Fired ceramic has the potential of being extremely hard and resistant to abrasion and wear. Special abrasion resistant ceramic products are made from highly specialized materials and fired to exacting requirements. Calcined alumina, for example, can be cast and fired to very high temperatures to produce surfaces with exceptional resistance to abrasion. Likewise, aggregates and bonding frits are employed by the abrasives industry to make all kinds of abrasive products (products designed to abrade others).
The hardness of pottery is mainly dependent on the development of aluminum silicates during firing (i.e. mullite crystals). This requires temperatures high enough to melt fluxes to allow them to dissolve quartz and other minerals. It also requires time to allow these processes to complete. Likewise the glaze components most resistant to abrasion are refractory alumina and silica, and the higher you fire the more you can get into your glaze and yet still get it to melt.
Absorption is the degree of moisture that will soak into plaster when casting, or into bisque when glazing or decorating with nonfired colors.
Various oxides can be considered basic (RO, R2O group) or acidic (RO2). The greater the proportion of oxygen the more acidic an oxide is.
Add-ons are greenware or leatherware parts that are added to the main casted piece ie: handles to cups.
Aging refers to the process of allowing newly mixed casting slip to set, undisturbed, for a period of time. This process allows the ingredients to homogenize for best casting qualities.
Airbrushing is the process of applying color with the use of a small air-pressure gun. It is used for shading and general decorating.
Without a regular structure. Amorphous minerals do not have a repeating crystalline matrix. Glass, for example, is amorphous because it is cooled in the kiln quickly enough that no crystals have an opportunity to form.
Antiquing is a decorating process in which you remove applied color to accentuate detail.
An analysis is typically used to compare the oxide content of materials. An analysis is expressed in percent and each number represents the amount by weight of the oxide. An analysis will often show LOI as one of the items, this represents the volatile portion of the material that is lost during firing. An analysis can be converted to a formula by dividing each oxide amount by the molecular weight of that oxide.
The relation between the volume of a mass and the volume or water absorbed when the mass is immersed.
Banding refers to applying color to ware in decorative bands. Usually done with the aid of a banding wheel.
A Banding Wheel is a hand-operated turntable used in banding and other types of decorating.
See Acidic.
Many people in Europe use the term "bat" to refer to kiln shelves (i.e. UK). Thus "bat wash" is kiln wash. However in North America, "bat" most often refers to wood, plastic, or plaster disks which are used on the potters wheel. A bat is held in place by pins, an interlocking arrangement, or glued on by slip. Ware can then be thrown on the bat and the whole thing removed to make another piece.
Binder is material, such as gum arabic, added to hold ceramic ingredients together.
The practice of prefiring greenware without glaze to make it impervious to water, easy to handle, or vitrify it. Generally, bisque is clay that has been fired in a kiln. Glaze is then applied and it is fired again. Low fire bisque is typical for pottery and ceramics while vitrified bisque or high fire bisque is done for bone china and some types of stoneware. Low fire bisque should be fired as high as possible to burn away all carbonaceous matter, yet low enough to provide enough absorbency to make glaze application easy. High fire bisque is done to mature the body (i.e. bone china) and subsequent firing is usually done to apply a low fire glaze. Such glazes must have special additives to make them gel and stick to the ware (i.e. calcium chloride, gum); these glazes take much longer to dry.
Black coring usually occurs during a reduction firing and is a result of fast firing and/or lack of oxygen in the kiln between 1,300° F (700° C) and 1,650° F (900° C) (usually in the bisque firing). If body carbon fails to oxidize to CO2 it steals oxygen from Fe2O3 (reducing it to FeO, a powerful flux.) This FeO will then flux the body, sealing it and preventing the escape of remaining carbon in the body. This produces the characteristic 'black core' you see on ware cross section. The more iron in a body, the greater the risk of this problem if firing is not right. Once iron is reduced to it is very difficult to reoxidize it back to Fe2O3.
Note that electric kilns can also produce this problem, depending on the carbon and iron content of the clay, density of the pack, available airflow, and speed of the firing.
Blistering refers to the appearance of broken bubbles found on the glazed surfaces of fired ceramic pieces.
Mechanical mixing of a clay slurry. Blungers or "mixers" of all different types are available. Some are simply a high-speed propeller on a shaft, others employ complex arrangements of paddles and container shapes. Clay slurries (slip) require the application of considerable amounts of energy to achieve a slip in which water has penetrated well between all particles.
A mixture of raw and/or refined clays with possible additions of flint as a filler and feldspar as a flux. In typical vitrified bodies the larger grains of quartz and clay remain unmelted giving rigidity to the mass while the fluxes melt to a glass which binds these grains together. Earthenware bodies develop much less glass bonding and depend more on sinter-bonding and are therefore weaker.
"Buff" is a term used to describe the color of a non-vitreous or semi-vitreous fired clay that has enough iron to take away its claim to being white yet not enough to make it a brown or red burning ceramic. Buff coloration is generally a straw color or yellowish white. Grey-white firing bodies are not usually referred to as "buff" firing because the grey coloration is associated with vitrification, especially in reduction.
The practice of slow firing greenware through the critical temperature surrounding the boiling point of water. This is done to prevent cracking and explosions associated with steam trying to vent out of the greenware which is either not completely dry or has a thick cross section. Kilns are often candled overnight on very low heat and then the firing is continued in the morning.
Glazes with variegated patterns of grey and black from carbon trapped below the surface.
The effect is created by fuel firing without adequate oxygen in early stages to build up soot (carbon) on the surface of ware. As the firing continues, the carbon trap glaze begins to melt before the carbon sitting on the surface burns away. Carbon is a refractory material and will stay in a glaze as long as there is no oxygen to combine with it. Typically this type of glaze includes soda ash or other soluble alkaline fluxes which will migrate to the surface of the raw glaze as it dries, forming a crust of alkalis which will melt earlier than the rest of the glaze, thus facilitating the carbon trapping.
Forming pottery by pouring deflocculated (water reduced) clay slurry (slip) into plaster molds. The absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks slightly and can be removed from the mold. There are many high-tech variations to this process and a wide variety of materials can be cast to make tiny and very large shapes.
Ceramic pertains to products made from clay and simular materals treated by firing in a kiln, as pottery and brick.
The term "ceramics" has come to signify the slip casting industry that uses talc-ball clay slurries to cast greenware for firing at low temperatures. The term "pottery" is used to refer to those individuals and companies who fabricate their own ware using plastic clays of all types and at all temperature ranges.
Clay Carbon is carbonless paper used for transferring designs onto greenware (unfired clay).
Cleaning greenware mainly pertains to the removal of the seam lines left from the casting process. A good cleaner will also remove any sags or imperfections. This process is achieved with the use of special "cleaning" tools (a metal scraper), sanders, sponges, and/or carving tools. The quality of the cleaning greatly effects the quality of the ceramic piece. This is one step that should be done well without removing the detail of the piece.
