GLOSSARY OF TERMS

Index

Adobe Surface clay which is suitable for making sun‑dried bricks. Rather non‑plastic and contains a high percentage of sand.

Air Brush Tool which is used with compressed air to apply slips and glazes to wares.

Alumina (AI 0 ) A major ingredient found in all clays and glazes. It is the chief oxide in the neutral group (R O ) and imparts greater strength and higher firing temperatures to the body and glaze. When added to a glaze, it will assist in the formation of mat textures.

Ash Generally, the ashes of trees, straw, leaves, and so forth. It is commonly used in the Far East to provide from 40 to 60 percent of high temperature glaze ingredients. Depending upon the type, it will contain from 40 to 75 percent silica, from 5 to t 5 percent alumina, and smaller amounts of iron, phosphorus, lime, potash, and magnesia.

Bag Wall A fire wall in a kiln which channels the course of the flame.

Ball Clay An extremely fine‑grained, plastic, sedimentary clay. Although ball clay contains considerable organic matter, it fires white or near white in color. It is usually added to porcelain and whiteware bodies to increase plasticity. Often included in glazes to assist suspension and provide adhesion before firing.

Ball Mill A porcelain jar filled with flint pebbles and rotated with either a wet or dry batch of chemicals insides. It is used to blend and to grind glaze and body ingredients.

Batch A proportioned mixture of raw materials.

Bentontie A highly plastic clay originating in the decomposition of volcanic ash. Up to 29'o can be added to all bodies to improve plasticity. Up to 3% can be added to glazes and engobes to help keep them in suspension. It is best to add the bentonite to the water and mix thoroughly before adding it to the batch.

Bisque Pottery that has been fired to a red heat but not yet glazed.

Bisque Firing, The first firing of ware without glaze. 850 C.‑1000 C. (1562 F.‑1832 F.) Too rapid a rise in temperature at the beginning of this firing will cause steam pressure to build up and possibly shatter the clay. In this firing, the bound water which is part of the crystal structure of clay is driven of , changing the clay to pottery. This addition begins gently

Between 350‑450 C. (662‑842 F.) and increases to a maximum at 600 C. (1112 F.). From this point on there is little change of escaping steam and temperature rise inside the kiln can be accelerated faster.

Bone China A hard translucent chinaware produced chiefly in England. The body contains a large amount of bone ash (calcium phosphate)

which allows it to mature at cone 6 (2232 F.). It is not very plastic and tends to warp during firing.

glistering Bubbling formed during fast glaze firing by expanding air or steam unable to escape; usually avoided by a slow steady firing.

Bloating The unwanted blistering of the body caused by trapped gases; carbon monoxide and dioxide, sulphur or fluorine, within the clay body.

Blower Electrical blower or fan attached to the burners which blows a mixture of gas and air into the kiln chamber.

Bone Dry State of dryness of greenware when it is ready to be bisque fired.

B.T.U. British Thermal Unit. The amount of heat necessary to raise one pound of water one degree Fahrenheit.

Burnishing Polishing leather‑hard clay or terra sigillata by rubbing the surface with a smooth pebble or metal spoon or dry chamois.

Calcine To heat a ceramic material or mixture to the temperature necessary to drive off the chemical water, carbon dioxide, and other volatile gases. Essentially a purifying process. Some fusion may occur in which case the material must be ground. This is the process used in the production of plaster of Paris, Portland cement, ceramic stains, and so forth. 2) A ceramic material or mixture fired to less than fusion for use as a constituent in a ceramic composition.

Calorie Metric unit of heat. The amount of heat necessary to raise one gram of water one degree centigrade.

Casting Slip A clay and water suspension used in the process of slipcasting.

Celadon grey‑green, jade‑like, transparent stoneware glaze. Afeldspathic type‑glaze in which the color is derived from iron oxide fired in a reduction atmosphere. Originated in China during the 9th century.

Centigrade C. Degrees. Units for the measurement of temperature Celsius scale in which the freezing point of water is 0 C. and the boiling point of water is 100 C.

