MARAGON - ARTS & CRAFTS
MARAGON - ARTS & CRAFTS
Casting, Moulding & Modelling - Materials & Kits
Casting, Moulding & Modelling - Materials & Kits
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GLOSSARYAnvil and Beater An anvil is a pebble or piece of wood used to beat pot walls. The anvil is used inside the pot, a beater or paddle is used outside. The process compresses the clay, thins the pot wall and enlarges the pot. To assist the clay to stretch without cracking, the anvil is often ´pecked´ that is, it has its surface covered with small chipped holes.
Armature The improvised hollow framework or cage (usually covered with a fabric/bandaging of jute, cotton or hemp) to which wet casting plaster is applied in building up a plaster-of-paris statue, etc.
Ball clay A plastic secondary clay, usually quite refractory, and firing to a fairly pale, off-white colour.
Bat wash/Batt wash A mixture of refractory material(e.g. Aluminium hydrate) and water painted on to kiln furniture and/or clay to resist fusion of touching surfaces during firing
Bentonite A highly plastic collodial clay-like material which is a useful plasticiser and suspender
Binder A polymeric compound (e.g. starch-based adhesives) used to strengthen unfired glaze surfaces
Biscuit firing A preliminary firing of unglazed ware beyond the point of ceramic change, but below the point of fusion, so that the ware remains porous. Biscuit may refer to finished unglazed wares, despite the fact that these may be fired to vitrification.
Bisque/Bisc Generally refers to unglazed ware fired to maturation (and in some cases, to vitrification)before the application of a glaze whose maturing temperature is lower than that of the body. A general term for used to describe a preliminary firing of unglazed wares.
Bloating Blistering of a pot wall during firing, caused by the expansion of unreleased gases after fusion has begun.
Boxing pots The stacking of pots rim to rim and foot to foot for drying, storage and firing.
Burnishing The technique of burnishing pottery can be traced back to ancient times. Burnishing involves no more than rubbing the clay surface with a smooth tool to produce a mirror-smooth surface. In reality it has a compressing effect on the clay particles. It can be done when the clay surface is leather hard and up untilit is almost completely dry. Most clays are suitable for burnishing although the finer the clay the smoother the burnished surface. Suitable tools for burnishing include: Smooth rounded beach pebbles, The convex side of metal spoons and smooth knife handles. After the pot is smooth, draw your design with lead pencil then scratch around design with a knife. Designs can either be geometric or organic. Once the pot has been blackfired it can be left without further treatment or polished with oils to enhance the shine.
Calcine Heat treatment of a material in order to alter to improve its behaviour in use. Calcining may purify a material, for example, or it may reduce its plasticity and shrinkage.
Casting plaster See Plaster-of-Paris.
Cation A positively charged ion.
Ceramic change The point at which chemically combined water is driven off from clay molecules, so that the clay becomes pot, and can no longer be slaked down. This change is permanent and irreversible, and is widely said to take place at 573degrees C..although it is, in fact, an ongoing process which may begin as low as 350C., and may still be taking place a temperatures as high as 700C
Celadon A high firing glaze of Chinese origin which is characterised by the effects of reduction firing on small amounts of iron in the glaze, giving a rang of colours from pale grey or blue-green to a deep olive green.
Chuck A made or found object used to support a pot whose shape makes it difficult to invert for processes such as turning or banding. Typically a chuck will be a heavily thrown hump or bowl which is allowed to stiffen in situ on the wheel head before resting a pot on or in it.
Colloidal Colloidal particles are particles so fine that, even though they may be of a material much heavier than water, they remain in suspension and will not settle. In a clay body, such particles improve plasticity, and in a glaze they aid suspension
Crawling A glaze fault most common in very viscous or opaque glazes which is characterised by a rolling back of the glaze during firing, leaving bald patches of pot body exposed.
Crazing A glaze fault caused by excessive shrinkage of the glaze in relationship to the body, and which is characterised by a network of fine cracks all over the fired surface.
Cristobalite inversion A sudden change in the physical structure of silica in its cristobalite phase. This change causes sudden expansion on heating (with corresponding contraction on cooling from alpha to beta form and takes pace a temperatures between 220C and 280C(227C is widely taken as a guide).
Deflocculant Generally soluble alkalis added to a glaze slop which bring about ion exchange, thus causing clay particles in the suspension to repel each other. This repulsion reduces friction between the particles, thus increasing the fluidity of the suspension without the addition of more water.
Dunting A crack caused by firing and /or cooling stress in a pot.
