Glossary A-M

				Alkali					Used here to mean an oxide of potassium or sodium which were important fluxes in ceramic glazes and some
									specialised body types such as porcelain (q.v.)


				Alkaline glaze					a glaze in which the principal flux is an alkali. See box p.75. It contains varying quantities of soda or potash in some form. The firing temperature is adjusted according to the variations in quantities and the physical characteristics of the raw materials. When small quantities of copper oxide are present it will give a rich turquoise colour.




				Alumina					Aluminium oxide (Al₂O₃). A major compoment of pottery clays, present in amounts of c.10-40 weight per cent. The higher the alumina content, the more refractory (or difficult to melt) a clay body tends to be. It can be present in glazes, due to reaction with the body during firing, as well as in raw materials such as clay and feldspar.




				Amphora					Pot form used in Mediterranean countries for holding liquids. Characterized by two handles linking the neck to the body of the pot.



				Antimony					A metallic element (Sb) not widely used in early metallurgy but which was used to make opaque and coloured glazes and glasses from the C16th BC through to the early first milennium AD, when its use in glass began to be supplanted by tin. Fine calcium antimonate particles in a glaze make it opaque white, and lead antimonate is opaque yellow (artists’ term ‘Naples yellow’). Antimony oxide is poisonous in lead glazes.





				Applied decoration					Addition of clay or components made of clay to the surface of a pot for decorative purposes.
				Applied decoration


				Arabesque					Ornamentation derived from a mixture of formal geometric and foliage patterns, characterized by flowing linear designs of leaves and scrolls.



				Ash					The material left when wood or other plant matter is burned and all the carbon is lost. Ashing concentrates a range of elements present in only minor quantities in the original plant, particularly lime and the alkalis. In northern Europe, wood ashes are rich in potash, while the plant ashes used in the Med and Nr Eastern regions are rich in soda.




				Ash glaze					A glaze formed by the interaction between the surface pf the ceramic and ash which may be carried over from the fire accidentally with the kiln gases, or deliberately sprinkled on the surface of the pot. (effect?)



				Bag Wall					A lightly constructed wall inside the firing chamber, built to force the flame up towards the roof before being reflected down among the pots.



				Barbotine					Decoration formed by piping soft clay through a nozzle as though icing a cake (see slip-trailing); generally reserved for describing Roman pottery.



				Biscuit-fired					The name given to a ceramic body which has only been fired once and on which there is no glaze. A biscuit firing usually varies in temperature between about 950^oC and 1100^oC, but may be higher. Intentionally unglazed porcelain and stoneware bodies are sometimes termed '‘biscuit'’.




				Body					General term for the clay from which a pot is made.


				Burnishing					Sometimes known as polishing. Production of a smooth, sometimes glossy surface by polishing the leather-hard (unfired) pot with a smooth-surfaced hard tool such as a pebble or bone. The burnishing compresses the clay minerals to produce a very dense continuous surface which has a dull gloss. It can be used to fix pigment particles such as hematite by rubbing them into the surface. Burnishing with a flat or narrow tool can produce a faceted effect.





				Calcareous clay					A clay containing more than about 5% lime, usually in the form of calcite (q.v.). Clays of this type were widely used in the ancient Mediterranean and Nr East and in the manufacture of wares such as terra sigillata. see boxes pp.54, 117, 136, 151.



				Calcite					Naturally occurring calcium carbonate (CaCO₃), the main constituent of limestone and chalk, which is present in calcareous clays.



				Carbon					Element present in most clays which gives a black colour unless it is burnt out during firing. (See box p.151) Pottery can be deliberately blackened by firing in a smoky fire, which impregnates the surface with carbon (soot).



				Carinated					A pot shaped by joining a rounded base to a bowl of straight sloping walls.


				Celadon					A Far Eastern stoneware with a green or green-blue glaze. See box on p.103.


				Cementation					See box on p.106.


				China clay					See kaolin.