Ceramics Clay products made permanent by heat. The word comes from the Greek "keramos" meaning potters clay and the ware made from it. Glass, enamel, and cement are also considered ceramic.

Chamber Kiln A type of kiln, developed in China, which has several chambers linked together on a sloping site. This design usually makes use of downdraft circulation, with the exhaust heat of each chamber being utilized for warming up the next.

Chamber The inside space of a kiln.

Chemical Water Water (H O) chemically combined in each molecule of the glaze and body compounds. At approximately 450 C. (842 F.) during the firing cycle this water will begin to leave the body and glaze as water vapor. The clay cannot be slaked down into workable clay again after this. Chemical water is present in powder forms of clay while "mechanical" (water from the tap) is added to powder clays to make them workable.

Chuck A thrown form which is centered and stuck onto the wheel head where it is used to hold a pot for trimming.

Chun A pale, blue, opalescent stoneware glaze first made in China (11th Century). Feldspar type glaze fired in a reducing atmosphere. For maximum effect, the glaze must be applied thickly.

Clay Basically, a decomposed, feldpathic, granite‑type rock. To be classed as a clay, the decomposed rock must have fine particles so that it will be plastic and the fluxes present in the parent feldspar have been leached out by the action of water. Clays should be free of vegetable matter but will often contain other impurities which affect their color and firing temperatures. They are classified into various types, such as ball clays, fire clay, and slip clays. Pure clay chemically is AIO 2Si0 2H0. This formula is often used as a generality for most clays.

Clay Body Generally refers to a combination of clay ingredients calculated to mature at a desired temperature and to have desired working characteristics, such as good for working on the wheel, or slip casting or handbuilding, etc. or color characteristics.

Clearing Short oxidation period after glaze reduction when excess gases are burned off to prevent scumming on the glaze surfaces. Done during final stages of high fire reduction.

Coiling A hand method of forming clay objects by building with ropelike rolls or coils of clay.

Commercial Glazes Premixed glazes which are manufactured by such companies as Duncan and Ananco and can be bought at greenware shops.

Cones See pyrometric cones.

Cone Pack Unit of pyrometric cones set in position in a coil of clay which are placed in the kiln to read proper temperatures in different parts of the kiln chamber.

Crackle Glaze A glaze containing minute cracks in the surface. The cracks are decorative and in the case of raku firing are accentuated by heavy reduction. These cracks are caused by the different rates at which the body and glaze cool and contract after firing.

Crawling Aparting and contraction of the glaze on the surface of ceramic ware during drying or firing, resulting in unglazed areas. Caused by too heavy application of glaze or from uneven contraction rates between glaze and body or by dust/dirt on the unfired bisque surface.

Crazing An undesirable and excessive crackle in the glaze which penetrates through the glaze to the clay body. It should be remedied by adjusting the glaze or body composition to obtain a more uniform cooling and contraction rate.

Cristobalite The crystalline form of silica stable at high temperatures; chstobalite is formed when quartz is heated at temperatures above about 1200 C.

Cross Draft Kiln A kiln in which the hot gases travel through the ware across the chamber from a pierced bag wall to exit flues in the opposite wall.

Crystalline Glaze Glazes which contain crystals within the glassy matrix of the glaze. The glaze ingredients generally used are iron, lime, zinc or rutile with an alkaline flux. A slow cooling cycle is also necessary for the development of the crystals.

Cutting Off Process of removing a piece from the potters wheel by cutting beneath the object with a wire a string.

Damper A device for adjusting or for closing the opening in the stack of the kiln to stop the escape of heat.

Defloculant Sodium carbonate or sodium silicate used in a casting slip to reduce the amount of water necessary for fluidity and to maintain a better suspension.

Delft Ware A light‑colored pottery body covered with a lead‑tin glaze with overglaze decorations in cobalt on the unfired glaze. Delft was first made in Holland in imitation of Chinese blue and white porcelain during the mid‑16th century.

