art

bone china

A clay body created 18th century Britain as an attempt to duplicate the translucent ability of Oriental Porcelain, whose formula was kept secret form Europe. This claybody is difficult to work with on a potter's wheel and is most conducive to slip-casting or press-molding

stalin

A compound added to glazes to add coloring. Sometimes applied directly onto a clay body without mixing with a vitrifying glaze

casting slip

A cream-like mixture of clay and water used in the process of producing ceramic objects by means of pouring the slip into a porous mould. Water is drawn from the slip and into the mould, leaving a thin shell of soft but non-liquid clay around the inner surface of the form. When the slip achieves the proper thickness, the excess liquid is poured out and the remaining slip is left to dry completely. It is then removed from the mould, cleaned up, glazed and fired. This quick and simple process is used for mass produced ceramics leaving only the glazing as an opportunity for uniqueness. The term slip casting is also used to describe the process

contraction

A decrease in size due to a temperature change. It should not be confused with shrinkage, which is a non-reversible occurrence. Contraction is reversible with a reverse in the temperature change. As a piece of ceramics heats and cools, it expands and contracts. The rate of expansion and contraction of a glaze must be compatible with that of the claybody, otherwise flaking off or separation of the glaze may occur. In pottery, this should be a concern when considering glaze fit and ovenware.

potter's wheel

A device used by a potter to rotate a lump of clay on top of a disk, which allows a skilled craftsperson to create a variety of cylindrical shapes for a wide array of functional objects. This activity is called throwing. Although the "knack" for creating a pot on a wheel can be acquired in a relatively short time, creating high-quality, aesthetically pleasing, well-proportioned pieces takes many years of experience and long hours of practice

decorative

A general term describing a ceramic piece that has no purpose other than art or decoration. Typical examples include sculptures, and wall hangings. Some seemingly functional pieces are only decorative pieces are actually decorative only because of how they are produced.

functional

A general term that refers to any ceramics piece that is not purely decorative and has some intended use. Typical examples include teapots, bowls, mugs, and vases. Some seemingly functional pieces may actually be decorative only because of how they are made, or because of the type of claybody or glazes used. For example, the artist may create decorative teapots that are porous, or may glaze bowls with glazes that are not food safe or that cannot take fluids well. When shopping for pieces for a specific purpose, make sure to confirm that they can, indeed, be used for that purpose

ware

A generic catch-all term referring to ceramic pieces. It is usually combined with adjectives to form compound words such as kitchenware, dinnerware, earthenware, stoneware, and ovenware

salt glaze

A glaze that is derived from the introduction of salt (usually common table salt) into the kiln atmosphere. The salt quickly decomposes and vaporized and combines with alumina and silica from the clay in the pieces, creating a glossy surface. This salt glaze actually adheres to everything within the kiln chamber so the wares must be placed on stilts or have the bottoms coated with materials resistant to the salt vapors. This renders the kiln a salt-glaze-only kiln. The one place this vaporizing glazing action does not occur is inside pots, unless they are shallow forms with a wide opening. To overcome this, many potters coat the insides with glaze prior to the firing. Decorative slips are often used on the outsides of the pieces. Pieces created with a salt glaze are sometimes referred to as saltware, and the process, because of the sodium in the salt, is also referred to as sodium firing or sodium glazing.

coiling

A hand building technique where snake-like pieces of clay are placed in a spiral formation, thereby building a cylindrical shape. The coils are then smoothed out so that the piece takes on a continuous contour. Only rarely are the coils left showing. Because coiling is a slow process and coiled pieces have an increased chance of weak points at any of the joins between the coils, coiled pieces generally do not lend themselves well to functional pieces

wedging

A manual process of preparing clay for use by a potter. Similar to kneading dough, wedging accomplishes three things: a) it removes air pockets present within the clay mass, b) it helps to align the individual clay particles making the process of throwing slightly easier and the clay stronger, and c) it is a final mixing process that tends to even out water concentrations in the clay as well as homogenize the various ingredients in the clay mixture

slip

A mixture of clay and water usually with coloring agents in the form of metallic oxides. Mostly seen as brushwork, slips are best applied during the greenware stage of drying. There is also a process of creating pottery from slip called casting slip, or slip casting

glaze

A mixture of various materials and colorants, which are ground into a fine powder, mixed with water, and applied to ceramic pieces. This mixture, when exposed to high temperature during firing will melt and vitrify, thereby forming a glass-like surface that is fused onto the ceramic piece. Glazes can be applied to dried unfired ceramics (greenware), or to ceramics that have been already been fired. Some complex pieces involve various cycles of glazing and firing to produce the artist's intended effect. Glazes are usually referred to by the temperature, or cone, at which they melt. For example, a cone 10 glaze, which is a high-fire glaze.

