DIY Ceramics from Dug Clay
Digging in the Dirt
While I was out doing trail work in White Clay Creek State Park, I came across a downed tree whose upturned root system had exposed an ample amount of gray clay. I used some of it to fill a muddy puddle in the trail, and it hardened up beautifully. That got me thinking about a video I saw on primitive technology, and I filled a couple of ziplock bags with the raw clay to experiment.
A little Googling told me I needed to screen and filter the clay, but all I needed for that was a bit of window screen, a bucket and an old bedsheet. I mixed the clay with water until it was completely dissolved, then let it sit for an hour or two. Any sand or rocks settled to the bottom while the finer clay particles remained suspended in the water. I then carefully poured the clay water through a tripled aluminum window screen, being careful to stop before I poured out the sediment at the bottom. A surprising amount of organic bits were caught in the screen, including bits of sticks, roots and leaves, and even a few unlucky bugs. I repeated this process a few times until I wasn't seeing sand or bugs anymore.
Now, I had to remove the water I'd added. The leading experts in digging your own clay all recommended filtering through a doubled bedsheet. I expected this process to be faster, but it actually took many days of dripping into a bucket to bring the clay to the consistency of a thick slip. Once it seemed to be more solid than liquid, I spread it out on board to dry enough to be workable. A plaster block would have been better, but I'd already made quite a mess of my workshop at this point without introducing plaster.
At first glance, the clay looked pretty much like what I expect clay to look like. Once I started building, I did note that while my clay had adequate plasticity (it didn't crack or crumble), it felt a little "mushy". Building anything thin and tall tended to cause it to sag, so I scaled down my test pieces, and also formed a little bowl over a form of window screen to keep it from deforming while it dried.
Fire Makes it Good
After doing a couple of small POC tests with a propane torch on a dried coin-sized piece of clay, I determined that it was actually going to fire into ceramic. More Googling yielded news that a wood fired backyard kiln was capable of reaching low-fire temperatures. Some people even used a barbecue grill and charcoal with some success.
I decided to go the "brick kiln" route. I had a low pile of dirt in my yard that I dug out a bit, then constructed a "tunnel" I could build a fire in, a little shelf for the pieces to sit on, and a chimney. I used some leftover unrefined clay as mortar to seal the gaps between bricks.
I followed a suggestion that I bake the pieces in a 190°F oven for a little while before firing just to make sure they are completely dry. Then, I placed them in my kiln and started lighting things on fire. Before long, I had a good fire going with a draft pulling air in the front and flames licking out of the chimney. I didn't have a good way of observing my pieces in there to see if they reached the required red glow. A pyrometer would also have been helpful. Instead, I tried a bit of forced air with a fireplace bellows to make sure I got as much heat as possible out of my wood fire. I later tried an air compressor, which was less work, but the noise spoiled the primitive technology atmosphere. A foot-operated bellows would be ideal.
I kept the fire going strong for about an hour before letting it burn down to just coals. It had gotten late, and I wanted to let it cool as slowly as possible, so I placed bricks over the chimney and front, and buried the whole kiln in dirt until I didn't see any smoke. I checked on it an hour later, and the ground was quite warm, but with it all safely contained underground, I went to bed and let it cool overnight.
Digging the kiln out in the morning, the bricks were still a little warm, but everything was cool enough to handle. My pieces had not cracked, and a gentle flick made a "ting" that told me they were no longer just dried clay. I brought them in to give them the final test. Would they dissolve in water? Rinsing and scrubbing with an old toothbrush told me I'd actually fired the clay - chemically removing the molecularly bound water to turn clay to ceramic. Success!
Learnings and Future Work
The low-fired unglazed pieces are quite porous, and while they will hold water, they are soft enough to be scratched with a fingernail. Fire bricks would do a better job of keeping the heat in the kiln instead of wasting it by warming the cement bricks. A better forced air system and a digital pyrometer would give me more control over temperature.
To address the mushy clay problem, I experimented with adding some grog (powdered fired clay), which seemed to help a little. I read that some potters mix some commercial clay into their home-dug clay to make it easier to work with. My clay seemed pretty high in mica (I can see it sparkling), and probably lacks some other key ingredient to give it more structural body, but it isn't bad for coming straight from the ground!
Glazing and decorative slips will be my next challenge. I did play a little with adding some cupric nitrate, but the end result when fired was just a black smudge. I did a reduction firing as a test (dumping wood shavings into the kiln before sealing it to cool) and got a solid black finish that doesn't come off on your hands. Getting a real raku finish would be amazing if I could somehow pull something like that off in my back yard without frightening the neighbors.