Della Robbia Ware Ceramic sculpture of glazed terra cotta, generally in relief, produced in Florence by Lucca della Robbia or his family during the 15th century. The glaze used was the lead‑tin majolica type developed in Spain.

Dioxide An oxide combining two atoms of oxygen with one atom of the other element.

Downdraft Kiln A type of kiln in which the hot gases, after rising upwards on entering the chamber, are drawn downwards through the ware before escaping through holes in the floor of the kiln or floor level flues.

Dunting Cracking of pottery cause by stresses which form during firing and cooling. Dunting point‑573 C. (1063 F.) This type of cracking can also be the result of opening the flues and cooling the kiln too rapidly.

Earthenware Low‑fired pottery (under2000 F.), usually red or tan in color with an absorbency of from 5 to 20 percent.

Egyptian Paste Earliest glazes ceramics known to man. Invented in Egypt around 4000 B.C. Type of self‑glazing, non‑plastic clay body in which soda is added to the batch. This soda develops into a crust on the surface of the wares while they are drying and during firing this same crust melts to form the glaze surface. This is a low‑fire ware.

Electric Kiln A type of kiln built of soft brick which is heated by electricity running through kanthal of nichrome elements or through silicon carbide rods called glo bars.

Element Coils made from nichrome or kanthal wire through which electricity is run to fire electric kilns.

Enamel I Enamels are essentially the same as glazes, except that they are melted onto metals rather than onto ceramic bodies. Enamels are usually melted at lower temperatures than glazes.

Encobe A layer of slip applied to ware to change the color or texture of the body. A prepared slip which is half way between a glaze and a clay; contains clay, feldspar, flint, a flux, plus colorants.

Eutectic The lowest melting mixture of two or more ceramic substances. This is always lower then the melting points of the individual materials.

Fahrenheit F. Degrees Fahrenheit. Fahrenheit scale is a scale of temperature measurement which is an extension of the thermometer scale 32 F. to 212 F. for the freezing point and boiling point of water.

Faience Earthenware covered with a lead‑tin glaze. A French term for earthenware derived from the Italian pottery center of Faenza, which during the Renaissance produced this ware partially in imitation of Spanish majolica ware.

Feldspar One of the most important glaze materials. It is used in almost all glazes, and in high‑fired glazes it is often the principal material and provides the principal flux. Feldspar is made up of (a) an alkaline portion consisting of sodium, potassium. or calcium, singly or in combination; (b) alumina; and (c) silica.

Feldspathic Glaze A glaze which contains a high percentage (50‑100%) of feldspar. Feldspathic glazes are usually stoneware temperature and are stiff glazes with a milkiness due to minute bubbles.

Feldsoathic Materials Materials which tend to have a dominance in potash or soda. Names of such materials does not necessary indicate the presence of feldspar, such 3s cornwall stone, nepheline syenite, spodumene, etc.

Filler 1) A material of little or no plasticity which helps to promote drying and control shrinkage in clay bodies, such as grog, sand or whiting. 2) A non‑plastic addition to a clay to change the fired state for reasons of porosity, glaze fit or whiteness (flint, quartz, feldspar, whiting and calcined china clay.)

Fire Box Combustion chamber of a gas, oil, or wood‑fired kiln, It's location depends on the type fuel and the shape of the kiln and draft system.

Fireclay Refractory clays, high in alumina and containing impurities, which vitrify below 1300 C. (2372 F.). They fire anywhere from a pale buff to almost whit and tend to be speckled with iron particles. Fireclays contain between 10‑40% of alumina and between 40‑80% of silica. Used in the manufacture of refractory materials, such as bricks, muffles, and kiln furniture.

Firing The process of conversion from clay to ceramic. It involves heat of at least 600 C.(1112 F ). During firing clay is changed into a stone‑like material; organic matter is burned away. The colors of the mineral used change and a layer of glass known as glaze is fused to the surface of the ware.