clay

A naturally occurring inorganic substance composed of very small "plate- like" particles. These particles, when mixed with water as a lubricant, can slide past each other with relative ease. Known as "plasticity" or "workability", this gives clay its unique characteristic. Clay with finer particles is said to be more plastic than coarser clays but there is also more shrinkage during the drying and firing process. The various classifications of clay are determined by size, color, chemical make-up and purity

form follows function

A philosophy of design that is used to determine the design characteristics of a piece produced for a particular purpose. The design or form of the piece is defined by the intended use in balance with aesthetic considerations, instead of being defined purely by aesthetics. If the function is less than acceptable, then that form must be corrected when used for subsequent pieces, even when the corrections remove aesthetically pleasing aspects of a piece. For example, a teapot may have a beautifully spiraled spout that is dribbles. Under this principle, the design of the teapot must be modified so that it functions correctly.

burnishing

A process by which leatherhard or blackhard clay is made smooth by rubbing it with a hard smooth object like a stone, spoon or piece of glass. This procedure gives the piece a polished look. Burnished pots are usually unglazed but sometimes fine slips are applied to add to the decorating. Burnishing not only adds a glossy surface, it also contributes to the durability of the clay by making it more resistant to water absorption.

throwing or wheel-thrown

A process of producing pottery by use of a rapidly rotating disk referred to as a potter's wheel. The procedure involves placing a lump of clay in the exact center of the wheel head, creating an opening in the exact center of the clay. The size of the opening is gradually increased and the sides are pulled up until a cylinder is formed. A sufficient amount of clay is left for the bottom of the piece. The sides and mouth of the piece are then shaped, and the piece is removed from the wheel. Once it dries to a leatherhard stage, the potter can then go about trimming away excess clay, adding handles, spouts or attachments, and carving, cutting, or re-shaping the piece. An experienced potter can make this process look easy, but it takes many years of dedication and long hours of practice to become efficient at throwing and creating balanced, beautiful pieces

grog

A sand-like substance that is added to a claybody to add workability and strength to the clay. Grog is actually high-fired clay that has been ground down to a granular state. Because it has already gone through the firing process, it lessens the shrinkage of the claybody. In clays that require great resistance to thermal shock, such as Raku and flameware, large amounts of grog are usually present

whitehard

A stage in the drying of a piece of pottery where it is fully dry and has very little moisture. At this point it is ready to be fired in the kiln. It is important that a piece be as dry as possible before firing because moisture trapped inside the clay can explode as it heats in the kiln

blackhard

A stage in the drying of a piece of pottery where it is no longer malleable, but it has not yet dried out completely. The moisture content still in the piece gives is a dark tone

leatherhard

A stage in the drying process when a clay object can be carefully handled without danger of the shape being deformed, but the clay is still pliable enough so alterations can be made if desired. Many ceramic artists take great advantage of this relatively short period of time to add personal and unique characteristics to their creations. It is also the stage when attachments, such as handles, are added to wheel-thrown pieces

greenware

A stage in the production of pottery where a vessel and been from and is going through the drying process necessary before it can be safely fired. There several sub-stages in greenware. They are: Cheesehard, Leatherhard, Black-hard, White-hard. Several decorative techniques are possible during these different stages of the drying process

stoneware

A strong, hard, vitrified ware, usually high-fired above 2,200 F, in which the claybody and glaze mature at the same temperature, forming an integrated clay-glaze layer. This high-firing process brings the clay to a point of maximum solidification without danger of distortion, creating pieces very suitable for kitchenware and other functional pieces

pinching

A technique of building pots entirely by molding the clay with the hands without coiling, using slabs, or throwing. Called pinching because it usually starts by a potter inserting a finger into a ball of clay and pinching the walls to thin and shape the pot.

earthenware

A type of claybody usually associated with low-fired ceramics. Earthenware tends to be more porous than higher fired clays. The relatively low physical hardness of both the clay and the glaze tend to make earthenware less durable then higher fired claybodies and less appropriate a choice for functional pieces.

temperature

A very important consideration in the production of ceramics. The temperature at which the claybody and glaze of a piece need to be fired is determined by a variety of factors and in turn determines attributes of the finished piece such as durability, porosity, density, and color. In general, low-fired pieces tend to be less durable but have more colorful glazes, while high-fired pieces are more durable, less colorful, and work better as functional pieces.

