BY Will Levi Marshall
Comments to will.phoebe@btinternet.com

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Alkaline glazes: The alkaline fluxes are Lithia (Li20) , Soda (Na2O) and Potash (K2O) . When alkali's are the predominate glaze flux a characteristically bright transparent and runny glaze results. The colour response from the alkaline glazes is excellent although small additions of Barium help. High thermal expansion of the alkali's makes crazing almost inevitable. Alumina: Al2O3 A refractory . Alumina Hydrate is often used to coat kiln bats stopping glazes running and adhering pots to the bats. As a bat wash small additions of a plasticizer such as bentonite or china clay allow easy application. Ash: Often used as a glaze ingredient. Ash is usually high in potash and calcium, although different plants produce ashes with different characteristics. As a rule of thumb the faster growing the plant the higher the Silica content. Rice husk ash is nearly pure silica. A classic ash glaze will be green and runny although this is not always the case.

Ball clay: A 'secondary' Clay with a fine particle size. Used as a large part of throwing clay bodies to create plasticity, however too high a proportion of ball clay can cause excessive wet to dry shrinkage. Barium carbonate: BaCo3 A flux which is unusual as it enables the potter to make bright colourful matte glazes. If ingested it is highly poisonous and its use on eating or serving surfaces is not recommended. Bentonite: A clay like mineral formed from volcanic ash. Properly termed Mountmorillonite, it is extremely plastic and 1-2% is often used to suspend particles of glaze in water for ease of application. Similar additions of bentonite also helps increase the plasticity of clays. Bisque or Biscuit: The first firing a pot undergoes to prepare it for glazing. In industry this is done at a higher temperature than the subsequent glaze firing, the reverse is usually true of the studio potter. Burnishing: A technique where the Leather Hard clay is polished with a hard instrument to force the smallest clay particles to the surface creating a soft sheen. This surface remains after the pot is fired so long as the firing temperature is kept below 1100oC.

Celadon: A felspathic Reduction glaze which originated from China and was perfected in the Song Dynasty. The glaze, coloured with small percentages of iron, goes from jade green to subtle greys and occasionally blue. High Soda base glazes encourage apple greens. High Potash glazes moves the colour towards blue, blue greys although for a really good blue there must be no titanium (often traces are present in clays). High calcia moves the colour towards greys/greens. Clay: AL2O3 2SiO2 2H2O. The decomposition of Granite through the process of Kaolinization creates clay. Clay is a mineral with a plate (platelet) like structure; it is these plates, (about 0.5 microns across) when lubricated with water, slide against each other to form the plastic mass we know as clay. 'Primary ' clays are those found close to the area of Kaolinization and hence the purest (China Clays). Secondary clays are those moved by water away form the site of Kaolinization and get progressively more plastic and less pure (Ball Clays, fire clays, Earthenware's and marls). Cobalt: One of the strongest colouring oxides used by the potter. Cobalt creates a dark dense royal blue in most cases. Historically used in China as a painting pigment on Blue and White wares. Cones: Pyrometric cones are composed of glaze material and designed to melt and bend at specific temperatures. By observing them through a small 'Peep Hole' in the kiln it is possible to ascertain the exact conditions in the kiln. Cones are a better indicator than temperature alone as the degree of glaze melt is a combination of time and temperature ('heat work done'), thus a fast firing needs to go to a higher temperature to get the same results as a slow firing to a lower temperature. Crystalline glazes: Most glazes have no easily visible crystal structure (see Glaze). Crystalline glazes have large and dramatic crystals upto about three inches across. A high Alkaline low alummina glaze is vital for crystals to develop. Additions of Zinc and titanium also help seed the crystals. An extremely slow cool of the kiln is necessary, to allow the crystals to grow. Because of the low allumina content in crystalline glazes they are very runny, often pots are supported in the kiln on stilts to avoid them adhering to the kiln shelves, the stilts can be broken off after the firing.

