Finding and Processing Clay
Just basic info for now, will expand this when I have time. There is also a good web site about processing clay here.
Deposits of clay are widespread, but finding one near you can be tricky. The soil around here is mostly glacial till, which looks and acts a lot like clay but seems to have too much silt in it to be really good for making cob or pottery. I kept an eye on road cuts while driving around the area and found several areas where there in an exposure of soft, crumbly soil with little growing on it. This is a good sign, but you still need to test it. While the soft soil at the surface looks tempting, I've found it to be poor clay (I think the rain washes out the finer particles, leaving it enriched in silt). So scrape off the top 1-2 inches with a shovel, and fill a bucket with the material underneath. In some cases, the lower material is so hard it seems like rock, but it should be soft enough to cut with a shovel (may need a little force).
Once you have your material home, spread it out to dry. I have some plastic bins that work well, but you could also put it on a tarp or a sheet of plywood. Just need to be able to spread it out while keeping it clean. Leave it in the sun until it seems fully dry, which could be a couple days or a couple weeks, dependin on how dry it was when you got it. It can help to break up any larger chunks. If you're not sure if it's fully dry, give it a few more days.
Once the lumps of clay are fully dry, they need to be hydrated. I fill 5-gallon buckets about 2/3 full of water, and then add about 1/2 bucket of clay to each bucket. You want plenty of water, so the clay will make a thin soup once dissolved. As you add the clay, you should see bubbles coming up from the chunks in the water. I like to let it sit overnight, and then give it a good stir (more air bubble should come up). Let it soak for a few days, giving a good stir every day or two. You can continue as soon as the mixture becomes thin and soupy, but it improves with longer soaking.
If the clay had lots of sticks/rocks in it, and/or if you're planning to use it for pottery, I recommend purifying the clay (see next step). If the clay seems pretty clean or if you're planning to use it for cob, you may be able to use it as-is (skip to "Drying clay").
Purifiying Clay - there are two main methods, which can be used separately or in combination. 1) Pour the clay through a screen or sieve to remove sticks, rocks, etc. You don't need a super-fine screen, and it works best if the clay is very thin and soupy. 2) Stir up the clay, let it settle briefly, then carefully pour off the liquid, leaving behind any sand/silt which has settled out of solution. Stirring up and pouring off may need to be repeated, depending on the purity of clay desired.
Drying clay - Once you have the clay as clean as you want or need it to be, let the clay settle overnight and pour off as much of the water as you can. Repeat until you have removed most of the water. The fairly wet/liquid clay remaining can be used as-is for cob, but for pottery you will need to remove more water.
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