Finding and Processing Wild Clay for potential use
Finding and using wild clay can be an exciting and rewarding experience. Even if it doesn't prove to be a great clay for throwing or hand building you might discover your own unique surface treatment material.
Identifying clay as opposed to dirt.
Clay will usually appear as a distinct layer in both color & texture from the topsoil. It is a much finer particle size than dirt. It is very dense and hard when dry and feels slippery when damp. Dampen it and rub it between your fingers, if it is clay it will feel very slippery, not crumbly. You’ll know it is clay!
Clay is found all around us and may prove to be useful in different ways. Some clays that make good earthenware bodies will melt and become a glaze in a high temp firing. They may be useful as a component in a mixed clay body. Some found clays will make a good terra sigillata or engobe surface coating for color or textural contrast on your current clay body and not be useful at all as a clay body themselves. Look for clay where any digging is being done. Also look in places where water has cut into riverbanks. In some areas just digging a few inches down with a shovel may reveal a clay deposit.
Collect your clay from as pure a source as possible; look for a place that appears to be consistent in color & texture; try to scoop it with as few roots, leaves, dirt, and rocks as possible. Get at least 3⁄4 of a 5 gallon bucket full for testing. You can put it in a box or bucket.
Let it dry out. It helps to break it up and spread it out in a thin layer for maximum exposure to air. Try to find a warm, dry place to do this. If necessary, put it on trays in your oven at 200 degrees fahrenheit for a few hours.
When clay is dry, crunch it up with a hammer in a low box. Wear a dust mask.
Fill a 5-gallon bucket to about 1/3 full of water.
Pour in your clay until the bucket is about 2/3 full.
Let it soak for several hours.
Stir the mixture using a whisk and then use a mixing blade on an electric drill. You should end up with clay slurry, kind of like heavy cream.
*Make sure you save at least one pound of dry clay for a terra sigillata test.
Run the clay slurry through a hardware cloth sized screen.
Then run it through a window-sized screen. If it is too thick to go easily through the screen stir more water into the slurry.
Throw away the debris collected in the screens.
This should be sufficiently screened for a clay body.
*If desired you can also run it through a 60 mesh screen for a finer clay. Do run about one quart through a 60-mesh screen and save for testing as an engobe.
You now need to dry the rest to a workable state by spreading it out on an absorbent surface. Bisqued clay, a cloth sling or a plaster slab will all work. Watch it closely so that it doesn’t get too dry.
You will end up with enough clay to make test bars and also some to try forming a few pieces to find out how workable it is. Test your clay for shrinkage and porosity. We generally test at bone dry, cone 06, 5, & 9.
Terra sigillata is a thin solution of super fine clay applied to dry greenware before bisque firing. It burnishes very easily as soon as it dries to the touch and even though it is not vitreous it imparts a very pleasant shiny texture to the clay surface. It is used as an alternative to glaze or stain and offers both textural and color contrast to the clay surface. It works nicely in a variety of low temp firing methods ( cone 08-cone 1 or so) and because it is still porous it is not good for utilitarian ware but is especially attractive in pit, smoky, or smudge firings. It generally does not retain it's wonderful sheen at high temperatures.
To Make Terra Sigillata
1) Put 1 quart water + 2 ml sodium silicate in a clear container and mix well
2) Add 1 pound of dry clay to water & sodium silicate, wait a few minutes, mix well.
3) Let it settle for 18-20 hours. You should see two or three distinct layers in the container: a thin layer of water on top, terra sig in the middle, & coarse particles on bottom. If there is a layer of water on top, pour it off or use a basting bulb to remove it. Now pour off ( or use the bulb) the middle layer of terra sig for use. Discard the material that settled to the bottom.
For some clays that are more coarse you will need to double the above recipe to get enough terra sig to test. Every clay will be different. Some clays don't easily show distinct lines between layers after settling, making it difficult to determine how much to use. It is better to err on the side of decanting less terra sig in order to avoid the more coarse particles in the bottom layer.
We usually apply Terra sigillata with a fine brush to bone dry greenware. It will be quite thin compared to engobe and should quickly and easily burnish to a shiny surface when lightly rubbed as soon as it is dry to the touch. Two or three coats should be enough. Applying to a dusty surface or too thickly can cause problems with cracking. I usually use a thin piece of plastic over my finger for burnishing.