Delft: Tin glazed Earthenware with elaborate painted patterns. The technique is often described as Majolica. The term Delft derives from the area in the Netherlands where the work was produced in the 16th century. Various colours were used to decorate on the creamy opaque glaze but the Cobalt blue is most characteristic. Dolomite Matte: CaO3 MgCO3. If large amounts of dolomite are used in a glaze (15-20% ) the high magnesium content usually creates buttery satin/matte opaque glazes which give pastel colours. One exception is Cobalt which can give purple in a high MgO glaze. Dunting: Cracks which occur on a pot being fired either on the heating cycle or the cooling cycle. They are usually caused by the Silica inversion at 573oC (Alpha to Beta phase) or the Crystobalite (one of the 'phases' of silica) inversion at 226oC, in both cases there is an expansion and contraction of around 2-3% in the heating and cooling cycles.

Earthenware: A low fired form of pottery (below 1100oC) . The clay can be any colour although iron red is usually associated with Earthenware. The low temperature vastly expands the range of glaze colours available which are often Alkaline or Lead based (Also see Clay and Slipware). Enamel: A form of low temperature glaze that is applied on top of an already fired higher temperature glaze. Enamels are often Lead based as it is a flux which works at a low temperature.

Faience: See Delft. Feldspar: One of the predominat naturally occuring fluxes used primarily in Stoneware glazes. The three most commonly used feldspars are Potash feldspsars K2O Al2O3 6SiO2, Soda Feldspars Na2O Al2O3 6SiO2 and Lithium Feldsapars Li2O Al2O3 8SiO2. Feldspars can be the only flux present in a stoneware glaze although this is uncommon and additions of calcium usually supplement it. Firing: The process which changes clay into ceramic. Up to 600oC the chemically bonded water in Clay is driven off (AL2O3 2SiO2 2H2O - AL2O3 2SiO2). This is an irreversible change.

Glaze: A super cooled liquid, with a random molecular structure and high viscosity at normal temperatures, super cooling is relative to geological cooling. A random molecular structure is the result of fast cooling so crystals cannot develop (the exception being Crystalline glazes). Granite cools slowly (geologically speaking) so we can easily see the crystals in polished granite, glaze cools quickly so the molecules do not have a chance to crystallise. A high viscosity means it does not run off the pot. (well it may in the kiln but not in the kitchen!)

Inlay: A decorative technique where a pattern is carved into the clay at the Leather Hard stage and a contrastingly coloured soft clay is forced into the decoration. When the clay is a little drier the excess is scraped of to reveal the pattern. Iron Oxide: Fe2O3 is one of the potters favourite colourants, when combined with the right glaze and firing iron oxide can produce greens, browns, blacks, yellows, oranges, subtle blues and greys (see Celadon). Most of the best colour responses for Iron in a glaze need a Reduction firing. Iron is also a useful colourant in clay bodies and is best introduced by adding high iron clays to the clay recipe.

Jiggering and Jolleying: A process where plastic clay is forced onto a spinning plaster mould with a a metal template. A Jolley uses an exterior plaster mould and an interior template, the reverse is true of Jiggering. Jun or Chun: A Stoneware glaze which is an opalescent milky blue in colour. The effect is a result of particles in the glaze scattering the light and reflecting only the blue wavelength, the other wavelengths are absorbed. It tends to work best on dark clays as they aid the absorption of the rest of the spectrum of light.

Kaolin: A china clay in its purest form Al2O3 2SiO2 H2O. See Clay. Kiln: Basically an insulated box which is heated to fire pots in. They can be either, cross draught, down draught, or updraught. The draught refers to the direction the combustion gasses have to travel from input to exit flues, since no combustion takes place in an electric kiln there are no input or exit flues and they are genuinely heated boxes. The fuels used to heat a kiln are gas, oil, wood, coal (now almost obsolete) and electricity. Each fuel source used to fire a kiln offers different possible outcomes for the pots fired in them. The maximum operating temperature for most pottery kilns is about 1300oC although many Woodfired kilns may be fired up to 1350oC.

