Tuesday, July 8, 2014

Homebrew Pottery - Finding and Refining Local Clay

In this long term project, I'm going to make a drinking mug completely from scratch. I will dig my own clay, make my own additives, refine my own high temperature fuel, fire it all in a homemade kiln, and apply homemade glaze then fire it again. In the process, I hope to get more fully in touch with all of the items and the effort that go into a single, simple tool in my life. This is a project about attaining a broader perspective.

Also... I get to play in the mud.



There are many kinds of clay, and you should be able to find some sort nearly anywhere you live:

"Earthenware" clay is easy to find most places. This is clay that has a significant number of impurities, notably iron oxides. As a result, it falls apart at lower temperatures than higher grade clays do. It also turns red when fired from the iron content (or black in an oxygen-starved environment). Earthenware won't vitrify (turn to a glass-like state during heating), so the products aren't waterproof unless you glaze them.

"Stoneware" clay (also called fire clay) is a purer clay that can be fired to much higher temperatures, where it can vitrify and be watertight naturally. You CAN also glaze it if you like. It is white or light gray in color and is usually deeper underground.

"Porcelain" clay (or china clay) is an almost pure "kaolinite" mineral of silicon and aluminum and oxygen, which allows for the highest firing, whitest, slightly translucent pottery wares. This is rarer than fire clay, harder to work with, and you're very unlikely to find any on your own.

Personally, I was only able to find local earthenware clay. Not too pretty right out of the ground:

For earthenware, just look around your neighborhood or yard for areas that LOOK like clay when wet (pretty technical stuff, I know). A good time to notice this is after a storm. Also check any bluffs/cliffsides/ravines that show you a cross section of soil. River or stream banks are also good bets.

Another method is to go to this website (http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm), hit "start WSS," highlight your neighborhood as an area of interest, then go to the "Soil Data Explorer" tab and run a map of "clay content" - it asks you for a depth first, so choose something like 0-12 inches.  You will see a colored overlay of your neighborhood with % clay of nearby soils. Find a spot where you might get permission to dig that has a lot of clay, and start there!

Once you find what you think is a high clay content soil, get it sort of wet, and roll a "snake" between your hands about half an inch thick. Try wrapping it around a finger. If it wraps without breaking apart, then you have a good amount of clay in that soil. You can still work with lower content soils, it will just take longer.


Chances are, if you dug up some local clay, it's going to be impure. The process of purifying is pretty labor intensive, but also entertaining! You get to explore your inner child and go play in the mud a lot.

You will need: A couple of 5 gallon buckets (if you used one to dig clay, that counts as one), some twine or rope, straining cloth like muslin, a sieve or loose burlap, an old large spoon, and one or two nice wide wooden boards or a piece of plywood for drying.
  1. With the clay in a 5 gallon bucket, pour in about 2 parts water for every 1 part soil you dug.
  2. Use your hands to break up any big chunks underwater. You want the smallest possible sized bits of clay. Be careful NOT to compress clay together! This will will slow everything down, because water won't get to your clay particles as easily. Just loosely break up clumps, and swish water into all the nooks and crannies.
  3. Let this sit for a full day (you may want to put a lid on it if it's outside, due to bugs and leaves and stuff). The smaller the bits you crumbled up, the faster the clay will "slake" into fully wetted clay water.
  4. The next day, stir up the mixture just a bit to get settled clay off the very bottom. The idea is to get tiny clay particles in suspension again, but then wait just a couple of minutes for sand and silt to settle out again.
  5. What you're going to do is pour the clay into some muslin or similar cloth to let it drip out excess water. A small scale version I did looked like this:

  6. But it's more efficient to do everything at once. Since then, I've changed to 5 gallon buckets with muslin tied around their rims, with the water dripping into the bottom of the bucket. So tie off some muslin VERY SECURELY (4-5 tight wraps of twine, it needs to hold heavy weight!) around the rim of a bucket, leaving about a 1/3 bucket height worth of room at the bottom for dripping. Then position a strainer and pour your clay water through it into the cloth, to remove the floaters. As soon as you see anything other than clean, smooth looking clay water coming through, STOP pouring:

  7. Discard everything left in the first bucket, which should be a bit of clay, lots of sand, rocks, etc. Also discard whatever the strainer caught (usually remaining floating things). From my small scale versions, this is what some of the leftover material in the bottom looked like:

  8. You may want to repeat steps 4-7 if you still feel sand or sediment at the bottom of your bucket, but really it's not that big of a deal.
  9. Around 1-2 days later, check on your clay. A lot of water will have dripped off, but it should still be fairly wet. If it is dry enough to spread onto a board without running off the side, then you're ready for the next step. Usually a "milkshake" consistency is enough to make it not flow off the board. If it is still too wet, let it sit another day. Note: the clay will compact a bit on the bottom near the cloth. It may look like pure water on top, but be milkshake consistency or thicker underneath still, and it might be ready. Check the actual clay.
  10. Now take a spoon or your hands and glop your clay onto your wooden boards or plywood, and spread it maybe 1/2-1 inch thick. Scrape off as much as you can from the cloth.
  11. Set the boards outside in full sun if available, and they will dry out the clay rapidly. A fan blowing over them works pretty well, too, at night, but sunlight is much less tedious. In sun, check every 10 minutes or so and remix if necessary to make sure that the edge clay isn't drying too quickly. With a fan, every 20-30 minutes. You don't want it to ever get crumbly-dry.

  12. When the clay is of a good strong pottery building consistency, put it in a plastic bag or wrap in plastic wrap or similar, so that it stops losing moisture. Now you should be able to store it and treat it like any store bought earthenware clay. When it's all dry, it works just like commercial clay! Here's some freshly sunned clay:
  13. Be sure to "wedge" your clay before building with it, like you would a commercial clay. If you don't know what I'm talking about, wedging means just mixing it together to make it an even consistency. There are many ways to wedge clay, and I can't explain them as well as online tutorials can. Check out several methods here: wedging/kneading methods. Then build stuff:
    (I know, I'm a masterful potter, eh?)


I will go over the remaining steps to making pottery wares (assuming I can get them to work!):
  • Tempering the clay: adding crushed pottery bits, shells, or sand to the clay to make it less likely to crack. This requires methodical testing to find the right amount to add for a given clay.
  • Making some sort of kiln: I will be attempting a wood or charcoal fired kiln to continue with the "from scratch, if possible" theme. In another post, I may also attempt making my own charcoal. Ideally, the kiln would be made out of homemade refractory bricks, but I might need a source of fire clay first, if I'm to do that.
  • Firing the clay, and possibly glazing it usefully somehow, although I understand glazing is difficult to do with low temperatures and with non-exotic, local materials. We shall see.