Refractory ceramic coatings are sprayed onto the insides of kilns and on elements to achieve several goals. Coated elements are claimed to last up to 10 times longer. Coated kilns are more efficient and last longer. Coating technology has been developed into successful commercial products by Feriz Delkic (pronounced Ferris) of International Technical Ceramics, Inc. Although ceramic coatings are expensive, they do save money in the long run.
A measure of the reversible volume or length change of a ceramic material with temperature. The more it expands during heating the more it contracts while cooling down. Glazes that do not have a similar thermal expansion to the body cause problems like crazing, shivering, and weakened ware.
Coil pottery or method is 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 into an even wall. Coil pieces can be almost any shape and any size.
Colloidal particles are so small and light that they do not settle in water. The movement of water molecules is enough to keep them in suspension. It is important to remember that colloidal particles occur in a suspension, not a solution. A simple way to tell the difference is to shine a beam of light through the liquid. If you can see the beam, it is a suspension.
Color wash is the term for a color and water solution used for shading and antiquing.
A pyramid-shaped ceramic device used to quantify the amount of heat delivered by a kiln. These devices are formulated from different mineral mixtures and numbered accordingly. They used in a kiln, a pyrometric cone deforms when the proper heatwork is achieved. Various cones are made to either trigger mechanical shut off's or to act as a witness to kiln conditions.
A stand or rest for holding witness cones during firing. Plaques are important to assume that cones are placed at a consistent depth and angle firing after firing. These days, most witness cones are self supporting and do not require a plaque.
An electronic device attached to a kiln. Controllers are usually capable of firing a kiln to a specific schedule and can shut it off at the right time, soak it for a specified period, and cool it down at a controlled rate.
A type of glaze that is intentionally crazed. Stains and other colorants are often rubbed into the crack lines to heighten the effect. Crackled glazes typically severely weaken ceramic ware, especially if it is thin, low fired or porous. These type of glazes are not "food safe" due to the fine cracks that can trap bacteria.
A special kiln shelf that has legs (usually three) and stacks by interlocking with others. Cranks are used to fire plates and tiles (one per crank). They are employed to overcome poor use of space in kilns when trying to fire flat objects.
Cratering refers to moon-like craters that may appear on a glazed surface.
A condition where fired glaze separates into clumps or islands leaving bare clay patches showing in-between. More prevalent in once fired ware. There are many causes for crawling (typically glazes shrink too much during drying and don't have a good bond with the bisque). Sometimes this is due to contamination on the bisque surface.
Some times glazes are made to crawl intentionally. One technique to make this happen is to add 15-20% magnesium carbonate (testing required to determine amount) to a low fire transparent glaze.
Small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. A glaze of higher expansion shrinks more than the clay to which it is attached and therefore crazes.
There are many treat-the-symptoms approaches to crazing but the bottom line is: If there is a thermal mismatch, it will reveal itself sooner or later no matter how you adjust firing or glaze thickness to hide the problem. If crazing is visible, it is an indication of a significant problem. This is because long before crazing becomes visible, serious strength problems result where glaze and clay are not expansion-compatible.
Crosshatch is a decorating technique that calls for applying alternate coats of color at perpendicular angles.
Crystals form in glazes during cooling depending on the oxides present. All glazes have the potential to crystallize if cooled slow enough. Crystalline surfaces form well in fluid glazes low in alumina because it is easier for the component oxides to migrate to the site of formation. Glazes prone to crystallization have a distinct "zone of crystallization". If the kiln is cooled slowly during this critical period of freezing they will grow.
Most crystals have a halo of a different color than the glaze. This surrounding area is reduced in crystal forming oxides and is thus a depletion zone. In theory larger crystals should grow at the expense of smaller ones in a "survival of the largest" situation.
Crystals demonstrate the phenomenon of phase separation, where a glass melt separates into two or more liquids. Coloring materials tend to preferentially and selectively gather at one of these, (it is not uncommon to see a crystalline glaze where one coloring oxide colors the crystals, another the glassy areas).
Crystal formation can be considered a mechanical imperfection in the glass since it is disrupting the homogeneity of the matrix and imposing discontinuities between glass and crystal phases.
For the best crystals, slow the firing at the peak to make sure all materials are fully dissolved in the melt and then cool to the point where the crystal forming material precipitates out into crystals floating on the surface. Once experience reveals at what temperature they grow best you can soak the kiln there for maximum benefit. Some people even reheat the ware to just below this temperature and they will grow further.
Some materials, especially those with high melting temperatures, seed crystals (give them a place to start) in melts with a chemistry that can supply the seed material. A good example is the formation of a calcium silicate matte with the addition of wollastonite (calcium silicate). Zircon materials and tin are other examples. Other materials will crystallize well if oversupplied.
An accurate electronic kiln controller is a must to make results repeatable.
For more information see also:
A method of printing designs using ceramic inks onto specially prepared paper (a glue, then a base coat of clear material is applied followed by the inks). The design on the clear material is then transferred to glass or ceramic. To apply the decal you put it in water for a minute or so to loosen the glue enough to slide the clear decal onto the ceramic surface (slightly porous paper is used so that water can soak through it).
Decal ware must be fired to precise temperatures to develop and mature the color properly. The are many mistakes to be made in application and firing that will compromise the quality of the final product.
The process of making a clay slurry that would otherwise be very thick and gooey into a thin pourable slurry. Deflocculants (or electrolytes) are liquids or powders added in small amounts and they work their magic by imparting electrical charges to clay particles making them repel each other. The opposite of flocculation.
To deflocculate a slurry properly it is very important to be able to measure its specific gravity and viscosity accurately. Yet it is very common for slip casters to be tied to a recipe and have little understanding of how to control their slip. Many will work for years with substandard slip without knowing it, others will throw away all scrap rather than reprocessing it simply because they do not understand slip therology.
It is common for potters to mix slips using clays intended for modeling or sculpture. Far better casting mixes can be made using mixes of materials that emphasize permeability instead of plasticity. Once you have used a slip properly formulated and deflocculated for casting, you will never go back to using an inadequate slip.
Sometimes glazes are deflocculated to reduce their water content, this is most likely where glaze is being applied to once-fire ware.
Check out the recipe section in our Education Center.
The crystallization of a ceramic melt during cooling. Devitrification is desirable in achieving certain visual effects (e.g. matteness) and undesirable when a transparent or glossy transparent glaze is desired.
Dimpled glaze surfaces are those that have tiny holes that do not go down to the ceramic underneath (a pinhole). These holes look as if they were produced by a pin-point being pressed into the surface of the glaze when its melt is very stiff. Dimples are considered a glaze imperfection and are usually caused by firing too quickly.
Dipping is a method of applying glaze by immersing a piece in a container of glaze.
Drybrushing is an effect achieved by applying nonfired color very lightly with an almost dry brush.