Lead: Historically one of the most common low temperature fluxes. It produces rich colour responses particularly with copper, iron and manganese. Lead is a forgiving flux which suffers slight over or under firing well. It cannot be used in high temperature glazes as it becomes volatile. As a predominate flux Lead is best used in glazes fired to below 1100oC. Lead is a cumulative poison and consequently its use as a flux has diminished, however if it is bonded in a glaze with at least two Silica molecules (Lead Bisilicate PbO 2SiO2) it will not release into food. Certain glaze colours are impossible without lead, such as chrome oranges. In Reduction Lead is of little use as it becomes volatile. Leather Hard: A stage in the drying process of clay when the clay is pliable but strong enough to handle. It is ideal for Trimming and the addition of appendages such as handles and spouts. Relatively wet clay can be attached to the pot at this stage and the resulting bond will not form cracks. Lustres: There are two types of lustre in use today by potters. Firstly and, most commonly, commercial lustres, these are metallic chloride compounds suspended in an oil/resin based medium which is applied to the already glazed and fired pot. In a subsequent firing (to around 750oC) the resin/oil burns out creating a Reduction which draws the oxygen out of the metallic chlorides to create a thin film of pure metal on the surface of the pot. Secondly there are 'transmutation lustres' where the reduction is provided by the kiln (often a wood kiln). In this case the metallic oxides are mixed into a paste with clay and painted onto a already glazed and fired pot. The glaze on the pot must be a low fired glaze, (usually a tin glaze) which will soften slightly in the subsequent lustre firing. During the lustre firing (temperatures between 600-1000oC) a heavy Reduction is introduced converting the oxide in the paste into a metal which is then trapped by the softening glaze. After the firing the clay paste residue is cleaned off to reveal the lustre trapped in the glaze.

Magnesia: See Dolomite Mattes. Maiolica: See Delft. Manganese Dioxide MnO2: A glaze colourant, it usually produces warm browns that can move to plum in high Alkaline bases. Manganese used in large quantities as part of a glaze or slip recipe can produce a metallic bronze black not unlike Lustre, (Lucie Rie used this phenomenon to great effect). Mountmorillonites: See Bentonite. Mullite: 3Al2O3 2SiO2 A crystal that has a needle like structure and because of this it adds strength to high fire clay bodies. Below 1000oC mullite will not form. For practical purposes a temperature of at least 1150oC is needed to strengthen the finished pot with mullite crystals, consequently it is of little use in low fired wares.

Nephaline Syenite: See Feldspar (Soda). Neriage: Coloured clay rolled out like a stick of Blackpool rock and then sliced thinly and joined to form a decorative slab which may be formed to any shape (term originally Japanese). Nickel Oxide: NiO A rather plain glaze colouring oxide which usually produces greenish browns. Remarkably nickel at 1-5% in a high barium glaze, pinks, reds, and blues are possible (additions of Zinc help encourage this colour response).

Ochre: The term usually describes an impure high iron clay, often locally won. Can be used as a Slip or Glaze colourant. Once-Fired: A pot that has undergone a single glaze Firing. The glaze is applied directly on to the dry or Leather Hard pot thus avoiding the Bisque firing. This approach, although offering certain economic and aesthetic advantages, can create technical problems for the potter. Oxidation: A Firing where there is either no combustion taking place (electric Kiln) or where there is sufficient oxygen in the kiln to allow a the fuel to burn cleanly. The atmosphere of the kiln (Oxidation or Reduction) dramatically affects the resulting clay and glaze colours, for example; copper in oxidation is green (as is copper oxide) in Reduction it becomes red (more like copper metal).

Pinching: A method of forming clay which is well described by its name. Plaster: 2CaSO4 2H2O. An invaluable mould making tool for the potter, also used extensively in industry. It can be poured or carved to virtually any shape. When it is dry it can be used to press clay into or Slipcast with. Plasticity: The plastic properties of clay are principally determined by the size of the platelets (see Clay). The smaller the platelets the more plastic the clay is. Aging or souring is also relevant to a clays plasticity, with time bacterial action creates a colloidal gel which aids the lubrication of the platelets. Porcelain (also see kaolin): A white highly vitrified clay body that is translucent where thin (often fired up to 1350oC). The translucency is a result of silica glass fused into the fired clay. To achieve this a high amount of flux is added to a kaolin based clay body. The flux to clay ratio is often flux > clay, indeed some of the original Chinese porcelains had as little as 20% clay like minerals. The low clay content makes porcelain very difficult to throw and trimming wares is almost unavoidable. At the home of porcelain, Jingdezhen (China), all the pots are throw in small thick sections, joined and trimmed. Accurate trimming is regarded as more of a skilful than throwing. The plasticity of porcelain can be improved by small additions (2%) of white bentonite.