Earthenware A clay body which is fired to relatively low temperatures (below 1150C) and which, after firing remains porous in its unglazed state.
Dunting points These are the points at which silica inversion takes place during firing and cooling. The significant points are quartz inversion (around 573C) and cristobalite inversion (around 227C).
Egyptian paste a prepared body which contains soluble alkalalis which migrate to the surface by evaporation and thus deposit fluxes which will fuse with the body during firing, hence its description as a self glazing clay.
Electrolyte A liquid or solution which is able to conduct electrical currents.
Engobe The word engobe is often used to refer to decorative (as oppossed to casting) slips, but may also refer to any material which is neither a pure clay slip nor a glaze but which is used to cover clay.
Feldspar One of a group of rock-forming minerals, the most abundant group in the Earth´s crust. They are the chief constituents of igneous rock and are present in most metamorphic and sedimentary rocks. The type known as moonstone has a pearl-like effect and is used in jewellery. Approximately 4,000 tonnes of feldspar are used in the ceramics industry annually.
Fibrous plaster Plaster-of-Paris that incorporates a small proportion of fibre - usually glass-fibre - as reinforcement; the loose fibre is mixed in with the wet plaster then poured into the mould. Mainly used in producing long slender items, or thin fascias or tiles, etc. in the building industry.
Flexural strength (A term usually associated with cast materials such as concrete or plaster). The resistance to fracture failure due to bending. Other types of fracture force include shear, tension, torsion and compression, but flexural strength is the main concern with projections and narrow areas, such as the limbs of statues, etc.
Flocculent/flocculant Acids (or salts which in solution behave as acids) such as sodium silicate which alter the electrostatic charges of fine particles in a suspension, reducing their mutual repulsion, and thus increase their forces of attraction. This causes particles to floc together, increasing friction and viscosity, thus apparently thickening the suspension without removing any water from it. See also Deflocculant.
Flux Oxides (usually alkaline) which, when combined with the acid glassformer, silica, encourage and lower the temperature of ceramic fusion during firing.
Friable Easily crumbled.
Frit/fritt Materials which have been prepared for use by melting (usually together with at least one other material-often a pure silica-to form a fused compound) and grinding. This may be done for a number of reasons, including: reducing solubility, safely containing poisons, reducing volatility
Galena Lead ore: historically a commonly used flux in earthenware glazes, but which is poisonous, and therefore rarely used in its raw form today.
Glaze Transparent vitreous coating for pottery and porcelain, which gives the object a shiny, protective and/or decorative finish and helps to keep it from leaking and chipping. Glaze is applied by dipping a formed ceramic body into it or by painting onto the surface. It is fixed by firing in a kiln. Different mineral glazes will combine chemically with different bodies (according to the minerals present in the clay). Glazes may be alkaline, lead, leadless, tin, salt, or feldspathic. Glazed pottery is first known from the mid-to late-Neolithic in Egypt, where glass was first made.
Gloss glazes produce a shiny glass-like finish
Glost Originally a second firing of ware to a lower temperature than the preliminary firing, but now used (most certainly amoung studio potters) to refer to any firing of glazed ware.
Grog Crushed or finely ground fired clay/clayware incorporated into the body of new work usualy for the purpose of strengthening and giving rigidity - similar to aggregates in concrete.
Hakeme/hakame A Japanese way of freely brushing slips onto a pot so that it is decorated with bold stirations around the from. The same technique may be used with slip glazes.
Inert material Material which is passively resistant to any change. In ceramics, we particularly refer to materials which are relatively unaffected by the action of heat or water.
Ion exchange An exchange of ions (electrically charged atoms) which alters the electrostatic charge and thus the behaviour charge and thus the behaviour of particles in a suspension.
Jigger/Jolley Moulds Soft clay is forced into/onto a spinning plaster mould in combination with a template. A Jolley mould uses an outer plaster mould and an inner template, the jigger mould uses an inner plaster mould and and outer template.
Kaolin Group of clay minerals, such as kaolinite derived from the alteration of aluminum silicate minerals, such as feldspars and mica. It is used in medicine to treat digestive upsets, and in poultices. Kaolinite is economically important in the ceramic and paper industries, giving paper its smooth writing surface. It is mined in the UK, the US, France, and the Czech Republic.
Maturity in ceramics when referring to glazes is the point at which they reach their maximum hardness during the firing process.
Mould oil See Release agent
Negative The opposite profile of the original or final object. The impression of a coin in plasticene is a negative, so too is the external mould or receptacle for any casting in clay, concrete or plaster, etc.