Firing Cryptocrystalline native silica. Almost pure silica containing less than 5% impurity in the form of calcium carbonate. In bodies it gives a whiteness, hardness and a resistance to crazing. In glazes it provides extra silica when needed and is often used to balanced the silica amount when adjustments are made for purposes of glaze fit.

Flocculant A soluble material which is added to a suspension to increase the viscosity. The following are flocculants‑‑vinegar, gelatine, epsom salts (magnesium sulphate) and aluminum sulfate. When using aluminum sulfate or epsom salts as flocculant add 1/2 to 1 percent.

Flocculation The action of altering the physical properties of fine particles in a suspension so that they no longer repel one another but aggregate into larger particles and settle by gravity. Process used by a potter in which a flocculant is added to a slip or glaze to thicken it.

Flues Passageways around the kiln chamber through which the heating gases pass from the fire box to the chimney.

Flux Relatively low melting compound in a glaze such as lead, borax, soda ash, or lime are including the potash or soda contained in the feldspar which acts as a catalyst on flint to make glass.

Free Silica Silica existing in a clay or body in a separate crystalline or cryptocrystalline state, such as sand, flint and cristobalite. Free silica influences a body's refractoriness, vitrification range and its susceptibility to dunting.

Frit A partial or complete glaze which is melted and then reground for the purpose of eliminating the toxic effects of lead or the solubility of borax, soda ash, and so forth.

Fusible Clays Opposite refractory clays. Clays which do not withstand high temperatures but break down and melt before 1300 C. (2373 F.). Often highly plastic containing many impurities, often in the form of fluxes.

Gas Burners .Parts to gas kilns which direct the fuel into the chamber of the kiln. They can be put into two general categories‑‑ atmospheric burners, which use natural air intake, and forced air bumers, which use electric blowers to speed up the heat of the flame.

Glaze A liquid suspension of finely ground minerals which is applied by brushing, pouring, or spraying on the surface of bisque‑fired ceramic ware. After drying the war is fired to the temperature at which the glaze ingredients will melt together to form a glassy surface coating. Egyptian alkaline copper glazes developed as early as 4000 B.C. Lead glazes from China developed as early as 200 B.C. High‑fire glazes made from feldspar, wood ash and clay in China as early as the 8th century A.D.

Glaze Fire A firing cycle to the temperature at which the glaze materials will melt to form a glass‑like surface coating. This is usually considerably higher than the first bisque fire.

Glaze Fit The stress relationship between the glaze and the body of a ceramic product.

Grinding Wheel A tool typically of cylindrical shape made of abrasive grains bonded together. Powered by electricity, it is used to remove sharp, unwanted edges of fired clay and glazes from pieces.

Grog Clay which has been fired and then ground into granules of more or less fineness. Grog provides texture, opens the body to help in uniform drying and cuts down shrinkage in clay bodies. From 20 to 40% grog may be used in a body depending upon the amount of detail desired and whether the pieces are free standing or made in press molds.

Gum Arabic or Gum Tragacanth Natural gums used as binders to enable the glaze to adhere better to the ware. Binders are necessary for fritted glazes containing little or no raw clay. They are also useful when a bisque fire accidentally foes too high, or in reglazing. One disadvantage of using gums is that their presence in a glaze causes spoilage. This can be prevented or delayed by adding a few drops of formaldehyde to the glaze.

Hard Brick Dense refractory brick used in building kilns. They have inferior insulation properties compared to soft brick.

Heat A physical form of energy generated by combustion, chemical action or friction and measured by calories or B.T.U.'s

High‑Tgmg2rature Glaze High‑fire glaze. A glaze which matures above 1200 C. (2192 F.). Also called stoneware and porcelain glaze.

Hump A large ball of clay centered on the wheel head from which several small forms are thrown and cut off.

Hydrate A compound involving water. Clay is hydrate because there is water chemically combined in its crystals.