Dryfooting refers to leaving the bottom of a piece unglazed so that stilting is not necessary.
Cracking that occurs in ceramic ware that is cooled too quickly. Dunting can exhibit itself as simple hairline cracks or ware can fracture into pieces. Ware of uneven cross section, ware with glaze that fits poorly, or large pieces (i.e. large flat plates) are often subject to dunting. Ware with high amounts of cristobalite or quartz undergoes sudden volume changes when heated or cooled through the inversion temperatures of quartz.
A clay fired at low temperatures (cone 010-02) where it does not develop maturity (vitrify). Earthenwares are porous and therefore not as strong as stonewares and porcelains (sintering is the primary particle bonding mechanism). Earthenware glazes are usually very bright colored and if the glazes are properly fitted, earthenware can be quite strong and functional.
Terra Cotta is a special type of earthenware where red burning clay is used.
A element is a high temperature coil or coils inside the kiln that produce the heat.
Embossment is a raised design on a clay piece.
A white or colored slip applied to clay as a coating or by slip trailing, usually at the leather hard stage.
Engobes are formulated with less clay content than slips. A slip shrinks with the ware as it dries. A engobe formulated for application to dry or bisque ware cannot shrink excessively. Also, a higher percentage of flux in an engobe helps it to bond to the surface during firing. A slip, on the other hand, bonds to the surface during drying by the interlocking of the flat clay particles at the interface between the damp clay and wet slip.
The lowest temperature at which a mix of two materials will melt. Often the temperature is an anomaly, that is, it is much lower than the melting temperatures of only slightly different mixtures.
Lead-tin solder is an example. Lead melts at 327° C, tin at 231° C. The lowest melting combination is 67% lead, 33% tin (180° C).
Non-eutectic mixtures have a melting or softening range. Such mixtures do not flow well until thoroughly heated past the softening range. This softening phenomenon is what makes glazes hang onto the ware.
The removal of the seam left by the mold in slip casting using a fettling knife. The seams are then finished with a sponge. Fettling is also refered to when removing the spare.
A Ferrule is the metal band of a brush just below the bristles.
A brick capable of withstanding high temperatures without deforming. 'Insulating firebricks' have the additional advantage of acting as good insulators due to the large pockets of air in the matrix of the brick. There are many different kinds of firebricks available, some very expensive. Types are categorized for their heat duty and the types of materials and atmospheres they must come into contact with.
Firing refers to the process of heating the ceramic piece in a kiln. This is done to turn greenware into bisque or for applying glaze, overglaze, decals, or precious metals such as gold.
Flash is the undesirable transference of a soft glossy sheen onto unglazed ware when high-fired glazed and unglazed ware are fired together.
Flashing refers to shiny edges on ware, often produced by overfiring.
The opposite of deflocculation. The process of making a ceramic glaze or clay slurry which would otherwise be thin and liquid into a gel. This is typically done to improve suspension properties or allow application without problems of running and dripping. However flocculated slips have a high water content and thus a higher shrinkage.
Flow is the term used when referring to the running or moving qualities of a glaze.
Flowing Coat is the term used to describe applying color with a well-loaded brush.
Food safe is a term used to declare a finished ceramic or pottery piece as "safe" to be used with food. There are many circumstances that prevent a finished piece from being food safe. These include such examples as the lack of glaze, lead amounts in the glaze, use of crackle glazes, and crazing to name just a few.
The Foot refers to the bottom of a ceramic item.
A substance that lowers the melting or softening temperature of the mix or compound in which it is present. The degree of melting that occurs depends on the particle size of the powders present and the melting temperature of the individual particles. It also depends on whether material particles present are premelted and whether they soften or melt suddenly. Fluxing oxides are those of the RO group and include ones like K2O, Na2O, CaO, Li2O, MgO. B2O3 is actually considered a glass former but it is also regarded as a flux by virtue of its low melting temperature.
A formula is typically used to evaluate the oxide content of fired glazes and glasses. Each value in a formula represents a number of oxide molecules and formulas are typically unified on the fluxes. Formulas do not usually show LOI because they are used to model the fired product and predict properties based on oxide content. A formula can be converted to an analysis by multiplying each oxide amount by the molecular weight of that oxide and then calculating percents.
A ceramic glass that has been premixed from raw powdered minerals and then melted, cooled by quenching in water, and ground into a fine powder. Huge quantities and varieties of frits are manufactured for the ceramic industry every year by dozens of different companies.
Although the fritting process is expensive there are many advantages to using frits in glazes, enamels, etc.
-To render soluble materials insoluble
Often very useful oxides (i.e. boron) are contained in high proportions in raw materials that are either slightly or very soluble. These normally cannot be used in glazes because they have adverse effects on the slurry's fluidity, viscosity, thixotropy, or make it difficult to achieve or maintain the desired specific gravity. In addition soluble compounds are absorbed into porous bodies during glazing and this compromises the body's resistance to bloating and warping and the glaze's homogeneous structure. Fritted mixes containing these materials renders them insoluble and inert.
-To improve process safety of toxic metals
Some materials contain undesirable and unsafe compounds. The fritting process drives these off. Many other materials are unsafe in the workplace and fritting decreases their toxicity for ceramic production workers. Lead is a prime example. Lead frits decrease the process toxicity of raw lead compounds. Barium is another example. However the fritting process has no effect on whether or not a fired glaze will leach or not. This is a function of its chemistry, unbalanced and unstable glaze formulas are just as likely with frits as without. The primary safety benefit for frits is thus for workers who use frits in manufacturing.
-To reduce melting temperature and improve melt predictability
Since frits have been premelted to form a glass, remelting them requires less energy and lower temperatures. Frits soften over a range of temperatures (in contrast to crystalline raw materials that melt suddenly) and lend themselves very well to production situations where repeatability and ease-of-use are necessary.
-To avoid volatilization of unstable substances
Most raw ceramic materials contain sulfur or carbon compounds as well as H2O. These vaporize at various temperatures as materials decompose and are driven off as gases during firing. This volatilization activity has a detrimental effect on the glaze surface and matrix. The fritting process drives off these compounds and glazes are thus much more defect free.
-To achieve homogeneity
Other than dissolution and very localized migration, fired raw glaze melts do not mix well to create an evenly dispersed oxide structure. The fritting process employs mechanical mixing to assure a completely homogenous glass that will exhibit the intended properties.
-To achieve oxide blends that are difficult or impossible with raw materials.
Many glaze formulations cannot be achieved with insoluble raw materials (i.e. high borax, high sodium). Frits employ soluble materials to make almost any combination possible.
-Improve the quality of decoration
Over and underglaze colors work better with frits than raw materials because the former are cleaner, less reactive, melt evenly, and have a more closely controlled chemistry. This means colors are brighter by virtue of compatible chemistry, by better glaze clarity. Edges of colors also tend to bleed less and color quality is homogeneous rather than variegated (although variegating materials can be introduced to introduce this quality if desired).
