MIX YOUR OWN CLAY
Potters use clay in many forms. We have a huge selection of pugged clay and slip, but sometimes commercially available clays don't offer features that a potter may be looking for such as being locally sourced, or a certain plasticity, glaze fit, or strength. Mixing your own clay body takes a little experimentation and some trial and error, but has the potential to yield high rewards.
Clay Basics – Local Clay
Pure clay is also known as hydrous aluminum silicate (Al2O3-2SiO2-2H2O) and makes up nearly 75% of the Earth's crust. Because it is so common, the most basic clays can usually be found within walking distance of your studio. The easiest places to find clay are by the sides of rivers and streams (look in the outside bends where the water has eroded the topsoil) or 5 feet below the surface of the ground (especially under soil that is sticky).
Once you hit clay, you can do some basic tests to determine if it is usable. First: try to roll a coil. The more plastic (flexible) the clay, the thinner the coil. Second: rub the clay between your fingers. If it feels smooth and round, it will throw easily. If it feels sharp and gritty it will be more difficult (but may have some desirable properties when fired).
After you dig up your clay, cut it into 1-2 inch pieces and put it in a bucket of water for 3 days (or until the clay is fully slaked). Mix the clay and then pass it through a 30-50 mesh screen to remove unwanted pebbles, sand, and other materials. Then pass it through an 80-100 mesh screen to remove finer grit. After drying to the desired wetness, the clay will be ready to use.
You will need to experiment in the kiln to find the right bisque and maturation temperatures, as well as shrinkage and glaze fit, and you will most likely have to add additional clay and chemicals to achieve the desired final properties.
Here are some basic troubleshooting tips to help: if your wet clay doesn't hold its form enough, add ball clay. If your clay is too plastic, or too sticky to work with, add fireclay. If your clay is too dark, matures at too low a temperature, is brittle when fired or shrinks too much, add kaolin.
Below is a tried and true earthenware recipe for use with local clays.
Local Earthenware Clay
Edgar Plastic Kaolin
Check out more clay recipes in the following book:
Conrad, John W. Advanced Ceramic Manual: Technical Data for the Studio Potter. San Diego, CA: Falcon, 1987. Print.