Opacifier A material added to a glaze in order to render it opaque. Opacity is achieved by the deflection of light, either by the suspension of vey fine bubbles or particles in the glaze, or by the formation of crystals in the glaze.
Opener Openers are used for similar purposes to grog(see above). Grog is itself an opener, but other materials such as sand, sawdust or straw maybe used.
Oxidation firing The firing of ware in a kiln where there is an ample supply of oxygen in the chamber, so that combustion is complete, facilitating the release of volatile gasses, and causing metals in the clay and glaze to give their oxide colours.
Parting agent See Release agent
Pinholing a glaze fault which causes pitting of the glaze surface. It is caused by bubbles rising to the surface during drying or firing, and being prevented from healing over because of the viscosity of the glaze melt, which may be caused either by its composition, or by underfiring.
Pipe A ‘pipe' is the metal rod that runs through the center of lamps through which the cord passes and it is threaded on each end.
Plaster-of Paris Calcium sulphate hemihydrate (manufactured through a crushing/milling and heating process from the rock-mineral, 'gypsum', originally sourced from the Paris region in roman times); this powder is used in art and engineering work to produce casts and moulds The origin and degree of refinement of these plasters varies considerably, but when mixed with water they all exhibit a flow stage then set hard within a relatively short period - usually less than 30 minutes at room temperature - in contrast to building plasters which, although chemically almost identical, are mixed- or coarse-grained powders containing varying amounts of other minerals and impurities, but most obviously 'stand-up' when mixed with water.
Plasticity The characteristic property of clay which allows it to be a mouldable solid. Plasticity in clays depends on a number of criteria, including the purity of the clay, uniformity and size of the particles, and the bonding of particles through souring.
Porcelain Porcelains are usually prepared white clay bodies which firm to a hard, vitrified finish. They are particularly admired for their translucent quality when finely potted and appropriately fired.
Pottery Clay that is chemically altered and permanently hardened by firing in a kiln. The nature and type of pottery, or ceramics (Greek keramos,”potter´s clay”), is determined by the composition of the clay and the way it is prepared; the temperature at which it is fired; and the glazes used.
Pyrometric Cones These are used to accurately measure the amount of heat work that has taken place in the kiln. They are made from precisely measured combinations of clay and flux materials which reliably begin to ruse and bend at a predetermined heat.
Pyroplastic The condition of a clay body in the kiln when heated to vitrification. At this point, any impact upon the pot may alter its shape, and, ultimately, it may begin to sag under its own weight.
Quartz inversion A sudden change in the physical structure of silica in its quartz phase. This change causes sudden expansion on heating (with corresponding contraction on cooling) from its alpha to its beta form and takes place at temperatures between 550C and 575C (573C is the usual given temperature).
Raku A Japanese word which loosely translates as ´enjoyment´, and which strictly only refers to pottery by the potter who holds the Raku title. In the West, however, the work has become associated with a particular technique which generally involves placing pots into the already hot kiln, and often then carbonising the pots by removing them directly from the hot kiln to a bin of combustible material such as leaves or sawdust.
Ram pressing The mechanical/hydraulic pressing of ceramic shapes with permeable dies. Gypsum plaster is commonly used, but more permanent materials have been developed and are being used where applicable. Release of the pressed shape is obtained by means of fluid pressure forced through the permeable die. An extremely wide range of bodies have been used successfully from all clay to all non-plastics with artificial plasticizer added. Grog refractory type bodies are used with equal success and a long die-life. In general the clay charge is a "stiff mud" consistency (slightly stiffer than normally used for jiggering) and have water retention properties sufficient to permit the body to flow easily during the pressing cycle. Each ram die consists of two members; usually a male and a female. They are encased in metal die casings and provided with alignment pins. The plaster die is usually internally reinforced with metal.
Raw glaze Although this term is frequently used by studio potters to refer to the practice of glazing pots when they are raw, it strictly (particularly in industry) denotes a glaze which is, itself, composed of raw, unfritted materials.
Reduction firing: The firing of ware in a kiln where there is an insufficient supply of oxygen in the in the chamber, so that combustion is incomplete, causing the flames to draw on the combined oxygen in the fabric of the ware, which has the effect of reducing oxides to their respective metal forms.
Refractory Resistant to high temperatures.