The Frit market is driven by large customers who need certain formulations and by the prepared glaze industry. Availability of smaller quantities of frits are generally determined by what industry is using. Since the Frit market changes with time, so does the availability of frit types.
Some frit companies, such as Fusion Ceramics, freely supply the chemical analysis of their frits. Others such as Ferro are more guarded and either provide no chemistry or approximate analyses (although they were more forthcoming with this data in the past). The latter practice makes little sense since it partially defeats the whole purpose of using frits, namely, having control. It also works against the general trend of using ceramic calculations to take control of glaze properties.
Furniture are implements used in a kiln to make full use of a kiln's capacity (shelves, posts and stilts).
A glass is actually a "super-cooled liquid", it is the opposite of a "crystal" in which molecules have opportunity to orient themselves in the preferred pattern during freezing. In a glass the random orientation of molecules is frozen into the solid.
"Ceramics for the Potter "University of Toronto Press" 1952 called it "silica and two or more bases, which are combined under heat to form a molten solution. On cooling, the solution becomes so viscous that the molecules cannot move about freely enough to form crystals before the state of rigidity is reached. If glass were allowed to cool slowly, it would be as crystallized and as opaque as granite - it is the fast cooling, with the viscosity, that makes glass transparent. Glass is, in short, a solid solution."
In 1945 the American Society for Testing Materials suggested the following definition if glass: "Glass is an inorganic product of fusion which has cooled to a rigid condition without crystallizing."
In 1962 the British Standards Institution adopted the same phraseology.
Later more complex methods of producing this state led to revisions such as:
"Glass is a non-crystalline solid" and....."glass is and x-ray amorphous material which exhibits the glass transition.."
A thin glassy layer formed on the surface of fired ceramic. Glazes are a finely ground mixture of mineral and man-made powders tuned to melt and flow at a specific temperature. Many clays will melt well at higher temperatures and thus qualify as 'slip' glazes. Glazes are normally mixed with water, suspenders, and hardeners to make them harden on drying and produce a suitable consistency for application by painting, dipping, or spraying.
Glazes are often classified (e.g. unleaded, raw, fritted, and/or foodsafe) to designate type within a specific industry or type of ceramic ware.
Glaze Brush is a brush with long full hairs for the application of glazes and underglazes.
Glaze Butting is the term used to describe placement of two or more glazes in proximity on the same piece.
See Sgraffitoing.
The relationship between the thermal expansion of body and glaze. Ideally a glaze should have an expansion that is slightly lower than the body so that contraction during cooling puts the glaze under compression and thus prevents crazing.
Glaze trailing refers to the use of a fine-tip squeeze bottle to "trail" one glaze over another to create a design.
'Gloss' refers to how shiny and light-reflective a glaze is. Glazes high in glass former (SiO2, B2O3) are glossy. Those high in Al2O3 tend to be matte. Fluid glazes can crystallize to a matte surface if cooled slowly or a glossy surface if cooled quickly. The SiO2:Al2O3 ratio is taken as a general indicator of glaze gloss, ratios of more than 8:1 are likely to be glossy.
Graining is the process of creating a wood-grained effect using thinned, nonfired colors applied in long, uneven patterns.
A granular material made from crushed brick, refractory rock, or other pre-fired ceramic product. It is added to bodies to reduce drying and firing shrinkage and thermal expansion, increase stability during firing, and to add texture.
When the wet clay piece is removed from the casting mold and dried, the piece is now refered to as "greenware" (unfired clay). The piece is still very fragile at this stage and caution when handling should be exercised. Once cleaned, the piece can be underglazed and fired or directly fired to bisque.
A Greenware Saw is a small tool having a serrated edge for cutting dry greenware.
Grit Cloth is an abrasive cloth used for cleaning greenware or bisque.
A Grit Sponge is a square sponge that has an abrasive surface on one side.
Hard Bisque is ware that has been fired to witness cone 04 or hotter. (See Soft Bisque.)
Hard Spots or sometimes refered to as "hot spots", are areas that will reject color, and sometimes cause ware to have bare spots. Commonly caused by improper greenware casting. This happens when the chemicals in the slip collect against the mold wall during the pouring of slip into the mold. Excessive sponging or sanding while cleaning the greenware is another cause.
Holding the kiln temperature at a desired temperature. This is also refered to as "dwell" time or soaking.
Heatwork is the combined measure of time and temperature. Both time or speed of the firing along with the final temperature determine the amount of heatwork accomplished. Pyrometric cones deform at particular heatwork values.
High-Fire refers to ceramic articles or glazes that are fired to witness cone 4 or higher. Like stoneware and porcelain.
See Hard Spot.
Gypsum Cement. Hard and strong. Used in high quality art and novelty casting. Can't be worked with a template or formed by hand. Forms exceptional details. Very resistant to water absorption. 10,000 psi..
Immature bisque is ware that has been fired cooler than witness cone 06. Also known as "soft bisque", the process is known as a soft fire.
Incising refers to the technique of cutting a clay surface to create a design.
A process for forming pottery on a mechanized wheel. A profile describing the outside shape of the ware is used to force the soft clay against a rotating plaster mold describing the inside shape. Flat ware is commonly jiggered.
Kaki (persimmon) glazes are high fired reduction transparents with a generous supply of iron oxide (12% or more) which forms a microcrystalline surface. If cooled correctly they exhibit "red" coloration. The atmosphere during cooling should be reduction. The iron content of the body and level of reduction affect the fired results. Tenmoku glazes are closely related, however they have less iron oxide (around 10%) so that crystallization occurs on the edges of pieces.
A kiln is the furnace used to fire ceramic pieces. They come in various sizes and may run on electricity, natural gas, propane or even wood.
Kiln furniture consists of implements used to make full use of a kiln's capacity (shelves, posts and stilts).
Fume results when a clay or glaze decomposes during heat-up in the kiln and forms a vapor. These vapors can deposit as very tiny particles on walls and other surfaces. The individual particles are so small that they can be inhaled deep into the alveoli (air sacs) of your lungs.
The most common fumes associated with kiln firing are sulfur and it is easy to identify from the odor. Many clays contain it as do fuels. Some metal oxides volatilize and form fumes (notably copper, zinc, manganese) that can lead to metal fume fevers of different types. Manganese in particular can be quite toxic.
A Kiln sitter is a mechanical device that shuts the kiln off when a small pyrometric cone receives the proper amount of heatwork.
Kilns vents are hoods with an exhaust fan that are suspended above an electric kiln. They suck air from around the kiln and expel it through a hose to the outside. Some vents also suck air through a hole in the bottom of a kiln. Vents are intended to remove all harmful kiln fumes from the room.