crazing

Also referred to as "crackle" and "spiderwebbing", it is considered a defect in the glaze brought about by a difference in the amount of shrinking in the clay and the glaze. Usually undesirable, it is sometime used as a decorative element, but should be avoided for pieces intended for dinnerware and kitchenware.

slab

Also slab built. Any one of various techniques for creating ceramic objects that do not involve the use of a potters wheel. In this technique, the clay is pressed into thin slabs that are then cut, assembled, and shaped into the desired form

pottery

Although this term is usually used interchangeably with ceramics, it more precisely refers to ceramic objects that have a container shape, such as pots, planters, and tureens. It also can be used to refer to factories that produce pottery pieces

shrinkage

An irreversible reduction in the size/volume of a ceramic piece or glaze which is caused during the drying and firing process. This differs from expansion and contraction, which occurs naturally as a piece heats and cools. All pieces usually undergo a small amount of shrinkage. It takes a lot of experience and skill to select and work with the right types of clays and glazes so that shrinkage does not deform or otherwise damage a finished piece.

Maiolica

An opaque, lead-glazed, earthenware originated in the Mediterranean area. Should not be confused with Majolica.

hand building

Any one of various techniques for creating ceramic objects that do not involve the use of a potters wheel. These methods include coiling, slab building, and pinch pots. Hand built pieces are usually decorative instead of functional, primarily because the seams make them less durable and the unevenness of the surface makes them more difficult to clean.

claybody

As used by the ceramicist, a claybody, is a combination of various types of clay, minerals, and other materials. One material commonly added to the claybody is grog, which eases handling and increases durability. The recipe or formula is determined by the intended use of the final product to be produced. This is one of the reasons a combination of clays is used to create a claybody suitable to the ceramicists needs. Preparation of the claybody will arrange the clay particles alongside each other, making the clay more workable and durable. There are numerous types of claybodies used in ceramics today. The most popular of these include earthenware, stoneware, bone china, and porcelain. When purchasing a piece of ceramics, it is important to choose pieces made with a claybody appropriate for the piece's intended use.

ceramic change

Ceramic change A transition that takes place in clay when it is heated to approximately 1100 F or higher. At that temperature, chemically attached water molecules are separated from the clay particles. These clay particles are fused together and the ceramic object is permanently changed. Prior to the transformation, when clay objects are re-introduced to water the clay particles will slake (fall apart) and re-dissolve into the water. They can then be re-constituted into workable clay and used again. Low fired pottery is heated to only slightly beyond this point and will tend to be less durable than high-fired pieces

ceramics

Ceramics From the Greek word "keramos" meaning potter's clay; it refers to clay products made permanent by the application of heat (see ceramic change). It also describes the subject of study of a person known as a ceramicist. In addition to artistic endeavors, ceramics have many applications in industry where it is used for engine parts, electronics, medical equipment, and many other areas. When discussing art, this term is often used interchangeably with pottery. Ceramic artists, today, are sometimes referred to as "potters"

frit

Glaze materials that have been combined by melting and are reground to powder to be used in subsequent glaze formulations. There are several reasons why this process may be necessary. Some glaze materials are highly soluble in water. Soluble minerals cause problems for the potter when present in glaze recipes. The fritting process renders them insoluble. Another important reason is to "trap" hazardous materials, such as lead, which would otherwise be absorbed by the potter when working with finely ground glaze materials.

low-temp glaze

Glaze which matures at 1900 F and lower (cones 06 - 01). Considered a "soft" glaze, it is less durable than the underlying claybody. Due to the relatively low melting point, the presence of lead is almost definite. However, if the lead is used in a "frit", the danger of lead poisoning is greatly reduced if not altogether eliminated

high-temp glaze

Glazes that have a maturing temperature at or above 2200 F (cones 7 and up). High-temperature glazes are known as stoneware and porcelain glazes. They tend to be more durable but less vibrant in color. Functional pieces such as casseroles and dinnerware are usually better if they are high-fired ware. Sometimes referred to by the "cone" to which they are fired, as in a Cone 10 glaze.