Jiggering An industrial method of producing pottery. Forming a pot by using a spinning mold, which gives the inside shape. Used in making plates. The outside of the plate is formed by a metal profile which is fastened to a pivoted arm.

Jollying An industrial method of producing pottery. Forming a pot by using a spinning mold, which shapes the outside of the pot. used in making cups and deep bowls.

Kaolin China clay. The purest clay containing very little iron and therefore fires white. It has little plasticity and is high in alumina. Name derived from kaolin, a high ridge where the white firing clay was first discovered in China

Kiln Furniture Refractory shelves, slabs, posts and stilts for supporting ware in the kiln, made from sillimanite, alumina, zirconium silicate and silicon carbide.

Kiln Setter A device in an electric kiln in which a cone is place. When this cone mitts. the desired temperature has been reached and if set properly the kiln setter will automatically shut off the kiln.

Kiln Wash A protective coating of refractory materials applied to the surface of the kiln shelves to prevent excess glaze from fusing the ware to the shelves. Studio kiln wash is made of equal parts of kaolin and flint. Kiln wash used in the salt kiln must be made of 30% kaolin and 70% alumina hydrate:

Leather‑Hard Clay that is dried sufficiently to be stiff but is still damp enough to be joined to other pieces with slip. The ideal state for pots to be trimmed. At this stage of drying, the shrinkage of the clay has been largely completed.

Low‑Temeerature Glaze Low fire glaze. One which matures in the range up to 1050 C. (1922 F.)

Lusters A form of meticallic decoration or pottery or glass introduced in the Middle East before 900 A.D. Lusters can be made from metallic resinates, such as Cu, Mn, or Co in a solvent. The firing conditions must be reducing, this being achieved by the presence of reducing agents in the luster composition and/or by adjustment of the kiln atmosphere.

Majolica i Earthenware covered with a soft tin‑bearing glaze, often with a luster decoration. The ware originally come from Spain and derived its name from the island of Majorca. The unfired glaze surface is painted with metal oxides which sink into and stain the glaze during firing.

Mat Glaze A dull‑surfaced glaze with no gloss. Mat surfaces may be developed by the addition of barium carbonate, or alumina, and are the result of minute crystals developed in a slow cooling cycle.

Maturity The temperature or time at which a clay or clay body develops the desirable characteristics of maximum nonporosity and hardness; or the point at which the glaze ingredients enter into complete fusion, developing a strong bond with the body, a stable structure, maximum resistance to abrasion, and a pleasant surface texture.

Mullite Interlocking needlelike crystals of aluminum silicate (3AI 0 2Si0 ) which form in high‑temperature bodies between 1850 ‑2200 F. This formation is responsible for much of the greater toughness and hardness of stoneware and porcelain, and in particular for the closer union developed between the glaze and the body.

Neutral Fire An atmospheric condition in firing a kiln that is neither strongly oxidizing or reducing. Technically we do neutral or reducing firings. Oxidation implies adding pure oxygen into the flame.

Opacifier Minerals which are included in glaze recipes to render them opaque, such as tin oxide, antimony oxide,zircon, zinc oxide and titanium, along with zircopax, opax, and ultrox. As a general rule add 15% opacifier to the batch except when using tin oxide use 5%.

Overfire Firing higher than the required temperature and thus creating defects in the ware.

Overglaze Decoration A ceramic or metallic decoration applied and fired on a previously glazed surface of ceramic ware. Overglaze firings are at a lower temperature than the original glaze firing.

Oxidation A fire during which the kiln chamber retains an ample supply of oxygen. An electric kiln always gives an oxidizing fire though technically f could be called a neutral fire because pure oxidation is not fed into the kiln. Glazes fired in oxidation tend to be brighter that those fired in reduction. When using gas kilns for oxidation firings, the flame is kept as orange burning as possible (the gas and air combination are balanced), so as the interior of the kiln looks clean and not foggy or smoky.

Peeling‑ Separation of the glaze o slip from the body. Peeling may be caused when slip is applied to a body which is too dry, or when a glaze is applied too thickly or to a dusty surface.