Kiln vents also have other benefits. They help make for better temperature uniformity at low temperatures giving firings an even start. Below red heat the only way to get heat to move around in the kiln is to move the air (above red heat, the dominant way heat moves is by radiation). They also give better atmospheric uniformity. Continuously bringing in fresh air and expelling products of decomposition makes for more consistency from firing to firing.
A refractory powder that can be mixed with water and painted on kiln shelves to prevent ware and accidental glaze drips from sticking. Porcelain clays, for example, melt enough during firing that they tend to stick onto the kiln shelf. Certain clays contain soluble salts which fire to a glaze-like sheen, these also tend to stick ware to shelves. Kiln wash is refered to as "bat wash" in Europe
See crackle
glaze.
A Lace tool is a long, pointed tool used when applying thin strands of
Glazes are not as inert and stable as most people think. All are slightly
soluble and will thus leach into liquids they come into contact with. However
some glazes are dramatically more soluble than others and lead glazes have
gained this reputation. The subject of leaching and glaze safety is a complex
one and ceramic calculations and chemistry can be used to great benefit in this
area. Lead release in glazed ware is a critical thing to consider if you make ware
at low temperatures and employ lead based materials. The entire ceramic industry
has been forced to progressively move away from leaded glazes and colors over
the past decade. It is important to realize that using lead frits does not make
lead glazes safe, it just makes handling lead glazes safer. The solubility of
the fired glaze is dependent on its chemistry and the way it is fired. If you do
not know that formula for a frit you are using, check it out, do not assume it
is lead free. Lead release is normally tested by measuring how much of the glaze will
dissolve in a dilute acid mixture. Lead testing kits are available from www.axner.com
Partially dried clay ware. Leather hard pieces can still be trimmed, fettled
and are still flexible enough that pieces which have gone slightly out-of-round,
for example, can be squeezed carefully back into shape. This is the ideal time
to attach add-ons. Leatherware is a term used to describe leather-hard clay items that are damp
but firm enough to handle without losing their shape. A limit timer is a back up device for the kiln
sitter. The limit timer shuts the kiln off, using a timer motor, in the
event of a kiln sitter failure. A liner brush is a brush used for fine lines and design work. Loading refers to completely filling a brush with color. Loading also refers
to "filling" the kiln with ware. The amount of weight a material loses on firing. LOI is usually crystal bound
water or carbon material that burns away. Lusters are very thin coatings of metallic substances fired at comparatively
low temperatures onto an already fired glaze surface. They produce iridescent
effects. Low fired pottery employing a red-burning clay covered with a soft opaque
white glaze. Most majolica also has colored brushwork designs which are painted
over the dried glaze. The Majolica process is exacting and requires careful
technique and good technical understanding to make it successful. Metallic
colors are brightest at low temperatures and stiff-melt white glazes provide an
ideal canvas for them. Majolica Technique refers to applying underglazes in a design over an
unfired, nonmoving glaze. A term referring to the degree to which a clay or glaze has vitrified or
melted in the kiln. A 'mature' stoneware or porcelain clay is normally one that
is dense and strong, a 'mature' glaze flows well and heals imperfections to
provide a good covering. Like the term 'vitrification' mature needs to be taken
in context. A mature sintered refractory, for example is quite porous and would
be considered immature for other uses. Mocha diffusion is a technique of applying slips to ware so that one bleeds
or diffuses into another. Typically oxides are mixed with tobacco juice and
vinegar (e.g. apple cider works well) and a brush of the mix is touched to the
surface of a coat of WET and freshly applied slip (i.e. Universal white slip).
Modeling Clay is prepared clay used for hand modeling. A Mold is a hollow plaster-of-paris form in which articles are reproduced
using clay slip. (See Casting.) See Variegated. Nesting - Greenware that can be stacked only during a low fire bisque
firing. Nonmoving glazes are ceramic glazes that move or flow very little in the
glaze firing. Nontoxic is the term used to describe paint products conforming to U.S.
standard ASTM D-4236 to contain no materials in sufficient quantities to be
toxic or injurious to humans. Most glazes these days are lead free and
nontoxic. The practice of applying slip glazes to dried ware and firing one operation.
The once-fire process requires control of slip shrinkage, adherence, and melting
properties in order to avoid problems with crawling and blistering. Once fire is
popular in industry for everything from table ware to porcelain insulators. Do
not underestimate the difficulty of getting a once-fire process working well.
A glaze additive that transforms an otherwise transparent glaze into an
opaque one. Common opacifiers are tin oxide and zircon compounds. Opacifiers
typically work by not dissolving into the melt. Opaque refers to nontransparent color. More correctly 'Onglaze'. Decorative liquids applied over the fired glaze
surface. These include china paints, lusters, gold, and other metallics. Fired
at lower temperatures (e.g. cone 018). 'Overglaze' can also refer to the process of painting metallic oxides or
stain mixes over a raw glaze before firing. For example, this is done for
standard low bisque stoneware and for majolica.
Overglaze compatible is a glaze that will accept overglazes for a third firing.
Glazes that contain copper for added color are not overglaze compatible. A firing where the atmosphere inside the kiln has sufficient supplies of
oxygen to satisfy chemical reactions in the glaze and clay which use it.
Electric kilns are synonymous with oxidation firing, however they often have
"stagnant" air flow and thus may fire to a more neutral atmosphere
(direct-connected kiln vents improve this). An oxide is a molecule like K2O, Al2O3. They are the most basic form of
matter that kiln temperatures can normally decompose materials into. Thus for
calculation purposes we view fired glazes and ceramic materials as made of
oxides. An oxide is a combination of oxygen and another element (designated
"R"), there are only about 12-15 common oxides that we need to learn about. Each
has specific effects on a fired glass. Glaze formulas compare relative oxide
amounts. Oxides are divided into three categories that recognize their
functions. There is a correlation between the amount of oxygen in each class and
the contribution that class of oxide makes. Fluxes are designated RO,
intermediates R2O3 and glass formers RO2. A palette knife is a flexible knife with no sharp point used for mixing or
stirring color. Some casters also like to use them as a fettling knife. Clay particles are flat and are either randomly oriented or arranged in some
general pattern. The pugging process, for example, orients particles concentric
to the center of the clay slug. Likewise, throwing a vessel on the potter's
wheel lines up the particles. Rolling, casting, kneading operations affect
particle orientation. Particle orientation imposes an influence on a clay's
drying shrinkage (a piece will shrink more along one dimension than another) and
this needs to be considered when joining pieces to form objects or cracks will
result. One way reveal particle orientation is to freeze an undried. This should
reveal the stresses that result from differences in orientation. Pyrometric Cone Equivalent A measure of how refractory a material is. The measure is done by making a
small cone from the material and firing it till it bends. A typical stoneware
clay body, for example, might have a PCE of cone 20. A 'phase' of a material is a physically different molecular or crystal
structure induced by a set of conditions (i.e. temperature, pressure). Phases of
silica, for example, are chemically the same but have different physical
properties. If significant differences are imposed a phase will have its own
name (i.e. diamond, graphite are phases of carbon). If differences are not
significant an alteration of the same mineral name is used (i.e. alpha quartz,
beta quartz). It is important to realize that a phase exists as a recognition of
a physical change, not a chemical one. These changes are measurable by
instruments such as a microscope or dilatometer. 'Cristobalite' is a phase of
silica and has very different properties than quartz, however they are
chemically the same. The former can be created by submitting quartz to a high
temperature and holding it there. Catalysts encourage chemical reaction thus they would not be associated with
phase changes. However the term 'catalyst' is used to refer to conditional
changes that effect phase changes. Peepholes are openings in the side of kilns for ventilation and visual
inspection of the kiln during firing. Pinholes are tiny holes penetrating a glazed surface. Plasticity refers to the pliability of modeling clay.
