I currently live among the loess deposits on either side of the Missouri River basin. Has anyone had experience making cob with loess subsoils?
I understand that along the Yangtze River basin houses and other structures are traditionally excavated out of loess soils.
Soon, however, I will be moving to my family farm that has heavy clay soil with high shrink-swell capacity. How well do expansive soils work as the clay component of cob?
Also, there are several old gravel quarries near the farm and there are large piles of old limestone screenings. I know the owner and could probably obtain all I want for cheap or free. How would limestone screens mixed with clay and straw do for cob?
Since you haven’t got an answer, I’ll post a seed. Someone can jump in and set me straight if nothing else. ;)
I’m not a RMH person, but I’m a potter. I’m betting that most folks using local clays will be using clays similar to yours. Lots of shrinkage. That’s both a blessing and a curse. More plastic clays (to a point) are easier to work with. OTOH, since they do shrink so much, adding wet cob to your drier underlying mass will get you a lot of cracks. The straw should help and I would expect a finer chop would help more than long strands. From what I’ve been reading here, I wouldn’t think you could harvest any clay that would fail to hold up in the heat conditions for the stove’s mass components. For the fire box and stack of course, you’ll need refractory materials, probably premade firebrick. Insulating brick is softer, but holds the heat inside better. Hard firebrick isn’t as retentive, but it’s tougher. It sounds to me like folks use both kinds.
Location: Eastern Kansas
posted 2 weeks ago
Thanks Cindy. That's good information. I'm specifically thinking of construction applications but an RMH is also something I do want to do eventually.
I like your idea about using chopped straw versus long straw. It seems that that would allow for a higher percentage of straw in the mix and therefore more cushioning against shrinking and swelling.
Since I'm new to cob construction I have a question regarding straw also. It seems to me that the straw would break down very quickly and you would lose any binding property in a very short period of time and be left with only a network of elongated pores throughout the mass. Since this apparently is not a problem in real life, I'm wondering if someone could explain it to me.
I’m not sure what the straw is believed to be accomplishing in cob construction but I can speculate. First, in the dry clay, the straw is protected from many of the things that would otherwise break it down, so it probably does retain some structural utility. Since straw is hollow, it can also provide some insulative qualities. Even if it did ultimately break down, by then the clay would be dry, so the air spaces would remain. Finally, I’ve been told (haven’t tested it) that wet straw can have some adhesive qualities (like oatmeal does). If that’s the case, it would definitely help to hold an unfired structure together. (BTW, I’ve heard of people building the whole structure of clay, then setting a bonfire inside to dry and harden it. You’ve probably come across that too, but I just thought it was so cool!)
Cob is really messy looking to me, with all the whiskers sticking out everywhere. Chopped straw might mitigate that somewhat, but I’d be careful not to overdo it with the straw. Too much could weaken the clay. You can also add sand (not too much... I’d start around 15% by volume). That can modulate expansion/contraction as well. And for the whiskers, I’m sure a propane torch could help with that.
I missed the bit about the limestone screenings. Clay is really special stuff for something so common. Its individual particles are plate-shaped. This gives it its plastic qualities. It can only be formed by long millenia of erosion and shaping by the waters that carry it to its banks (where we ultimately find and dig it up). That’s why the porcelain and kaolin clays are less plastic. They’re found close to the places where they were first eroded, so no long trip down the river for them. Bottom line though, nothing but clay will work for cob. Nothing else has those properties of malleability and the power to hold the shape you form it into.
Location: Eastern Kansas
posted 2 weeks ago
Thanks for hanging in there with me Cindy. Maybe someone could post a link to some info on the science of cob and/or cob building for dummies?
Hi John, expansive clays are not ideal for cob construction. They can absorb large amounts from the air and weaken the structure with their swelling. Cob is generally 70-80% sand, fines, silt and 20-30% clay, though this differs from soil to soil. You want a good amount of straw as this provides the tensile strength of cob and allows the cob to fluctuate without cracking--think rebar in cement. Straw doesn't add anything to the insulation value of the clay/sand. Cob has a R value of .5-1 per inch with a much better U-value. You do not want to use chopped straw in building applications, the longer the better as this helps knit the whole structure into one piece--try tearing a handful of long straw in half and you get the picture.
These are the best books on building with cob. Your local library should be able to get you a copy if you do not want to purchase, but I highly recommend having your own copy to consult whenever you need.