I decided to try out the jar shake test to get an idea of how much clay I have to work with in the future. I live in Idaho so the clay content ended up being pretty high (about 70% clay).
There is a lot of info online on how to do the shake test, but not a lot of info on what to do with the data you collect from the test. Like, is there a way to calculate how much sand you might need if you have a certain percentage of clay in order to make a good oven/wall/house? Obviously, you'd still need to experiment/adjust a little bit after the fact but is there a way to use the test to give you an idea of where to start?
Having done the shake test, you now know your soil condition. That doesn't tell you what it is good for. Research cob building and that research should tell you the minimum soil conditions. If your soil is above that, have at 'er.
The thing is; clay can vary greatly in it's composition, some clays are great for adobe bricks or cob and some clay is better for plaster or pottery. Clay is formed by the degradation of the parent rock, so the constituents of the clay are highly variable according to the source.
In Idaho and most of the intermountain west, there is generally a buff colored clay that is wonderful for building with. It has a high lime content, so the bricks made from this clay are very strong and only need a little straw to help them not crack as they dry. This same clay makes a great mortar for the bricks and when a sharp sand is added, a good plaster.
Clay is processed by digging it dry and then adding 2 parts water to 1 clay. Put this in a cement mixer and tilt the mixer forward while running to get the big stuff out. Then screen the clay slurry(window screens on wooden frames) until you get the desired consistency, then put the whole mess into a pit lined with a bed sheet. The water will go through the sheet and you'll have perfectly hydrated clay on the bed sheet, scrape it off and mix in additives as necessary.
You can then press the mix into slop forms, pull the forms right up and let dry in the sun, turning every day until dry. You are ready to build a traditional horno!
As others have said, use the results from the shake test to educate you on how try improvements on your soil (adding sand, straw, additional clay, or what ever). Make test batches and see how well the results match your needs with each type of additive (start, of course, with the raw material and no additives).
My raw material sub-soil approaches 100% clay so my first test showed a nice hard (but excessive) compaction- that fractured too easily. with sand it compacts less easily. I am trying with fibers now (borax/lime treated paper, and cement additive fibers next).
You know you are rural- if you get a big black stain on the seat of your daily wear pants and no-one knows for a month...