Peep Hole (Spy Hole) A hole in the kiln wall through which one can observe the cones or the process of combustion.

Pin Holes I Imperfections in the surface of a ceramic body, glaze or enamel, characterized by depressions resembling pinpricks.

Plaster of Paris Hydrate of calcium sulphate, made by calcinating gypsum at 150‑160 C.

Plastic Clay Clay that is in a state in which it can be formed easily by moderate pressure and yet retain the new form without collapse.

Plasticity The characteristic property of moist clay that permits it to be deformed without cracking and to retain its new shape when the deforming stress is removed. Plasticity is associated with the sheet structure of the clay minerals and with the manner in which water films are held by the clay particles. The clays that are very plastic are: ball clay, Jordan clay, china clay, and some kaolins. Because of their high shrinkage these clays are not usually used alone.

Plasticize A material usually organic, capable of imparting plastic properties to non‑plastics or improving the plasticity or ceramic mixture.

Porcelain A vitrified, white and translucent ware fired at 1300 C.(2372 F.) plus. At this temperature, the body and glaze mature together to create a very thick body‑glaze layer. First made by the Chinese in the 8th century.

Porosity 1) The ability of a fired body to absorb water by capillary action. 2) A measure of the proportion of pores in a ceramic material.

Port Any opening in a kiln through which fuel or flame enters or exhaust gases escape or through which the progress of the firing is monitored.

Press Molding. Forming pots by pressing plastic clay onto absorbent plaster molds. As the clay stiffens , it shrinks slightly and can be easily removed from the mold.

Primary Clays Residual clays. Clays which can be mined from the position on which they decomposed from the parent rock. Tend to be relatively large in particle size and hence low in plasticity. Most important primary clay is china clay (kaolin).

Pyrometric cones Small triangular cones (1 1/8 and 2 5/8 inches in height) made of ceramic materials which are compounded to bend and melt at specific temperatures, thus giving a visual indication that the given cone temperature has been reached.

Quartz Inversion The temperature at which the flint (silica or quartz) is turned into a crystalline form. If this temperature of 1000 F. (573 C.) is passed too quickly in cooling the ware will crack (dunt).

Raku A low‑temperature technique of firing , invented by the 16th century Japanese potters. Traditional Japanese raku is associated with small, handbuilt bowls which are used in the tea ceremony. The firing technique generally involves a placement of pottery in a preheated kiln with tongs and removal from the kiln, again with tongs, when the glazes have reacher maturity. At this point, the ware may be placed in a combustible material such as leaves, straw, or sawdust which will ignite and smoke the ware. This smoking or reduction process will effect the color of both the clay and the glazes.

Reduction Fire Firings done to enhance bodies by bringing out the iron properties in the clay and glazes; consequently, richening the clay and glazes. Reduction firings can be done at any temperature at which clay and glazes mature. Reduction is brought about when the carbon in the fuel is combined with oxygen which bums‑the products of this action are heat and carbon dioxide. If not enough oxygen id present during this buming some free carbon is released (black smoke). This carbon monoxide will seize the oxygen induding the oxides (like red iron oxide) in ceramic materials. When these ceramic materials are deprived of oxygen, they are said to be reduced. The effect of this is to after the color of either the clay or glaze.

Rib A tool of wood, metal or rubber, which is held in the hand while throwing to assist in shaping the pot or to compact the clay.

Rubber Resist Latex mixture which is used to mask off areas on a piece before glazing in order for the surface beneath the resist to remain unglazed or to show underglaze layers. After applying the resist, allow to dry before glazing; when finished glazing gently pull rubber off of the piece before firing. Rubber resist can be thinned with water and/or ammonia. Brushes used in this resist can be ;;leaned with small amounts of lacquer thinner, hot soapy water or ammonia water.

Sagger A fired ceramic container to hold ware during firing and to protect ware from flames.

