Plasticity: The flexibility of the clay. Clay that is flocculated will be more plastic and crack less.
Epsom salt: Adding 2 1/2 ounces of Epsom salt (magnesium sulfate) per 50 pounds (a 5-gallon bucket) of clay will increase plasticity. Each clay body will require a slightly different percentage of Epsom salt due to the individual materials in the formula. Too high a percentage of Epsom salt can cause salt migration to the drying clay surface, which can result in blistering and carbon being trapped in the clay. Adding the Epsom salt to the water and then adding that mixture to the clay mixing operation can help ensure that the salts are dispersed evenly throughout the batch.
Flocculation: Epsom salt increases the attraction of clay particles in the clay and causes the clay to become flocculated. Clay particles are drawn together. The overall effect is a tight, plastic clay body with good throwing properties.
Organic Agents: Mold and bacterial growth in moist clay will increase plasticity since it increases the attraction of the clay particles. When mixing a 50-pound (5-gallon bucket) batch of clay you can add organic materials like these to the moist clay: 1/2 cup of coffee, beer, apple cider vinegar, grass clippings, sawdust, or 1 1/2 oz of yeast. You can add one or more or none of these things. Experiment and see what results you come up with. Try it without additions, then add them in one at a time to see what difference each one makes. All of these agents will start mold and bacterial growth in the clay mixture.
For thousands of years, people who produced pottery just dug clay from the ground and used it to make the needed bowl, jug, or pot. The process really didn’t change very much through the years because it didn’t have to. Potters learned from family and other similar makers in the community. The knowledge and skills were passed down through the generations because there was a need to know.
As societies became more complex and businesses grew, clay processing became more and more industrialized. People began to purchase clay and other materials from suppliers because it was easier and suppliers offered more varieties that couldn’t be obtained otherwise. Over time, people forgot where to find clay in the wild and what to do with it to make it usable. Today, because the Internet lets us purchase things from anywhere in the world, we are seeing an increase in sales from those who appreciate handmade objects. Potters who produce a lot of pots are spending a lot of money buying clay. Including freight costs, a thousand pounds of commercially produced clay could cost $500 or more. As a result, potters are looking for ways to keep some of that money in their pockets, which has led some to ask about where regional clay deposits used to be found and how local clay can be processed to make their pottery. I was one of those people for several reasons: saving money was one, but I’m also curious and learning about the way things used to be done intrigues me, so I started doing some research.
Finding and Developing Material
First look for river banks, streams, lake shores, ponds, and other natural water sources to find clay. Because Ohio was glaciated in the past, vast regions are covered in clay deposits, including my own yard! When you find something that looks like it could be clay (cracked and reddish, yellowish, or brown (1), dried hard or sticky), fill a clear container halfway with the raw material, fill it to the top with water, shake it up, and let it settle for a day or so. You will be able to see layers in the soil: rocks on the bottom, then clay and organic materials on top. The thicker the clay layer the better. My soil tests at almost 100% clay. You can test your clay’s plasticity by rolling out a pencil-thin coil and wrapping it around your finger. If the resulting looped coil has no cracks or only a few cracks, the clay is plastic enough, and you’re in business! Next, follow these steps.
Fill a 5-gallon bucket with the grass, sticks, rocks, and leaves included with the clay and add water to almost top off the bucket, leaving some room for mixing and expansion (2).
Add 2 cups of white vinegar to help dissolve any calcium, increase plasticity, and help with any odors you may end up with as organic materials break down.
Add 2 1/2 ounces of Epsom salt to the water in the bucket to increase plasticity.
Mix 1 1/2 ounces of bread yeast in 2 cups of warm sugar water to activate the yeast and add it to the clay bucket. Mix it together with an electric drill and a paddle mixer. This also increases the plasticity.
Cover the bucket with a lid and leave it alone for a couple of months to let bacteria grow that will eat all the organic material. The by-product from the bacteria eating all of the leaves increases the acidity, which also makes the clay a lot more plastic. You can even stir in a couple handfuls of grass clippings or sawdust if you want to feed the bacteria more! It will get slightly stinky and may even overflow the bucket a little (mine did), but that’s a good thing. It means it’s working!
Using the Clay
When I’m ready for some clay, I mix it up and push it through a plastic window screen with a credit card or wooden block to remove the rocks (3, 4). Then, thin it out with some water and screen it again through a 30-mesh screen (minimum)—60-mesh is preferred. Pour the clay slip out onto plaster bats and leave it for a couple of days to evaporate the excess water. Next, wedge and store the clay in a plastic tote with a lid to use as needed. If at any time you notice the clay cracking when you wedge it, add some more Epsom salt to it. It’s amazing how this pulls the clay particles out of suspension and clumps them together for use.
Fire the processed clay to test it. First, bisque fire it to cone 07 (1803°F (984°C)). Then, glaze and fire it again to cone 5 (2185°F (1196°C)). Caution: Protect your kiln shelves when firing an unknown clay sample. My clay begins to melt around cone 7, so I allow for temperature fluctuations in the kiln. It’s a beautiful terra-cotta color when bisque fired and an antique copper color when glaze fired. I first tested a small piece of the clay fired in a cone 5 firing. There was no cracking, some shrinkage, and reasonable hardness/durability. I made more tests, and fired those to cone 6 and cone 7. The clay began to deform at cone 7, so I use this as the top temperature it can be fired to.
All of the additive amounts mentioned here can be changed to better suit your purposes, situation, and type of clay. And, all the processes can be scaled up to handle larger batches. Hammering dry clay and screening it to remove impurities used to be commonplace. What was done once can be done again. Go out and dig some clay, save yourself some money, and learn to feel a deeper connection to the pottery that you make.
Sourced from the Nov/Dec 2019 issue of Pottery Making Illustrated.
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