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. A clay body which, when fired, becomes very mature and usually translucent.
Porcelain is normally quite white and fires to a very smooth pleasant surface.
Porcelain clays lack iron impurities and are ground to very fine particle sizes.
Plastic porcelain clays tend to be shorter than their stoneware or earthenware
counterparts. Porcelain casting slips achieve the whitest and most translucent
results. Porosity refers to the permeability of fired or unfired clay. The pore space
within a fired body can be measured by weighing a specimen, boiling it in water,
weighing it again, and calculating the increase in weight. Posts are columns of refactory material used to support shelves inside the
kiln. (See Kiln
Furniture.) Pouncing is a technique in which you apply color with quick up-and-down
movements with a brush or sponge. Pouring is the process of filling a plaster mold with casting slip (liquid
clay) to create a clay object. Once the plaster mold is removed, the clay object
is known as greenware
or unfired clay. (See Casting.)
Usually refers to a process of firing clay ware under primitive conditions,
usually in a fire or a fire pit. It requires a clay that will handle thermal
shock well (normally well-grogged). If you burnish your pots you will need to
consider whether the grog will mar the finish so it might be better to slip the
ware and burnish that. One challenge is generating enough heat to sinter the pots well. In a typical
open wood fire it is difficult to achieve temperatures more than a few hundred
degrees above red heat. Use of sawdust, hard wood, and various schemes to
contain the heat are all common. Firings may double as a social occasion and
take only a few minutes or they may be quite elaborate insulated hole-in-the
ground affairs that span several days. Books are available on sawdust and primitive firing. Propane fired gas commercial and home-built kilns are quite common. However
this process seems to generate more questions than any other, especially on the
subject of propane tanks. If the propane tank is not large enough, for example,
it will freeze up and be unable to supply the necessary fuel. People often
underestimate the number of tanks needed for a firing and the rate at which a
given tank can supply gas. The Clayart discussion group on the Internet has a lot of knowledgable people
in this area. A pyrometer is an instrument that indicates temperature in the kiln. See Cone. The raku process is an economical way of firing ware in reduction to achieve
metallic and carbon effects. Normally ware is heated in a kiln until the glaze
is melted to the desired degree then it is removed with tongs and put into a
container of organic material (i.e. sawdust) where it is cooled. The organic
material burns and uses the available oxygen in the container and the metallic
effects develop. Because of the heat shock during heat-up most people bisque
fire ware before rakuing. Raku ware is usually crazed and very porous and lacking in strength. Thus it
is only suitable for decorative ware. Metallic effects that looked great out of
the firing can tarnish and disappear with time and people have developed ways to
preserve these with various fixatives and surface treatments. Raku firings often generate a lot of smoke, an people have developed
different ways to try to contain this smoke. Unfortunately some have developed
and even promoted quite casual methods that pose significant dangers. Raku kilns
can emit harmful metal fumes (depending on how pots are decorated) and vapors of
chlorine and sulfur (from salts, chlorides, and sulphates). Some people have
been seriously injured in this regard. Inhalation of Chlorine can cause chemical pneumonia, immune reactions,
rashes, irritated mucous membranes, diarrhea, nausea, vomiting, cancer, brain
damage, etc. Smoke from raku is hazardous simply because ALL smoke is hazardous
and possibly carcinogenic. Incomplete combustion ("yellow" smoke) produces the
most hazardous substances in smoke (i.e. wood tar and thousands of other
compounds). While some people claim to wear respirators with "smoke blocking
filters" there is no such thing. This is why firefighters wear tanks of
compressed air. A kiln atmosphere which is deficient in free oxygen. This condition is
accomplished in gas kilns by increasing back-pressure or reducing the amount of
primary air available to each burner. The result is an increase in gases like
carbon, hydrogen and CO. These are very aggressive in wanting to combine with
oxygen. Hydrogen is small and particularly oxygen-hungry and can thus steal it
from within bodies and glazes. Reduction firing produces different colors and
visual effects because metallic oxides willing to give up oxygen convert to
their reduced or more metallic form. Good examples are copper which burns red
(it fires green in oxidation) and iron which becomes a powerful flux and
produces earthtone browns (it is refractory in oxidation). Because almost all
natural clays contain iron, reduction firing normally gives completely different
clay surface effects than oxidation. Many people do a period of oxidation at the end of a reduction firing to
clean the atmosphere and soak the glaze to heal bubbles that result from the
active volatilization (an accompanying bubble formation and surface disruption)
that reduction induces. In many cases color breaks in glazes are a result of
localized reoxidation of the melt surface. The effect depends on glaze thickness
and evenness of coverage. Tenmoku glazes are an example of this, the brown
thinner areas are oxidized. It can be a challenge to reproduce the same effects in firing after firing
using the reduction process. Many people have developed great skills in this
area. However the oxygen probe is promising to revolutionize reduction firing,
especially for small scale industry and hobby operations. It provides a direct
measurement of the amount of reduction and enables one to more easily maintain
the critical balance between oxidation and incomplete combustion. While these
devices are quite expensive there are very few people employing this process
that are not at least planning to get one. Reduction firings are not without hazard. Any form of incomplete combustion
can generate smoke and deadly gases. CO for example, is deadly and is colorless
and odorless. It is important that gas kilns be vented well and if possible that
a CO alarm be installed. The ability of a material to withstand heat without deforming or melting.
