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Kit Cornell Pottery

HOW TO FIND AND DIG CLAY
by Kit Cornell

Clay is imagination-inspiring, malleable, healing, abundant, accessible--and messy.
That’s the glory of clay.

What Clay is

The earth’s crust is made up of largely igneous rock, which, as nature breaks it down, becomes clay. It has been being formed for millions of years, since the time of the glaciers. The main ingredients in clay are alumina, silica and water.

Clay which remains in the place where it was formed is called primary clay. Clay which moves from its original location, picking up accessory minerals such as quartz, mica and iron, is known as secondary clay.

I
n its natural state clay can be easily molded and manipulated by the hand. This quality is called plasticity, and it is what makes clay so delightful to use. A second defining characteristic of clay is that when heated to at least 1200F, it becomes hard, or matures. Earthenware clay matures at 1200F-1800F; stoneware and porcelain at temperatures up to 2400F. Firing clay to it’s maturing temperature makes it vitreous--impermeable to water--and gives it permanence. Pieces of pottery made thousands of years ago have been found in archeological digs around the world. It provides a wonderful record of human history.

Where Clay may be Found

Naturally-occurring clay exists in many areas of the world.. With curiosity and ingenuity one can find out where nature has deposited it for our use. Look for it in lakes, ponds or at the seashore. Find it where road construction crews and builders have cleared away the topsoil. You might also visit your local historical association to see if a pottery or brickyard was ever in operation in your area. Or call the archeological organization nearest you to see if there are digs nearby. Ask your neighbors, town elders or children where they may have seen clay.

How to Recognize and Dig Clay


Clay may be found wet or dry or in any stage in between. In its dry state it may look like rocks; in its wet state, like mud. When looking for clay take a bucket, shovel, trowel and penknife. If you find dry lumps. take a chunk and scrape it with a knife. If it’s clay the fine particles will crumble off. Scrape some into a small pile and dampen with water to see if it dissolves .

If you’re in a wet area and must dig to get to the clay source, don’t stop too soon. It’s necessary to go beneath the layers of leaves, sand, stones and humus, clearing these away as you dig. You’ll know you may have arrived at clay by the fine particle size.
Take a small lump of the damp material , working it into a ball with your fingers. Roll it into a coil, then bend the coil around your finger. If it stays together and feels smooth and plastic, chances are you’ve found clay. Gather about a bucketful for processing and further testing. Be careful not to add stones, twigs or dirt as you do this.

If the clay is very workable, you may use it as it is. But usually processing is needed to remove extraneous matter. Here’s how to do it:
Dry the clay completely (on boards, plaster bats, etc. Break it up into very small (pea-size) pieces, using a hammer. Pour into a bucket as much water as you have clay, then sprinkle the clay into the water. Stir, adding more water to make a liquid mass. Let sit for at least a few hours (or weeks), then stir well and sieve through a sieve or window screen. Add enough water to keep the mixture moving through. Leave to settle in the bucket, then pour off extra water. Do this until the mixture is mud-like, then spread on plaster bat or board to stiffen up. When mixture is stiff enough to roll, fold and knead it on a plaster, wood or canvas surface. (This process is called wedging. ) To store, wrap tightly in several layers of plastic and place in a bag that is tightly closed. If your clay is too plastic or sticky, you may add sand, grog, finely ground shells or other materials to make it more workable. Clay may be dampened when stiff, and rewrapped and recycled. The clay may become more plastic with time.

How to fire Clay You Find

Clay that is not fired may be kept, but the pieces made will be quite fragile. Burnishing (polishing) them with a stone before they are dry will make them stronger, but still they will be easily breakable. You can try painting unfired (greenware) clay with water colors or acrylics.

Ideally, though, clay should be totally dried, then fired in a kiln. Kilns use electricity, gas, wood or other combustibles. Each method will give different results. You can even try firing pots in a wood stove or fireplace, heating the pots gradually. To fire pieces to maturity, you need to learn what temperature is right for each clay body. First, fire your pieces to a low temperature (1200F). If it still is absorbent, you can try firing it a little higher. But if you fire it too high the clay will melt into a pancake and make a mess of the kiln shelf. So only fire to high temperature if you are sure your clay matures at that temperature. Unglazed, fired clay is very lovely and emphasizes both the quality of clay and the form.

If you have access to firing, you may also glaze your pots, using minerals and colorants to paint or dip your pieces. Usually they will then be fired for a second time.

Some Cautions

If digging on private land, be sure to gain permission in advance of digging.
Do not dig clay in environmentally polluted areas.
Do not use commercial “clay” substitutes whose contents are not listed clearly and proven harmless.