courses; the walls can taper as they go up, but usually they are never less than one foot thick. A rule of thumb is that for every one unit wide the wall is, it can't go over ten units high, but in practice, according to our teacher Stevan, this rule is sometimes violated to no ill effect.
A thin coat of lime and clay is often used as a plaster on the outside and inside of a cob wall.
Cob houses usually have framed roofs, but it's possible to make an arch by corbelling the cob inward gradually. I'm not sure if cob vaults are possible, though.
Simone watched the cob demonstrations in a beautiful North African style robe that she sewed herself from fabric she bought in Ojinaga. (More about that later on my other blog, All Fibers, All the Time.)
Later that day we met with master adobe craftswoman Jesusita Jimenez. Jesusita was the project manager for Simone's house and several other adobe houses in the region. She is retired now from adobe building, but her influence is still felt in current adobe construction. Her craftsmanship, precision, and speed in adobe masonry are legendary. She is shown here with El Maestro, Sevan de la Rosa, our teacher during the adobe workshop. I'll write more about what Jesusita had to say in my next post.