Common Pottery Terms
Brief definitions of terms you'll see as you collect art pottery.
Coil method
One of the oldest ways of forming pottery. Long strands of clay which are laid on top of each other and joined through blending coil to coil. Coil pieces can be almost any shape and any size.
Crackle glaze
Minute decorative cracks in the glaze that are often accentuated by rubbed-in coloring material.
Crazing Crazing
The fine crackling you often see on most glazed art pottery. Crazing is in the glaze and is not detectable when you rub you fingernail over crazing. Art pottery crazing occurred during pottery production when the clay body and glaze cool at different rates. Crazing is a very common condition with virtually all glazed art pottery.
Earthenware
A low-fire clay. Porous and not waterproof. To be functional, it needs to be glazed.
Factory Glazed Chip
A factory-glazed chip is a chip that occurred on the piece prior to firing and glazing. Factory glazed chips are not post-production chips that have been subsequently repaired and reglazed. Factory glazed chips or mold flaws are more common on Roseville, Weller, and Van Briggle than other American art pottery such as Rookwood pottery.
Firing
Clay is hardened by heating it to a high temperature, fusing the clay particles. Primitive pottery is usually fired on the ground or in pits with whatever flammable material is available. Kilns allow a more efficient use of materials and more control over the atmosphere during a firing. The two basic atmospheres, oxidation and reduction, affect the color of the final piece.
Firing Lines
A factory firing line is a crack that occurred during firing of a piece of pottery. Firing lines are not damage that occurred after production. Firing lines typically occurred at weaknesses in the clay body. Particular areas susceptible to firing lines include hanging holes on Roseville wall pockets and hanging baskets, and the corners of Rookwood, Weller, Van Briggle, and other art pottery.
Glaze
A coating of material applied to ceramics before firing that forms a glass-like surface. Glazes can be colored, opaque, translucent or matte.
Glaze chip
A glaze chip is simply a chip, which does not impact the clay in any way and simply involves flaking of the glaze. Glaze chips are common on the high points of hand-tooled pieces such as Grueby, Wheatley, and similar art pottery.
Glaze Scale
A glaze scale is typically an area where the glaze has flaked from an edge or ridge of a piece of art pottery. A glaze scale can also occur on art pottery where the glaze is not adequately adhering to the pot. This type of glaze scaling can sometimes be seen on Roseville Rozane, Weller Louwelsa, and Owens Utopian where the pots have loose crazing or have been cleaned with harsh chemicals.
Glaze skip or glaze crawl
Glaze skips or crawls are areas where the glaze did not completely cover the pot leaving areas of exposed clay. Glaze skips are seen more often on early arts and crafts pottery such as Grueby, Hampshire, and Van Briggle.
Glaze pops
Glaze pops occurred during firing when air bubbles reached the surface of the glaze and burst.
Hairline Crack
A hairline crack is a crack that goes into or through the clay body and can be felt with a fingernail. In some instances, new collectors and less than reputable dealers will refer to hairlines as crazing. If the crack is into the clay and is detectable with a fingernail it is not crazing. Pottery such as Roseville, Weller, and Rookwood are susceptible to hairline cracks from minor abuse. Other pottery such as Van Briggle is less likely to be found with hairlines.
Kiln
The furnace in which ceramics are fired. Kilns can be electric, natural gas, wood, coal, fuel oil or propane. Materials used to heat the kiln can affect the work: wood ash can build up on the surfaces of a piece and form a glaze at high temperatures. Some potters introduce chemicals into the kiln to influence the effects of the firing. Famed ceramist Beatrice Wood achieved a lustre effect by throwing moth balls into the kiln.
Kiln Flaw
A kiln flaw occurred when pottery in the kiln came in contact with either another piece of pottery or the kiln wall. A kiln flaw or kiln kiss often resulted in glaze loss on one pot and possibly excess glaze blips on the adjacent pot.
Mint Condition
Mint condition implies that a particular piece of art pottery is in “as made” or “like new” condition. Mint condition items are always free of chips, cracks, repairs, or similar damage or wear. Normal crazing and typical factory flaws that are seen on Roseville, Rookwood, Weller and other American art pottery does not exclude a piece from being classified as mint. In all cases, if these factory conditions are beyond the norm typically seen on art pottery they should be mentioned in the description.
Oxidation (Compare to Reduction)
A firing atmosphere with ample oxygen. An electric kiln always gives an oxidizing fire. In a wood or gas firing, the mixture of fuel and air is perfectly adjusted to give a clean burn. Acoma whiteware is fired in oxidation.
Oxides
Metal oxides can be mixed with water and applied to the surface of clay. By varying the amount of material applied and rubbed off, the potter can achieve effects similar to stained wood. The most common stain is iron oxide (rust).
Peppering
Peppering is the minute black specks of carbon that is sometimes seen in white or light colored glazes. Peppering is a condition that can be seen in some examples produced by most American pottery companies including Roseville, Weller, Van Briggle and even Rookwood. Some Roseville patterns such as Primrose and Thornapple were much more susceptible to peppering than others. Minor peppering doesn’t usually adversely affect the value of piece.
Pinch Pots
Starting with a ball of clay the potter opens a hole into the ball and forms a bowl shape through a combination of stroking and pinching the clay. Many coil-built pieces are constructed on top of a pinched bottom.
Porcelain
True porcelain was being made in China and Korea around 960 AD. Porcelain is a combination of kaolin (a pure, white, primary clay), silica and feldspar. A unique aspect of porcelain is that it can be worked as clay, but when fired properly reaches a state similar to glass. Primary qualities of porcelain are translucency and whiteness. In the 17th Century, English potters invented Bone China to compete with the porcelain being imported into Europe.
Raku
Pottery is fired normally but removed when it is red hot and the glaze is molten. It is then usually placed in a bed of combustible materials and covered, creating intense reduction resulting in irregular surfaces and colors.
Reduction (Compare to Oxidation)
A firing atmosphere with inadequate oxygen and large amounts of carbon (smoke or unburned fuel). What would have been copper oxide in an oxidation atmosphere will be pure copper in reduction. Reduction allowed the Chinese to develop the sangue de beouf red glazes and gives Raku its metallic finishes. In Indian pottery, Maria's black pieces are the result of heavy reduction; the same piece in oxidation would be a terra cotta color.
Slab Built
Clay slabs are cut to shape and joined together using scoring and wet clay called slip. Slabs can be draped over or into forms, rolled around cylinders or built-up into geometric forms. Large forms are difficult because of stresses on the seams and because the slab naturally sags. Some potters get around this by working fibers into the clay body. The fibers burn out during the firing, leaving a network of tiny holes.
Slip
A fine, liquid form of clay applied to the surface of a vessel prior to firing. Slip fills in pores and gives uniform color.
Stoneware
A high-fire clay. Stoneware is waterproof even without glaze; the resulting ware is sturdier than earthenware.
Terra cotta
A brownish-orange earthenware clay body commonly used for ceramic sculpture.
Wheel thrown
The term throw comes from Old English meaning spin. A piece of clay is placed on a potter's wheel head which spins. The clay is shaped by compression while it is in motion. Often the potter will use several thrown shapes together to form one piece (a teapot can be constructed from three or four thrown forms).