Kiln shelves and firebrick are refractory. Many clays and minerals are also
refractory. A fireclay with a PCE of 35 is said to be a super duty fireclay. A rolling consistency refers to the consistency to which glazes are thinned
for rolling inside ware. Rolling glaze is a method of covering the inside area of ware by rolling
thinned glaze inside. Running refers to the fluidity of a glaze at the point of maturity before
cooling and hardening. Salt firing is a process where unglazed ware is fired to high temperatures
and salt fumes are introduced into the kiln chamber (normally by a spray in the
burner ports). The sodium in the salt combines with the silica and alumina in
the clay to form a glaze. Salt glazed ware often has marbled and variegated
surface effects and has a very distinctive look. Salt glazed ware is suitable
for functional use. Sodium vapor glazing using compounds other than sodium chloride (table salt)
is practiced by many people. Many books are available on this topic and an
increasing number of web pages on the Internet extol the process. There has been quite a bit of discussion about the safety and environmental
concerns of salt vs. soda firing. It does not appear to be a foregone conclusion
that soda is better than salt or even that chlorine is released in salt firings
(rather than HCL vapor). Recent reports on the Internet claim that measurements
done on kilns have demonstrated that salt firing is as clean or cleaner than
fuel reduction firing. Score refers to the process of scratching tiny criss-cross lines on areas of
greenware that will be fastened together (add-ons) with clay slip or an
attachment mix like Magic Mender, Patch-A-Tatch, or Quick-Fix to name a few. The Scrub-coat is a priming coat of thinned opaque underglaze or glaze. Sealers are spray or brush-on coatings for use over nonfired colors to
protect the surface and enhance the colors. A Seam is a ridge formed in greenware where mold pieces join. Sgraffitoing refers to the process of creating a design in ware by gently
scratching through applied color to reveal the color or the clay body beneath it
or to create carved designs. This is also known as "glaze carving". Shelf Supports are columns of refactory material used to support shelves
inside the kiln. (See Kiln
Furniture.) Shelves are flat slabs of special high-temperature materials on which ware is
placed inside kilns. (See Kiln
Furniture.) The common reduction fired Shino glazes in North America are mostly gloss or
semigloss, fat, white crackle glazes with some orange to red and dark red from
iron in the clay body or iron slips under the glaze. Shinos were born in Japan
as almost pure high fired feldspar glazes. Shino on porcelain requires
underglaze iron bearing slips, or in-glaze iron (i.e. from an iron-containing
kaolin). Shinos are fluxed with soda spars and nepheline syenite and do not
contain calcium because it dissolves the iron and inhibits the red color. Shinos
usually do not have added silica, the silica being contributed by the feldspars
and the clays in the glaze. Some Shinos contain Spodumene, which may be added to
balance the high expansion soda spars. A simple Shino recipe is 70% Nepheline
Syenite and 30% Kaolin. Shinos crawl, craze, and pinhole in ways that are often decorative and
pleasing. Carbon trapping is common with Shinos. The trapped carbon leaves
random dark, shadowy areas in the glaze. A defect in glazed ware. It is the opposite of crazing.
Shivering occurs when the glaze or underglaze and the clay body are
incompatible. The clay shrinks more than the color, causing the color to peel or
break away. Shrinkage is the reduction in size of a clay object as a result of drying and
firing. Sieves are usually made from bronze or stainless steel wires. They are
available in varying degrees of fineness and sizes are quoted according to the
size of the opening or in wires per inch. An 40-60 mesh sieve is normally
required to screen glazes to make sure they have no coarse particles that could
disrupt the fired surface. In order for a porcelain to fire speck-free it would
normally need to pass a 200 mesh ( about 75 micron opening) or finer screen.
Silica ( Flint ) - Material used in making kiln wash. Silica sand is used for
propping porcelain. Screen printing is a technique to reproduce multicolor designs on tiles and
flat surfaces. It is used in the sign painting industry and it is easy to find
books on the process. Ceramic inks are available from suppliers or can be mixed
from stain pigments and oil or glycerin bases. Screens may be printed "on contact" or "off contact" The latter gives a
sharper image, the screen is held off from the printed surface by a small
distance (1/8" to 1/4") and as the squeegee is pulled, the screen is stretched
down to make contact with the printed surface. After the squeegee passes the
screen snaps back up. Improvisation is often to key to this process and
incredible results are possible. A silk sponge is used for decorating. It has short hairs on its surface and
is soft when wet. Sintered clay has been fired high enough so that it no longer will slake or
break down when exposed to water. Bisque fired ware is sintered. However the
term sintering refers more to the particle bonding mechanism where particles are
not glued together by the melting of a flux. Rather adjacent particles bond by
the migration of species across the connection and by the deposition and buildup
of material that has become gaseous in the kiln atmosphere. Refractories are
often sintered to considerable strength. Sintered alumina bodies are very porous
yet they can have a 'ring' like that of a fine porcelain. 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. A slip or slurry is a suspension of clay and mineral particles in a water
medium. It is typically either:
Slip trailing is the process of applying slip in an applicator bottle to flow
on design for a raised effect. Smoking refers to the greying or discoloration of a glaze, caused by
underfiring. The practice of holding the kiln at final firing temperature for a period of
time. This is usually done to mature the clay and give the glaze opportunity to
flow and heal imperfections. The advent of electronic kiln controllers has made
it possible for anyone to soak. Soaking is especially advantageous for glazes
with a stiff melt (i.e. low temperature zirconia whites) and for porcelains that
require translucency, density, and glassy surfaces. Soft Bisque is ware that has been fired to witness cone 06 or cooler. See Immature
Bisque. A high fired ceramic that is vitreous or semi-vitreous, not translucent, and
often made of clays that are not highly refined. Stonewares can be brown, buff
or white. Stonewares commonly have some speck and some particulate material such
as sand or fine grog. The spare is the excess clay left behind from the casting process on the
ware. The spare is removed by fettling. A comparison of the weights of equal volumes of a given liquid and water. A
ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than
water. The best way to measure specific gravity is to weigh a container and
record its weight, then weigh the container full of water and then full of the
liquid of unknown specific gravity. Subtract the weight of the container from
each weight and divide the weight of the liquid being measured by the weight of
the water. Solvent is a dissolving agent used in antiquing and to clean brushes used
with oil-based colors. Spattering is a method of applying small flecks of color to ware, usually
with a bristle brush. Sponging refers to the use of a sponge to apply colors directly to the
surface of a piece. Staggering refers to the process of applying separate successive coats of
glaze by fractions of an inch to prevent glazes from flowing together. Stencil refers to the process of using paper or plastic perforated with a
design through which color can be brushed or sponged onto a surface. Stilts are supports used to separate a glazed article from a shelf during
firing. (See Kiln
Furniture.) Stippling is a method of applying color by pouncing the tip of a brush loaded
with color against the ware. A reduction fired transparent glaze with about 10% iron oxide that fires to a
variegated deep maroon to black and which breaks to iron-red crystallized areas
where thin. Kaki glazes are closely related, however they contain more iron
oxide so that crystallization occurs over the whole surface. Terra Cotta is red burning earthenware.
Red clays have more flux impurities and fire to a harder stronger matrix than
white burning materials at the same temperature. Still, terra cotta bodies fire
to a porous matrix at cone 06-04 and do not have anywhere near the mechanical
strength of vitrified stoneware bodies. Without significant additions of
expensive frits it is impossible to vitrify a body at these temperatures.
