How do you process wild clay to make it moldable?

Question

I want to try to make pots and bowls. Does anyone know how to get the impurities out and how to flame fire it ?Thanks

Answer 1

For a beginner, I strongly recommend that you purchase clay that is intended for pottery use. The problem with most naturally occurring clays is that they don't have a good mix of materials to provide good pottery characteristics. While it is definitely possible to use naturally occurring clay, it will make your work as a beginner much, much more difficult.

Learn how clay should feel and behave by using a number of different commercially produced clay bodies. Then go out and dig your own. The process for refining it is simple, though it takes a little time.

Dry your clay completely and break it into pieces smaller than about 1 inch in diameter. There's nothing critical about the size. Don't worry if there are some bigger chunks in there. While you're at it, remove any sticks, rocks and other trash. Again, there is nothing critical about cleaning the clay, as these materials will be removed by later steps. It's just a little easier to work with if you get rid of the big nasty stuff.

Fill a container with about 2-3 times as much water volume as your clay volume and pour the dry clay into the water. Let it stand until it stops bubbling. Then vigorously stir the clay and water.

Pour the clay slurry through a fine screen and into a second container. Twigs and other similar material will be caught by the screen. If the slurry does not easily pass through the screen, add more water and stir again.

Rocks and other heavy material will remain in the bottom of the first container. In addition, there may be more clay remaining that didn't get stirred up the first time. If so, add more water to cover the remaining clay, stir vigorously again and pour through the screen into the second container. Do that until nothing but rocks and sand remains in the first container. Dump the rocks and sand.

If you want a finer grained clay, you can stir and pass the contents of the second container through a finer sieve.

Now allow the container to stand undisturbed for at least a day. The clay will settle to the bottom and a clear water layer will form on top. Pour off the clear water, being careful not to disturb or pour off the fine layer of clay at the interface between the clay and water. If desired (or if you accidentally sloshed the container too much), let the container stand for another day or three and decant the clear water again.

Finally, allow the clay to dry to a workable consistency. This is the most time consuming part. Many people use plaster slabs to help absorb water from the clay. Others pour the clay goop into an old pillowcase (careful, it's very heavy!) or other porous container to help evaporate the water. Keep stirring or mashing the clay around during the drying so that it dries uniformly.

Eventually, the clay will be dry enough to work with. When it is no longer so sticky that it sticks to your hands, you are pretty close. Wedge (knead) the clay again to ensure that it is uniform and to help remove air bubbles.

An alternative method is to let the clay goop dry completely, break it up into very small pieces (less than 1/4") and mix water back in until you get a workable consistency. It is better if you do it the first way, though, where you don't let it dry completely. Your clay will be more uniformly wetted and will behave better.

Anytime that you are dealing with dry clay you should wear a respirator to avoid inhaling the dust.

The only way that you can tell how hot to fire the clay is to experiment. You will need to fire some of your clay along with some pyrometric cones to determine its maturation temperature. Most locally found clays will fire somewhere in the range of cone 1 to cone 6.

Answer 2

  Different clays have different properties and you may need to mix them to get the properties you want.  If it has a lot of dirt in it, you may have to wash it, working it to make clay slip and washing off the organics and dirt, then dry the clay to make a working substance.

  There a books on natural clay and firing pottery - it is complex.

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