Salt Glaze A glaze developed by throwing salt (NaCl) into kiln fired to maturity. The salt vaporizes and combines with the silica in the body to form sodium silicate, a hard, glassy glaze. The kiln interior becomes glazed itself which limits the kiln's use to the salt glazing process. Probably originated in the Rhineland during the 12th‑14th centuries.

Sawdust Firing A simple system of firing in which pots are set in sawdust which is lightly packed within a ventilated, non‑combustible container such as a metal garbage can. The sawdust burns slowly achieving sufficient heat to turn clay into a semi‑vitrified material. This type of firing produces heavy reduction.

Score The process of scratching the surface of moist greenware joints when building objects or attaching clay parts to one another. After scoring. the scratched areas are coated with slip, which creates a stronger bonded joint, and then press together.

Scum 1) A floating layer of unmelted material on the molten glass surface. 2) The light colored marks which appear along edges and as finger‑prints on unglazed ware and bisque, caused by soluble salts in the clay. Adding barium carbonate to the clay body will prevent scumming.

Secondary Clay Sedimentary clays. Clays which have been deposited in their present position by the action of water or ice. More plastic than primary clays. Most contain impurities of which iron is most common giving grey, cream, or brown colors. Also contain organic matter.

S’graffito Decoration achieved by scratching through a colored slip to show the contrasting body, glaze, or slip color beneath.

Shivering A glaze defect in which slivers of glaze shear away from the pot. What you do when you're cold.

Silica Silicon dioxide. The glassy substance which in the most important constituent of ceramic glazes and which is an integral part of clay and many minerals. occurs naturally as quartz rock, flint, and sand.

Slip Casting A reproductive process of forming clay objects by pouring clay slip into a hollow plaster mold and allowing it to remain long enough for a layer of clay to thicken on the mold wall. After hardening, the clay has shrunk and the object can be removed from the mold easily. Slips formulated for casting usually contain sodium carbonate or sodium silicate which function as deflocculants. Plaster molds used with slips which contain these deflocculants, tend to disintegrate because of the sodium.

Slip Clay A clay such as Albany and Michigan clays containing sufficient fluxes to function as a glaze with little or no additions.

Slip Trailer A plastic squeeze bottle or device for extruding a thin trail of slip or engobe to make surface decoration.

Slipware Earthenware pottery decorated with white or colored slops under a transparent lead glaze. Slipware was popular in the ancient Near East, from the 8th century onwards. English slipware was made in the 17th and 18th centuries. Most early American pottery was slipware.

Soak Soaking. To hold a kiln at one temperature for a period of time to effect a desired degree of vitrification, chemical reaction and/or recrystallization.

Soft‑Paste Porcelian A type of porcelain made from a body containing a gassy frit and fired at a comparatively low temperature (1100 C.). The most famous soft‑paste ware was that produced in the 18th century at the Sevres factory in France, and at Chelsea, Derby, Bow, Worcester, and Longton Hall in England.

Sprigging Applying clay in a plastic state to form a relief decoration. It may be freely modeled and then applied or it may be presses from a small, mold form.

Stacking Loading the kiln for maximum number of items with efficient distribution.

Stain in Sometimes a single coloring oxide, but usually a combination of oxjdes, plus alumina, flint, and a fluxing compound. This mixture is calcined and then finely ground and washed. The purpose is to form a stable coloring agent not likely to be altered by the action of the glaze or heat, While stains are employed as glaze colorants, their chief use is as overglaze and underglaze decorations and body colorants.

Stilt 1) A ceramic tripod upon which glazed ware is placed in the kiln. 2) The refractory posts used to support the kiln shelves.

Stoneware A high‑fired ware (above cone 8) with a slight or no absorbency. It is usually gray in color but may be tan or slightly reddish. Stoneware is similar in many respects to porcelain, the chief difference being the color which is the result of iron and other impurities in the clay.

Terra Cotta An earthenware body, generally red in color and containing grog. Often used in large sculpture and architectural forms.