intermediate glaze

Glazes that have a maturing temperature between 1900 F and 2200 F (cones 1-6). This is a wide range and not all intermediate glazes are interchangeable. A piece that has a good fit between the claybody and glaze within this range may be as appropriate and long lasting as high- fire wares, and are often more colorful

oxidation

In pottery this refers to a process that takes place during the firing stage of production. Oxygen in the air is allowed to enter the kiln to combine with elements in the clay and glaze. This is particularly important so that carbon and sulfur naturally occurring in clay can be "burned off". This takes place between temperatures of 1300 F and 2100 F. While all pottery firings go through a stage of oxidation, when it is allowed to continue throughout the entire process, it is called an "oxidation firing". An opposite effect is known as a "reduction firing"

porcelain

Introduced in Europe by Marco Polo after his excursions to China, true porcelain is a very high-fired (2300+ F) white ware, which, when thin enough, has a translucent quality. At these high temperatures, the body and the glaze mature together, creating a thick bonding layer. This gives porcelain great strength and durability. Due to many difficulties of working with porcelain, several imitations have been developed. These are referred to as china, bone china, and sometimes erroneously as porcelain. Although they may not have the same high qualities as true porcelain, they may be perfectly adequate for use and will most likely be less expensive

lead

Lead has traditionally been a key ingredient in many glaze formulations. Since these can leach into liquids and foods, you should take this into account when shopping for ceramic pieces to be used for food. The danger is in the cumulative effects that occurs with the repeated use of everyday functional pieces. Fortunately, most functional ware is fired to higher temperatures where lead, commonly used as a flux (melting agent) in lower fired glazes, will not be present or would tend to have been burned off during the glaze firing. If you have any doubt or concern over the possibility of lead poison, you should always inquire before making a pottery purchase

Lusters

Metallic materials applied on glazed surfaces. Lusters are pure metals dissolved by hydrochloric acid, which are then suspended in an oil base that can be applied with a brush. Precious metals are often used but whatever the metal, lusters are generally only used as highlights such as pinstripes or small applications. The firing is at an extremely low temperature (1200 F). The oil resin is burned out leaving, on the glaze surface, a very thin deposit of metal that can easily rubbed of with extended use of the piece.

black core

Occurs during the firing process when carbon inside the clay body has not been sufficiently eliminated. All clays contain some carbonaceous matter, which oxidizes during firing at approximately 1400 F producing carbon dioxide that escapes from the claybody. If the presence of oxygen is insufficient in the kiln atmosphere, the carbon cannot be eliminated from the clay and it is left in a weaker state. This tends to occur more during a "reduction" firing where oxygen in the kiln atmosphere is reduced.

Majolica

Originating in late 19th century England, Majolica has a pale yellow claybody with shiny colorful glazes. The surface was highly modeled providing many "pockets" where the glazes collected to create variations in color saturation. Majolica today still carries the connotation of brightly colored glazes but the surfaces of the wares tends to be smooth allowing for eye catching decorative possibilities. Should not be confused with Maiolica.

ovenware

Pottery made specifically for use in oven cooking. The claybody used must be capable of withstanding thermal shock. This should not be confused with flameware, which has the ability to withstand direct contact with a heat source such as an open flame. There are several factors to consider when selecting a piece for oven use. Beside the ability to withstand temperature changes, the piece should be designed for ease of handling, especially when hot, ease of cleaning, and have an appropriate shape

raku

Raku is a classification of ceramics that falls into the low-fire range. The term "raku" describes the piece as well as the firing process used to create it. Originally developed in Japan as a technique for quickly producing small functional vessels, in Western Civilization, because the process has been somewhat altered, raku ware is primarily created as decorative pottery. The most interesting aspect of the technique is that a piece is taken directly from the kiln into a raku pit while it is still very hot. The raku pit is lined with combustible materials, which immediately ignite. The pit is covered, and the resulting fire and smoke add a wide variety of finishes to the piece. The process is quick, exciting, and predictable only within a certain range of possibilities. The uniqueness or "one-of-a-kind" aspect of a raku piece is impossible to reproduce. This, along with its shortcomings for functional use, is the reason raku is popular primarily for decorative purposes.

reduction

Refers to a glaze firing process or a glaze mixture that is best enhanced when going through this type of firing. What is actually being reduced is the quantity of oxygen that is chemically bonded to any metal oxides in the clay or glaze mixture. To bring about the removal of oxygen molecules, when the kiln temperature reaches the melting point of the glazes used the kiln atmosphere is "flooded" with combustible material, such as gas or wood, thus causing the fire to pull oxygen from the pieces being fired. The duration of the stage varies, but it can be as long as an hour or more. The removal of iron oxide in clay causes a "fluxing" (melting) action thus creating a stronger bond between clay and glaze. In the glaze mixture, reduction brings about a wide array of colors depending on the combination of materials used in the glaze. In general, reduction fired glazes tend to have what is considered warmer tones than those of oxidation glazes. Both have the potential for beautifully produced pieces.