However many terra cotta clays do develop rapidly after cone 04 and turn from
red to brown in the process. It is possible to produce fired ware that rivals
stoneware in strength at cone 02-1, however few people do this because the clay
is so volatile, slight overfiring will produce warping or bloating. Besides
cost, one of the primary advantages of the terra cotta process in the warm red
colors of the raw clay surface. In addition glazed low fired terra cotta remains
red whereas at higher temperatures the glaze matures the surface and turns it
brown. Because terra cotta ware is weak and porous it is very important that the
glaze and body thermal expansions match. The clay-glaze interface is not well
developed (the glaze is not stuck on as well as stoneware) so a measure of
resistance to chipping and crazing can only be achieved by a well melted glaze
of low enough thermal expansion to resist crazing. In the past inexpensive lead
compounds were used on terra cotta because they contributed exactly these
properties plus they gave very bright and vibrant colors. Today boron glazes are
employed. While safer to use they do not have the ideal set of properties that
lead based compounds had. 'Majolica' refers to the use of a terra cotta clay with an opaque white glaze
decorated with colored overglazes. Today red clays are used in this process
because they provide maximum strength at low fire. In the past white low fire
materials were not available. Usually caused by removing the ware from the kiln to quickly. Thermal shock
occures when stresses imposed on a ceramic piece by the volume changes
associated with sudden shifts in temperature. Ceramic materials with good
thermal shock resistance are able to withstand sudden temperature changes
without cracking. Cracking usually occurs when one part of an item is a
different temperature than another part and therefore expanding or contracting
at a different rate. Fired ceramic does not withstand thermal shock nearly as well as other
materials like steel, plastic, wood, etc. Ceramic is hard and resistant to
abrasion but it is brittle and propagates cracks much more readily. A simple probe made from two kinds of wire (i.e. platinum, rhodium) welded
together. This probe is wired to a sensitive electronic meter that displays a
reading of the voltage it generates when heated. However the world of high temperature measurement and thermocouples is a
complex one. There are many kinds of thermocouples. Some generate a nice smooth
voltage increase that bears a direct relationship to temperature increase,
others require complex software to make the translation. There are also
different manufacturing processes, calibration techniques, response to different
atmospheres, abilities to measure different temperature ranges, different types
decay in their accuracy in different ways, variations in frequency of need for
recalibration, etc. Maintaining accurate pyrometers can be expensive and typical inexpensive type
K devices used in potters kilns are not accurate at higher temperatures (most
potters won't pay for the platinum/ 10% platinum-rhodium (type S) thermocouples
and control systems that really should be used, and the more expensive plated
switches and contacts). However the type K are more resistant to oxidation than
types E, J, and T at temperatures over 500C. Thixotropy refers to the way a slurry's viscosity changes with time and
motion. A good casting slip is the product of maintaining the specific gravity,
viscosity and thixotropy. It should have the required specific gravity, be tuned
to the needed viscosity yet gel after a set time to prevent sedimentation. Plastic clay is sometimes called thixotropic. This usually refers to material
that is very elastic, can be pulled and twisted like taffy, and does not set
until left still for a time. Tint is the process of lightly applying diluted colors over a base coat or
coloring a product with another product. Tipping involves touching tip of loaded brush with other colors for muted
shading or accenting. Translucent refers to transparent color, allowing color underneath to
show. An underglaze is a ceramic color used under a glaze. Oxide color mixtures
which are applied to bisque or greenware and over which a transparent glaze is
applied. To prevent excessive feathering of edges, underglazes are usually a mix
of metallic oxides and a fritted stable glaze. A unity formula is just a formula
that has been retotalled so that the RO group of
oxides total one (unity). This is also called a Seger formula and this standard
provides the basis for comparing glazes. Utility items are functional, rather than purely decorative items. Examples
of utility items would be plates, vases, pitchers, bowls and planters. Variegated or mottled glazes are those that do not have a homogeneous solid
color or character (i.e. like a ceramic sink or toilet bowl). Variations in
color and texture are highly prized by many ceramists. A variety of mechanisms
are used to create the variegation. These include crystal growth, addition of
speckling agents, phase separation, layering, and thickness variation of
translucent glazes. Venting refers to allowing moisture and gases to escape from the kiln during
firing. Viscosity refers to the rate of resistance to flow. 'Vitrification' is a process. As clay is fired hotter and hotter, it reaches
a point where, if cooled, it will produce ware of sufficient density and
strength as to be useful for the intended purpose. The intended purpose may well
require some porosity to gain another more important advantage (i.e. stability
in the kiln, resistance to blistering). However 'vitreous' ware is usually
functional, water proof, sanitary, hard, and strong. Ware that has fired dense
and strong is said to be mature.
Wedging clay is similar to kneading bread dough. Clay tends to set up over
time and the process of wedging it loosens it up. It is not uncommon for the
clay to soften dramatically on wedging, this is thought to occur because of of
mobilization of water between the flat particles of clay and the disruption of a
stable electrical charge pattern between water and clay that develops over
time. Wedging also performs the function of lining up the flat clay particles
concentric to the center of the mass allowing them to slip over each other more
easily in that direction. 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).
Lace Tool
Leaching
Lead Solubility, Lead Release in Glazes
Leather Hard
Leatherware
Limit Timer
Liner Brush
Loading
Loi
Lusters
Majolica
Majolica Technique
Mature
Mocha Glazes
Modeling Clay
Mold
Mottled
Nesting
Nonmoving Glazes
Nontoxic
Once-Fire, Once Fired, One Fire
Opacifier
Opaque
Overglaze
Overglaze Compatible
Oxidation
Oxide
Palette Knife
Particle Orientation
PCE
Phase, Phase Changes
Peepholes
Pinholes
Plasticity
Pinch Pots
Porcelain
Porosity
Posts
Pouncing
Pouring
Primitive Firing, Pit Firing, Sawdust
Firing
Propane
Pyrometer
Pyometric Cone, Cone
Raku
Reduction, Reducing Atmosphere
Refactory
Rolling Consistency
Rolling Glaze
Running
Salt, Soda Firing
Score
Scrub-Coat
Sealers
Seam
Sgraffitoing
Shelf Supports
Shelves
Shino
Shivering, Peeling
Shrinkage
Sieve, Screen
Silica
Silk Screen Printing
Silk Sponge
Sinter, Sintering
Slab Built
Slip, Slurry, Suspension
Slip Trailing
Smoking
Soaking
Soft Bisque
Soft Fire
Stoneware
Spare
Specific Gravity
Solvent
Spattering
Sponging
Staggering
Stencil
Stilts
Stippling
Tenmoku
Terra Cotta
Thermal Shock
Thermocouple, Pyrometer
Thixotropy
Tint
Tipping
Translucent
Underglaze
Unity Formula
Utility Items
Variegation
Venting
Viscosity
Vitreous, Vitrification
Wedging, Kneading
Wheel Thrown
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