Melissa and I are strong advocates of using what you have, to the extent that you can make it work, rather than focusing on getting “ideal” natural materials from a distant location. At home in Sacramento, our subsoil is rich in clay. Somewhat unfortunately, it is also rich in silt. We wondered, can you mix and build cob with this stuff, and if so, how, and what are the results like?
Compared to a high-clay, low-silt soil typically favored for cobbing, cob made with silty-clay soil takes more sand and a lot more dancing (x2 to 4 times as long on the tarp), goes from not wet enough to too wet in a narrow range of added water, can’t be built up in a lift on the wall very far (4 to 6 inches max) before it slumps down and sideways (“splooging”), is generally not plastic enough to take blending deeply into previous lifts without rupturing, and winds up much denser.
To work with this stuff, myself and Melissa along with our friend Effren Velasques developed what Ianto Evans has dubbed “Sacramento Cob.” Each lift is added to the wall in a layer 4-6 inches thick, with some splooging allowed. Instead of an Oregon cob “spine and rib”-style finishing for a lift, the upper surface of a Sacramento cob lift is formed into a series stubby ridges and pockets, with ridges a few inches high and several inches wide, and pockets a few inches wide and about 2 inches deep, or slightly more. Effren calls the pattern “tire treads”, which gives an effective visual reminder of what works. The lift is then allowed to dry enough to “set”. Sometimes our cobbing sessions may be days apart, time enough to allow the wall to dry very hard. The pockets allow easy re-soaking of the cob at the wall’s top to be ready for the next layer.
When it is time to add another lift, the top of a set lift is wet repeatedly a for an hour or so before the next lift is added to allow some of the set cob to soften up and get sticky before the next lift is added. The pockets hold water, potentially allowing deeper moistening if water is added the evening before another lift is planned, though we have not found this to be necessary to get solid connection between lifts. To the moistened surface of the set cob, we add clay slip, then freshly mixed wet cob, thumbing to make sure this new layer is thoroughly cross-integrated with itself, and thumbing downward where possible to enhance connection with the set lift. The integration zone between lifts is quite narrow, as shown, yet sufficient to bond. This may sound a lot like adobe construction. Unlike adobe, Sacramento cob has no slip planes between lifts - each lift, at a minimum, is locked to the one below by the ridges and pockets left on top of the previous lift.
The resulting wall is very dense, very heavy, and surprisingly strong. Ridges in the “tire treads” are very tough, and a lift-layer, even if visible as a discrete layer in the wall due to variation in the color of cob batches, will not lift off or separate from the layer beneath – they are truly united structurally. Sacramento cob will carve easily, and is very vulnerable to water. Unlike typical Oregon cob, Sacramento cob falls apart in a frothy mess very quickly when you get it wet. Its density means it offers less insulating value and more thermal mass value than typical Oregon cob. It is very easy to shape and carve, as it becomes friable when chipped at or abraded with a rasp.
We love the fact that our tiny cob house has arisen from the soil of our own yard, and the extra dancing is fun! So is carving, not as easy with tougher Oregon Cob. I also don’t mind not being in a hurry. If all this resonates with you, and what you have on hand is silty-clay soil, go ahead and give Sacramento cob a try.
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