Quartz: See Silica.

Raku: Originally a Japanese seal given to a prominent family of potters (1598) who developed the technique. The term describes a lowfire form of pottery where the pots are removed from the kiln as soon as the glaze has melted and then left to cool or doused with water. In the mid 20th century Paul Soldner introduced the now popular process of post firing Reduction. In this case the red hot pot is placed in a lidded bin filled with straw or sawdust . The glazes are dramatically altered by the Reduction particularly noteworthy are the colours achieved with Copper. Reduction: Also see Oxidation. A situation where too much fuel is introduced into the Kiln to be able to burn with the available oxygen, consequently oxygen is 'stolen' from the pots in the kiln, it affects the clay and the glaze colour. A good example is iron, which changes from Fe2O3 to FeO, even the tiny amount of iron present in porcelain changes it hue from a creamy colour in Oxidation to a slight grey blue in Reduction. Resist: A decorative technique where a wax based medium is used to create a pattern which is then covered in another coat of glaze or slip. The wax resists the subsequent coating creating the pattern. Paper stencils or tape can create a similar effect.

Saltglaze: Introducing salt into a hot kiln was probably first discovered in 12 century Germany. As the sodium volatilises it attacks the Silica in the clay (and the kiln itself), the result is a sodium alumino-silicate glaze. The glaze is characterised by a pitted orange peel surface although magnesium based slips may result in a smoother surface. The typical colours resulting from saltglaze are oranges (high alumina clays/slips), browns (high iron clays/slips), and blues (Cobalt). Other colours are possible although not much contemporary research has been done in this area. Soda is a popular modern alternative to salt, many potters believe that the gasses resulting from a salt firing are more harmful to the environment than those resulting from soda (it must be added that the jury is still out on this proposition). Additions of boron are also common in salt/sodaglaze, as boron is a glass forming oxide it results in a smoother surface. Notable exponents of saltglaze are Walter Keeler and Michael Casson. Sawdust Firing: Bisqued pots can be coloured by a Reduction firing in sawdust. This may be done in saggers (clay containers) packed with pots and sawdust and then placed in a kiln to fire, or the pots may be packed with sawdust in a bin; the sawdust is then lit and allowed to burn. The results are most dramatic if Burnished Earthenware is used and the colours vary from rich oranges to charcoal blacks. The temperature for this type of firing is usually below 1000oC. Silica: The primary glass forming oxide used in pottery. Boron is the other glass forming oxide used although more commonly as a flux than as a glass former due to its low melting point (577oC). A glass forming oxide must be present in any glaze and as Silica's melting point is 1800oC a flux is always present to reduce the melting point to a workable range. Pure boron glasses are water soluble so of little use but Boro-sillicate glasses have a very low thermal expansion and are the main constituent of 'Pyrex' etc. See also Dunting. Slip: A clay with enough water added to become a 'cream' like consistency. Most often coloured with oxides and painted or poured onto pots for decoration. Slipcasting: Plaster moulds are filled with a deflocculated slip; defloculation reverses the electric charges in the clay particals which reduces the water content in a slip to that of most plastic clays, around 30% of total weight. A common defloculant is Sodium Silicate. The remaining moisture is absorbed by the plaster leaving a sediment of clay over the entire interior surface of the mould. The excess slip is drained off and the cast can be removed from the mould soon after. This approach is used widely by industry and some studio potters. Slipware: A traditional English decorative technique associated with red Earthenware and Lead glaze. Coloured slip is piped onto the Leather Hard pot much like cake decoration. The most noted exponent of slipware was the the 18 century potter 'Thomas Toft', his dishes set a standard that few modern potters can compete with. Stoneware: Highly vitrified ceramics fired to above 1200oC. Most of the Silica in a fired stoneware body is melted into a glassy matrix and the resulting body is of high density and usually has a water absorption rate of less than 1%. Strontium Carbonate: SrCO3 Often regarded as a 'safe' substitute for Barium Carbonate in glazes although this is not yet proven to be always be the case.

