Many pottery shops that have classes seem to be a bit expensive. Since clay is relatively inexpensive and found all over the world, many people wonder why the cost is so high. Clay cannot be taken straight out of the ground and immediately used, especially on a pottery wheel. The clay must first be kneaded by hand to take out air and any other impurities. This takes a bit of time, but it is well worth the effort. Clay that contains air bubbles will cause problems when it is worked. An air bubble in the side of a piece may cause it to be weak or explode during kiln firing. Impurities can cause stiff spots in the clay and need to be taken out of the clay or distributed evenly through it.