Abrasion Ceramics 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.
Acidic Various oxides can be considered basic (RO, R2O group) or acidic (RO2). The greater the proportion of oxygen the more acidic an oxide is.
Amorphous 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.
Analysis 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.
Apparent porosity The relation between the volume of a mass and the volume or water absorbed when the mass is immersed.
Basic See Acidic.
Bat 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.
Bisque, bisquit The practice of prefiring ware without glaze to make it impervious to water, easy to handle, or vitrify it. Glaze is then applied and it is fired again. 'Low' bisque firing is typical for pottery and ceramics while vitrified bisque is done for bone china and some types of stoneware. Low 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' bisque firing 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 Black coring usually occurs during a reduction firing and is a result of fast firing and/or lack of oxygen in the kiln between 700 and 900C (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.
Blunging, mixing Mechanical mixing of a clay slurry. Blungers 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 require the application of considerable amounts of energy to achieve a slip in which water has penetrated well between all particles.
Body, clay body 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 stoneware, buff ceramicclay '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.
Candling The practice of slow firing ware 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 ware 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.
Carbon trap glazes 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.
Casting, slip casting Forming pottery by pouring deflocculated (water reduced) clay slurry 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. The Magic of Fire book has a chapter on understanding casting slips.
Ceramic Any man-made solid produced by the fusion of mineral substances in a kiln.
The term 'ceramics' has come to signify the slip casting industry that uses talc-ball clay slurries to cast ware 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.
Coatings, ceramic coatings,
rfc 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.
Coe, co-efficient of thermal
expansion 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 Making pottery by laying down layers of coils and working them together into an even wall.
Colloid 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.
Cone 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 are placed in a kiln so they can be viewed during firing and when a cone begins to bend it is closely monitored and the firing is terminated when it reaches a specific position.
Cone plaque A stand or rest for hold cones during firing. Plaques are important to assume that cones are placed at a consistent depth and angle firing after firing.
Controller 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.
Crackle glaze, craquele 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.
Crank, plate setter 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.
Crawling 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) in the Magic of Fire book.
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.
Crazing 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.
See the Magic of Fire book for many chapters on dealing with crazing. Ceramic calculations are very useful in dealing with crazing and the INSIGHT manual has an example of dealing with a specific problem.
Crystalline glazes 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 must to make results repeatable.
For more information see also:
The Studio Potter, Dec '96, Vol #25, #1, pg. 39-49
Ceramics Technical, Nov '96, #3"
Glazes for Special Effects, book by Herbert Sanders (1974)
Ceramic Review #137, Sept-Oct 1992, pg. 27-31
An article was advertised in the back of C.M. that sells for around $12.00 U.S. The potters name is Dan Turnidge, River Rock Clay Co., P.O. Box 3183, Salem, Oregon 97302 Phone (503) 581-3606
There are also web pages on the Internet dedicated to the production of crystal glazes. The can be found through the search engines.
Decal 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.
Deflocculate,
deflocculation 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 recipes 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 rheology.
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.
The Magic of Fire book has a chapter dedicated to understanding deflocculation and how to make slips work properly. It also deals with the basics of formulating a good powder mix for slips.
Devitrification 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 '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.
Dunting 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.
Information on understanding and preventing this problem can be found in the Magic of Fire.
Earthenware 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.
Encapsulated stains Certain coloring oxides are toxic and stain companies have found a way to embed particles of stain into transparent zirconia frit particles. The zirconia melts at temperatures higher than typical glazes so the effect is "encapsulation", even though no true capsule exists. These stains cannot be ball milled (for obvious reasons) and there is some need for caution in high temperature highly fluxes glazes. The best example of encapsulation is Cadmium red stains. While good red colors can be achieved it is expensive and a much higher proportion of stain is needed to get a bright color, up to 25%.
Engobe 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.
Eutectic 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 327C, tin at 231C. The lowest melting combination is 67 lead, 33 tin (180C).
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.
Fettling The removal of the seam left by the mold in slip casting using a fettling knife. The seams in then finished with a sponge.
Firebrick 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.
Flameware Flameware is pottery that must withstand severe temperature changes without cracking (i.e. stove top burners). Ceramic is much more susceptible to thermal shock failure than most other materials because of its brittle nature, lack of elasticity and tendency to propagate cracks. Ovenware is not nearly as resistant to thermal shock as flameware.
Flameware bodies are typically made from as high a proportion as possible of low expansion minerals like spodumene, kyanite, mullite, and pyrophyllite. Theoretically 100% of these minerals is the ideal but for practical purposes clay and other flux content is also needed.
Flameware cannot normally be glazed because it is not possible to make a glaze of low enough expansion not to craze.
Flocculate, flocculation 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.
Flux 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.
Formula 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.
Frit 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.
Glass 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.."
See "Ceramics Glaze Technology" for more information.
Glaze 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) to designate type within a specific industry or type of ceramic ware.
Glaze fit, glaze body fit 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.
Glossy '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.
Grog 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.
Jiggering 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 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.
Kiln fumes 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.
Kiln venting system 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.
Kiln wash 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.
Leaching, glaze solubility 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. The Magic of Fire book has much information on this subject.
Lead solubility, lead release
in glazes 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.
Testing kits are available for $25 and come with a neutralizer (sodium hydrate) and an indicator (sodium sulfuret). Both have warnings about flushing skin or eyes thoroughly with water if splashed on and drinking 3 or 4 glasses of water or milk if ingested (do not induce vomiting).