Tenmoku: A black Reduction glaze which breaks to rust on ridges. It is an iron saturate glaze with up to 12% iron oxide added to a Potash/calcium based glaze. Firing and glaze thickness are crucial to achieve the desired results which may vary from plum to deep black. Originally a Chinese term for a glaze perfected in the Sung Dynasty (960-1279 A.D.). Terra Sigallata: A Slip comprised of the smallest particles of clay which consequently resembles a Burnished surface. The technique was used to impressive effect in the Greeko-Roman period. Throwing: A delicate balance which defies gravity and centrifugal force as clay is coaxed up by hand from a spinning turntable. Tin Glaze: A creamy Opaque white Earthenware glaze which is usually Lead based. Additions of 5-10% tin oxide create the opacity in the glaze. Tin glaze is used as a ground for decoration, see Delft and Lustre (Transmutation). Also see Zirconium. Transfers: In industry usually called 'Decals'. The process involves an image printed with ceramic pigments onto a gel which releases from a paper substrate when immersed in water. The image can then be transferred to the pot, and provided there is no trapped air between pot and decal it can then be fired (usually to around 750oC) onto the surface of the pot. Turning: (Termed 'Trimming' U.S.A.). Certain forms made on the potters wheel will not support themselves unless excess clay is left at the base, alternatively, extra definition on the foot of a pot may be needed. The solution to both these problems is turning, which is done at the Leather Hard stage. The pot is inverted onto a potters wheel and a metal cutting tool is applied to the bottom of the pot until the desired finish is achieved.

Underglaze: Ceramic colours applied under a glaze, usually a clear glaze. Although a durable method of decorating, colours can run especially if colorants which double as fluxes are used. Unomi: A Japanese style of handleless cylindrical mug. The form is often termed a 'teabowl' by western potters although Unomi's are not used in the Japaneses tea ceromony.

Viscosity: The ability of a liquid to flow, the term is used by the potter in relation to glazes (see Glaze). Vitrification: The degree of melt in a Clay body as the Silica forms a glass with fluxes present. See Stoneware.

Water: H2O. A most important part of clay and is also used to suspend the particles of a glaze prior to application. The water in clay takes three forms; Water of plasticity, Pore Water and Chemically bonded water. Water of plasticity lubricates the Clay platelets. The pore water is water of plasticity that is trapped during the drying process in between platelets of clay. Pore water can cause problems in the Bisque firing even if it appears the pot is bone dry and caution must be taken up to around 150oC (Water smoking period). The chemically bonded water (Clay is Al2O3 2SiO2 2H2O) is driven off up to around 600oC, henceforth it becomes ceramic. See Clay and Firing. Wedging: A hand process used to homogenise the clay, thus making it workable. The techniques for wedging are called; Spiral wedging, Rams head wedging and wire/slab wedging. Both Rams head and Spiral wedging involve the folding of the clay on itself too build up an ever tightening spiral of clay platelets. Wire wedging builds up increasing layers of clay platelets and is the best for introducing other clays and fillers into a already plastic clay body. Woodfiring: See also Kiln and Ash. Wood is the original fuel used by the potter, and if significant temperatures (1200oC-1400oC) are achieved it has a dramatic affect on the unprotected pots. The results from woodash landing on clays/slips vary from subtle greys and browns to oranges and a runny green glaze. Careful packing of the kiln is vital if the best results are to be achieved as the flame leaves its mark travelling through the kiln. Usually large areas of the pottery to be woodfired are left unglazed as the interaction between clay and ash is more fruitful than its affects on a glazed surface.

Yixing: A type of finely crafted red Earthenware from China. Yixing has been made in Jiangsu province since the 16th century. Teapots are most characteristic of this type of ware , which are immaculately handbuilt.

Zinc Oxide: ZnO A flux which is used to great effect in Crystalline glazes in conjunction with titanium. Zinc's colour response is not that remarkable excepting that even small amounts of Zinc will turn Chrome brown (as opposed to green), many commercial stains are based on this phenomena. Zirconium Silicate: ZrO2 SiO2 An oppacifier used in glazes. Zircon is the colourant used in the bright white enamel on washing machines and other domestic appliances. Tin is a more expensive alternative, giving creamer whites which are more pleasing to the eye.