What is clay? The stuff we play with. Technically, it's alumina and silica mixed with water.
What is a clay body? A clay body is clay found in the earth, mixed with other compounds to make it suitable for different uses. For example, porcelain, stoneware and earthenware are all different clay bodies.
What is a slip? Slip is clay mixed with water. It can be used for decoration on leather hard pieces before they are bisque fired.
What is a glaze? A glaze is a liquid which is applied to bisque wear and when fired to a specific temperature, it's chemical composition changes to form a glaze (glass).
What does greenware mean? Greenware is pottery before it's bisque fired. Your work is very vulnerable at this stage.
When is a pot leather hard? Leather hard is the stage of drying when the pot is dry enough not to be damaged by handling however it is not dry enough to be bisque fired. This is the right time to apply a slip.
When is a pot bone dry? Bone dry is completely dry and ready to be bisque fired. If you are wondering if your pot is bone dry, hold the bottom of it up to your cheek, if it feels cold, it has moisture in it and it isn't safe to bisque fire.
Caution: If you fire a pot that is not completely dry, it can explode in the kiln and damage other work.
What is a bisque firing? Bisque firing is the first firing of your work. Wear is fired to just before vitrification. Earthenware is bisqued fired to around Cone 04. Stoneware and Porcelain are bisque fired from Cone 06 to Cone 04. Slips can be applied for this firing but no glazes.
What is a glaze firing? This is the final firing in which you add glaze to your work then fire it to a specific temperature depending on the clay body. Some people call this a 'gloss' firing. For example, stoneware, white stoneware and porcelain all vitrify when fired to cone 10. Glazed earthenware is fired to around Cone 05. Glazed stoneware and porcelain are fired in the range of Cone 4 - 11.
What does vitrify mean? Vitrification occurs when the clay body and the glaze become fused together. A vitrified piece is safe from bacteria even if unglazed. When stoneware, white stoneware or porcelain are fired to cone 10 they become vitrified thus safe as functional wear. However, earthenware and other low fire clay bodies don't vitrify thus they cannot be considered safe as functional pieces as bacteria can become trapped (or grow) in cracks in the clay or glaze.
What is an oxidation firing versus a reduction firing? Oxidation and reduction are high fire (cone 9, 10, 11) techniques for altering and setting the glazes in the kiln during a firing. Oxidation firing is when the air in the kiln contains oxygen. Reduction firing is when you reduce the amount of oxygen in the kiln.
Oxidation produces brighter shiny colored pots due to the oxides used. For example copper oxide will produce greens. Because oxidation occurs in atmospheres not deprived of oxygen, all electric kilns fire in oxidation.
Reduction yields a warmer clay body and textured glaze results. For example, copper reds and celedons work only in reduction. Copper oxide can produce brilliant red in a reduction firing.
In fuel burning kilns, the carbon from the burning materials combines with the oxygen in the kiln. When more carbon exists in the atmosphere than oxygen, carbon and carbon monoxide form and begin to take oxygen from all available sources including the oxides in the clay body and glaze.
What are some of the low fire techniques and how do they work? Salt firing, saggar firing, pit firing are all primitive low fire techniques. In pit firing the pots are laid in combustible materials which are ignited and used as the heat source. Materials such as cow manure and sawdust produce a strong reduction during firing. In sagger firing a large pot or container is used to hold the pottery and organic materials inside the kiln as it fires. In salt firing the salt is added to the kiln as it fires. Salt firing can also be adapted to cone 10 firing.
What is a flux? A flux acts as a catalyst for the glaze to change it's chemical composition in a firing from a liquid to a glass.
What is terra silgilatta? Terra Silgilatta is a slip made of fine clay particles. It can be polished to achieve a shiny look and provides greater waterproofness. This is used for low fire, at higher temperatures the particles would recrystalize.
What is burnishing? Burnishing is a finishing technique for pots. You press down or smooth out the surface of a pot using a stone or the back of a spoon. It's a low fire process in which pots are fired inside of a sagger which is filled with organic materials. You don't use any glaze on burnished pottery. The color of finished piece depends upon the reduction used.
What is raku? Raku is traditional Japanese low fire technique developed by the Raku family in the 16th century. Paul Soldner is know as the pioneer of American raku. American raku is different from Japanese raku because of the reduction used. The Japanese tradition calls for water as a reduction while most Americans use organic materials to reduce a piece. When your work reaches the desired temperature, you pull it from the burning kiln and place it in a container along with a reduction. For example, a barrel, can, or hole in the ground are typical containers. Some examples of reduction materials include sawdust, newspaper, straw, banana peels, seaweed, coffee grounds.
Glazes with coppers can produce flashes of metallic lusters. White crackle is another popular raku glaze. It's white but when the glaze 'cracks' in the reduction, the cracks fill up with carbon from the smoke causing the crackle look.
What are cones? Cones are little orange ceramic pyramids made from clay and glaze. Cones melt at specific temperatures indicating when to turn the kiln off at the end of a firing. Cones have numbers on them indicating at what intervals they will melt and bend. In a cone 10 firing you may use 3-4 cones. A guide cone, one or two numbers below your desired temperature, the desired temperature cone, and a guard cone to indicate if you have overfired. Cones must be placed in the kiln in front of a peep hole so you can see it while the kiln is firing.
How does the cone system work? The Orton Cone Foundation makes most of the cones used in the United States. The cones and their chart is available on this site for your review. Click here to view or print your own chart.
What is compressing and why is it important? Compressing is forcing the particles of clay closer together. When you wedge your clay you begin compressing it. You must continue to compress the clay as you center and throw by pushing directly down on the clay toward the wheel head. If you don't, small pieces of the clay begin to come off as you center. If you are throwing, you need to be especially careful to compress the lip of a cylinder or it can tear off, ruining a tall cylinder. If you are throwing a platter or dish you need to compress the center with a rib or your hands to prevent it from cracking while drying or firing.