Release agent Same as parting agent. Any treatment applied to the surface of a mould to prevent the cast material from sticking to it; usually a wax, oil, or greasy application which can be rubbed, sprayed or brushed on. depending on the casting medium, the release agent can be any of a number of proprietary products, but vaseline/petroleum jelly, soap, WD40, furniture polish, cooking oil, etc. can all be used. A release agent is not neccessary where the mould itself is a flexible or non-stick surface such as alginate and silicone or latex rubber.
Resist Any material which is used to create a barrier between the surface of a pot and an applied treatment, such as a slip or glaze coat. Resists may be either wax (hot paraffin wax or cold wax emulsion), latex (e.g. carpet glue, copydex etc.), paper (florists paper is good, but newspaper will do for making templates or stencils), or found materials such as lace or leaves.
Salt/vapour glazing The glazing of pots in the kiln by introducing damp salt (or other soda compound) into the kiln at high temperatures. The salt decomposes and volatilises, and the soda thus released into the firing chamber the kiln and furniture too), thus forming a glaze.
Scrim The narrow tape of fairly wide-mesh fabric (cotton or jute) used to reinforce slender plaster mouldings, or to strenghten joints and stress points in plaster-of-paris work.
Scumming Unwanted deposits of soluble salts which sometimes appear on the surface of a raw dry pot, or more significantly, on bisc-fired ware. After bisc firing, areas where scum is apparent may be slightly fused and therefore resistant to glaze take-up. Scum is unlikely to resist glaze take-up on a raw pot.
Setters Special refractories which are designed to support the shape of a pot during firing.
Setting The word which potters use to mean placing ware into a kiln.
Shop rotten A raw pot which is shop rotten is one which has been stored for a long time, and been subjected to extreme variation of ambient conditions which have weakened it to such a point that it becomes friable.
Short Clay A clay of poor plasticity and strength which tends to split when worked, and which fails to stand up well in throwing.
Siccative A drying agent; any material which tends to take up moisture from a slop or body.
Sintering The bonding of particles by heating, but without fusion, thought to be brought about by friction.
Slip A suspension of clay in water, usually quite creamy in consistency, which may be used either in conjunction with colouring oxides for decorative effect, or, with deflocculants in casting.
Slipcasting Plaster moulds (slip moulds) are filled with a deflocculated slip; deflocculation reverses the electric charges in the clay particles which reduces the water requirement of the slip to about one third by weight. The moisture is absorbed by the plaster mould leaving a layer of semi-hard clay over the interior surface of the mould. Excess slip is drained off and, although the time needed depends on the absorption of the plaster/water content of the clay and the general drying conditions, the layer of hardening/drying clay shrinks or retreats from the mould surface, so that the two can be separated simply by turning the mould upside down. The mould can then be dried, cleaned and reused. Complex or convoluted profiles can be achieved by using split moulds (e.g. a mould in two halves) which can be held together during drying with elastic bands, etc.
Slipware Earthenware pottery decorated with slips under a transparent glaze. A traditional English decorative technique associated with red earthenware and lead glaze. Coloured slip is applied to the leather hard pot much like cake decoration. Not to be confused with slipcasting
Slake To rehydrate. Clay will usually slake down´ if it is dried completely and then covered with water; as the water penetrates the pores, it causes the particles to move apart, and forms to disintegrate.
Souring A process of organic breakdown which releases acids into the water of plasticity in a clay body. These acids bring about the flocculation of fine particles, thus improving the body´s plasticity and strength. Stoneware: A high-firing clay body. Stonewares are fired to the point of vitrification, and are therefore much harder, stronger and more brittle than lower fired earthenwares
Suspender A material added to a glaze slop in order to facilitate the suspension of heavy particles in water. Suspenders usually act by the creation of a macrostructure which hold particles in dispersal.
Thermal shock Extreme stress caused to a pot by sudden or uneven change in temperature.
Thixotropy The property of some materials in a suspension which causes a thickening or gelling of the suspension when still.
Vicat The measure of the time taken for (e.g.) a casting plaster to acquire a particular hardness during the setting process. In controlled situations this involves measuring the penetration of a special needle under a known force, but in general parlance, the Vicat set-time is an indication of when fresh plaster can be safely be removed from a mould.
Viscosity The thickness of a liquid or its resistance to flow.
Vitrification The process by which silica is converted to glass by the action of heat and fluxes. A vitrified body is one in which some of the silica in the body has been converted from its crystalline to its glassy phase. If all of the silica in a body has been vitrified, it may slump in firing, and the finished pot will be very brittle.
Waster Pots which are rejected because of some fault which has become apparent after firing.In Cast ceramics a waster is the extra of the mold that is cut off.
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