One set of instructions, for example, says to wash the object, immerse in white distilled vinegar (5% acetic acid - any vinegar will do) for 18 hours (or longer), stir the vinegar, fill the supplied test tube to the etched line (about 2 cc), add 7 drops of neutralizer, invert to mix. After 2 minutes, add 7 drops of indicator and invert to mix. After 2 minutes a clear or milky solution is OK, any shade of brown indicates lead according the comparison chart provided.
Lab testing can be done at Elemental Research Lab, 309-267 West Esplanade, North Vancouver, B.C. Canada V7M 1A5 604-985-0445 (they test many metals). Also at Kirby Health Center Lab, 71 North Franklin Street, Wilkes-Barre, PA 18701 717-822-4278.
Leather hard Partially dried clayware. 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.
Loi The amount of weight a material loses on firing. LOI is usually crystal bound water or carbon material that burns away.
Lustre Colors Common materials used in lustres are stannous chloride, barium chloride, barium chloride, sodium chloride, bismuth subnitrate, silver, gold, and platinum. We do not provide a definition here however you can look for these books:
Ceramic Colours and Pottery Decoration by Kenneth Shaw, published by Maclaren and Sons Ltd., London, 1962, reissued 1968.
Lustres by Margery Clinton, published by BT Basford Ltd. London, 1991. This book covers how to make and use lustres.
Lustres, lusters Lustres are very thin coatings of metallic substances fired at comparatively low temperatures onto an already fired glaze surface. They produce iridescent effects.
Majolica, tin glaze
earthenware 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.
The Magic of Fire book has more information on the Majolica process.
Mature 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 glazes 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).
Mottled See Variegated.
Once-fire, once fired 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.
Opacifier 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.
Ovenware Ovenware (and flameware) clay bodies have a lower thermal expansion so they can withstand more sudden changes in temperature. Ovenware bodies should thus have much lower free quartz content and employ low expansion minerals like mullite, pyrophyllite, petalite, kyanite and spodumene. While many potters make ware for use in the oven using standard clay bodies, ovenware manufacturers (like Corning) would object to calling this 'ovenware'. This is because they dedicate considerable resources to producing bodies and glazes that have a much lower thermal expansion and therefore are much more suitable. Potters are able to get away with using standard bodies and glazes by making sure the glaze fits well (no crazing), avoiding high-quartz and highly vitreous bodies, firing evenly to reduce built-in stresses, maintaining an even cross section, avoiding angular contours and larger sizes with broad flat bottoms and telling customers to be careful about subjecting ware to sudden temperature change.
Glaze fit is a major problem in designing an ovenware body since common glazes will craze. It is much easier to make a low expansion clay body than a glaze, thus it is normal to compromise the lowest possible expansion on the body in order to get a reliable glaze fit. Low expansion glazes typically employ lithia and high silica and alumina and avoid sodium and potassium. It is much easier to make a low expansion glaze at high temperatures where silica and alumina can be higher.
There are two mechanisms to creating a low expansion body: By firing to form crystalline minerals that have low expansion or by employing mineral particles and fluid glasses in the body recipe have low expansion. The former produces a more vitreous body and requires much more expertise and test equipment. The later is a bit of a 'crowbar' approach and is dependent on not firing to full maturity (otherwise mineral species can be dissolved by the feldspar in the body and the low expansion effect lost). The latter creates a bit of a 'tug-of-war' in the body since some particles (like quartz) want to expand on heating and contract on cooling whereas others (like kyanite, mullite) want to remain stable. Furthermore, the glass that glues all the particles together introduces a third expansion dynamic to the matrix.
Overglaze 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.
Oxidation 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).
Oxide 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.
Particle orientation 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.
PCE 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.
Phase, phase changes 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.
Porcelain 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 The pore space within a fired body. This is measured by weighing a specimen, boiling it in water, weighing it again, and calculating the increase in weight.
Primitive firing, pit firing,
sawdust firing 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 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 knowledge people in this area.
Raku 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.
Refractory 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.
Reduction, Reducing Atmosphere
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.
Salt, soda firing 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.
Shino 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.
People who use Shino glazes then to be philosophers!
Shivering, peeling A defect in glazed ware. It is the opposite of crazing. Our Magic of Fire book has many chapters on adjusting the thermal expansion of glazes. Also check the Education area of this site.
Sieve, screen, lawn 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.
Silk screen printing 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.
Sinter, sintering 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.
Slip, slurry, suspension A slip or slurry is a suspension of clay and mineral particles in a water medium. It is typically either:
A glaze consisting primarily of clay ingredients. It is applied to once-fire ware. Slip glazes can be glossy or matte and any color or texture.
A clay slurry poured into molds to be cast into shapes. The slip is usually deflocculated to minimize water content and fine tune viscosity. The deflocculation process involves using special chemicals that enable you to create a fluid clay-water slurry with a very low water content.
Soaking 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.
Specific gravity 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.
Stoneware 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.
Tenmoku 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 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.
Thermal shock Stresses imposed on a ceramic 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.
Thermocouple, pyrometer 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 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.
Underglaze Oxide color mixtures which are applied to bisque or green ware 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.
Unity formula 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.
Variegation 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. Click here to read more.
Vitreous, vitrification '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, kneading 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.
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