Terra Sigillata A Sliplike glaze produced during the Etruscan and Greek periods. A deflocculant. such as calgon, is added to the slip so that the finer particles of clay can be decanted off and collected. It can be applied by brushing or spraying to greenware or bisque. After applying, the surface is burnished with a chamois or metal spoon. When dried, terra sigillata gives a wax like surface which is not completely waterproof. Terra sigillata made from grolleg clay, along with other types or kaolin clays, can be fired as high as cone 10. All other types of terra sigillata are best when fired at bisque temperatures.

Thermal Capacity The amount of heat that a clay product will absorb, usually expressed in B.T.U. per F.

Thermal Shock The stresses created within a ceramic object by sudden temperature change; may cause wares to crack.

Throwing The method of creating clay forms in which a ball of the prepared body is thrown on a revolving potter's wheel, where it is centered and then worked into shaped with the hands. A sponge, cut‑off wire, and rib are tools which are necessary to finish any pieces thrown on the wheel. A container filled with water to lubricate the hands while working the clay is also necessary.

Trimming Removing unwanted clay to achieve a particular form, thin a pot wall or create a foot, rim, etc. Usually done on thrown ware on the wheel, at the leather hard stage.

Undercut A cut that slants inward, preventing a poured or cast form from being removed from a casting mold.

Underfired Fred at a temperature lower than the intended or insufficient heat work. Underfired glazes are often dull. Transparent and shiny glazes tend to craze because they have not integrated with the body.

Underglaze Decoration A ceramic decoration applied directly on the bisque surface of ceramic ware and subsequently covered with a transparent glaze.

Updraft Kiln A type of kiln in which the fire enters the kiln at the bottom, passes upward through the war chamber, and escapes from, the flue at the top into the chimney.

Viscosity The nonrunning quality of glaze, caused by glaze chemicals which resist the flowing action of the glaze flux.

Vitreous Pertaining to the hard, glassy and nonabsorbent quality of a body or glaze.

Vitrification Stage of firing beginning at 800 C (1472 F.) when the soda and potash within the body start to flux the free silica. The body shrinks during vitrification. Vitrification strengthens the final product by welding the particle size together with glass (mullite crystals). The furthest stage to which a body can be taken without deformation.

Volatilization Action under influence of extreme heat of the kiln in which some glaze elements turn successively from a solid to a liquid, and finally into a gaseous state.

Warping Deformation of a clay shape caused by uneven stresses during shaping, drying or firing.

Water Glass Another term for a liquid solution of sodium silicate that is used as a deflocculant.

Water of Plasticity Lubricant water which provides the lubrication for the clay particles and enables them to slide past one another. Highly plastic clays require more water than low plasticity clays to bring them to the plastic state. Sometimes called 'mechanical water" or tap water as opposed to chemical water present in the dry form of clay.

(1000F‑1100 F.) .The heat rise must be gradual to allow atmospheric and chemical water to escape. In some cases there will be organic Impurities which will also bum out. releasing carbon monoxide.

Wax Resist Wax, either melted or as an emulsion which is applied to pottery to prevent slips or glazes from adhering at that point. Wax may be applied to either the raw or bisque ware, over or between two layers of glaze.

Wedging A general conditioning of the prepared clay body. Kneading or mixing plastic clay by cutting or rolling. This process compresses the clay and works air bubbles out or the body and develops a uniform texture or consistency.

Vitreous Pertaining to the hard, glassy and nonabsorbent quality of a body or glaze.

Volatilization Action under influence of extreme heat of the kiln in which some glaze elements turn successively from a solid to a liquid; and finally into a gaseous state.

Warning Deformation of a clay shape caused by uneven stresses during shaping, drying or firing.

Water Glass Another term for a liquid solution of sodium 'silicate that is used as a deflocculant.

Water Smoking The initial phase of the firing cycle up to dull red heat (1000F‑1100 F.) .The heat rise must be gradual to allow atmospheric and chemical water to escape. In some cases there will be organic Impurities which will also bum out. releasing carbon monoxide.