bisque

Refers to both a preparation firing process and the pottery piece that has undergone the firing. The firing is to a temperature that brings about a physical and chemical change to clay. Atomically attached molecules of water are driven off the individual clay particles and they are fused together transforming them into one piece. This intermediate step in glazed ceramics gives bisque ware the ability to absorb water of the glaze solution causing the glaze materials to adhere to the piece while it maintains its shape

cones

Small, elongated, three sided pyramids made of materials designed to melt at a specific temperature. Designed to melt at the same temperature as the glazes used. Cones indicate the progress of the melting. Cones are placed just inside the kiln during a firing so they can be seen through a peephole. The cones are one-time use only, and are available for a wide range of temperatures. They are numbered according to the temperature at which they melt. When discussing the temperature to which a piece/glaze is fired, reference is usually made to the cone used. The low fire range usually includes cones 06 through 01, intermediate fire includes cones 1 through 6, and high-fire includes cones 7 and up.

food safety

Some glaze formulations may contain lead and other heavy metals, which may leach into liquids and foods placed in contact with the glaze. Today's potters are keenly aware of this issue and are careful to use appropriate glazes and production techniques to eliminate these concerns. When shopping for pottery to be used for food, it is best to always ask and to shop at reputable stores that only carry pieces created by experienced potters

absorbency

The ability of porous materials to attract a liquid (water) into its air spaces. In the production of pottery this is useful for the application of glaze to bisque ware in preparation for the glaze firing

porosity

The ability to absorb water by capillary action. Over a period of time, this tends to weaken a claybody, and therefore should be a consideration when shopping for pottery. In high-fired ware, with a desired porosity of 1% to 2%, the concern is less important. However, low-fired pieces must be covered with a non-crazed glaze to minimize the effects of water absorption. This is not to say that low-fired ware should necessarily be avoided, but it may be less appropriate for some intended uses

glaze fit

The compatibility between the glaze and the clay used in the production of pottery. The match does not have to be exact but it must fall within an acceptable range. If not, various problems may occur with the piece, such as cracks in the glaze, shivering and shelling (glaze flakes off the clay) or dunting (various types of cracks in the claybody). The best way to avoid these problems when shopping for pottery is to deal with a reputable store or artist that will stand behind their pieces with a warranty and a good return policy.

dunting

The cracking of pottery caused by stress during the firing and cooling process. During the firing, clay goes through what is called a "silica inversion" at slightly over 400 F and again at slightly over 1000 F. This transformation puts a lot of stress of the clay. Another cause may be the different contraction of the clay and the glaze. All pottery goes through this stress but most are strong enough to withstand it. Dunting is not always evident immediately upon removal from the kiln. It sometimes occurs as much as a month or more later but it should not be confused with cracks caused by "thermal shock"

cheesehard

The first stage in the drying process of a piece of pottery where it is still soft.

kiln

The oven in which ceramic pieces are fired to convert them from unstable greenware into durable finished pieces. This term is derived from the Latin "culina", which refers to a structure built for the purpose of retaining heat that is introduced into the main chamber. There are a wide variety of kiln types from the earliest known open pit of pre- historic times to the sophisticated, efficient structures used by today's ceramicists. The many different types of kilns include: anagama, updraft, downdraft , gas burning, electric, wood burning, open Pit, centenary arch, raku, climbing, and salt

firing

The process of exposing ceramic pieces to high heat in order to convert them into durable finished pieces. During firing, the clay and/or glaze goes through a transformation whereby it is fused together into a solid piece. Unfired clay will dissolve in water, but the clay becomes impervious to water after firing. Although some fired pieces may still absorb water, this will have not an adverse effect on them.

vitrification

The process, induced by exposure to high heat, by which a material such as a clay or a glaze, melts and fuses together, thereby becoming solid and glass-like. This is what happens to ceramics and glazes during the firing process, and what converts a form made of soluble materials into an insoluble and permanent piece of ceramics

china

This term was originally used by the British for all ceramic imports from China and European imitations of it. Today, it implies a translucent white claybody covered with a glaze that is fired to a temperature lower than that to which the claybody is fired

flameware

Ware created with a claybody capable of withstanding extreme thermal shock produced by direct contact with an open flame or being subjected to a pre-heated oven. There are a number of factors that could classify a claybody as flameware, but the key factor is the ability of a piece to remain intact while different areas are experiencing a difference in temperature of several hundred degrees. When purchasing a piece for the purpose of such use, check the label or get some other form of confirmation. Flameware is sometimes referred to as "cookware", but it should not be confused with "ovenware" which cannot withstand a similar direct contact with a heat source