				China stone					Also known as porcelain stone, a rock composed of quartz and alkali feldspars used as a flux in porcelain. In China and Korea, porcelain stones commonly contain mica as well as feldspar and quartz, and sometimes kaolin; some were used as raw materials without additions of clay (see boxes pp.100, 184, 196, 203).



				Chromium oxide					Widely used as the basis of green and yellow pigments in modern glass and glazes. The pure oxide was not isolated until the late C18th and was first used as a ceramic pigment at Sèvres, the French porcelain manufactory, in the early 1800s. However the impure naturally occurring mineral, chromite, is quite widely distributed and was utilised as a black pigment in early Mesopotamian pottery (see box, p.42) and on Islamic pottery. Some Islamic green glazes may owe their colour to chromium, but this effect has not yet been fully investigated.




				Cinnabar					Naturally occurring mercuric sulphide (HgS), basis of the bright red pigment vermillion.


				Clamp firing					An open firing. Usually refers to the firing of bricks, but sometimes used to imply an open firing where blackening of the wares has been achieved by covering with excess fuel or vegetation (see smudging).



				Clay					A fine-grained natural material which when wet is characterised by its plasticity, the property which allows it to be deformed by pressure into a desired shape without cracking and to keep this shape when the pressure is removed. In addition to clay minerals (q.v.), clay typically contains quartz and may contain other minerals such as feldspar, calcite and iron oxide. The clay body must be carefully prepared to remove foreign matter such as stones or roots, and be mixed evenly throughout.




				Clay minerals					A group of very fine-grained minerals (alumino-silicates) which are the main constituents of clay. They occur as minute platelets which, when wet, slide across one another, giving the clay its plastic properties.



				Cobalt oxide					A blue pigment, used to paint under or over a ceramic glaze or to colour the glaze deep blue throughout (see box, p.67). It was imported from Persia into China where it was known as 'Mohammedan blue'.



				Coiling, Coil-building					Building a pot by rolling out coils or strips of clay which are then wound on top of one another (see box, p. 34)
				Coiling, Coil-building


				Colouring oxides					Various metal oxides when painted on or mixed into a glaze give different colours. Copper oxide gives green, manganese oxide gives brown or purple, iron oxide gives brown or green, cobalt oxide gives blue.


				Combing					Incised parallel-line decoration created by dragging a comb-like tool across the surface of the soft clay.


				Copper oxide					Used to colour glazes turquoise or green when fired in oxidation (q.v.). Also fired in reduction (q.v.) by Chinese and Korean potters to colour glazes red, or as an underglaze pigment. Analysis suggests that much of the copper oxide used by early potters was obtained by heating scrap bronze.




				Crackle					A deliberately induced type of crazing. Sometimes finely ground iron oxide is rubbed in and fused in an additional firing with another thin layer of glaze over the top.



				Crazing					A very fine network of cracks in the glaze, not to be confused with crackle. Crazing may be due to a wrong balance in the glaze mixture, or it may be the result of age and the effects of burial. (see boxes, p.75, 117)



				Deflocculation					The separation of the aggregate particles into the individual particles, as in a slip in which the proportion of particles in suspension is increased through the reduction in particle size.



				Dipping					Application of a glaze by immersion of the vessel in the glaze suspension.


				Dolomite					A naturally occurring calcium magnesium carbonate (CaMg(CO₃)₂).


				Earthenware					A clay-based ceramic body which is not fully vitrified (q.v.), ie. the glaze and body remain separate layers, and is therefore relatively porous. Earthenware bodies can be quite variable in colour and texture: by definition, earthenware includes the fine white-bodied glazed tablewares of Wedgwood as well as the low-fired common pottery of much of the archaeological record. It is fired at temperatures between about 850ºC and 1100ºC, and not exceeding 1150ºC. It may be glazed or un-glazed.





				Efflorescence					Migration of water-soluble compounds to the surface of a pot as it dries (see boxes, p.106, 136)


				Enamel					Glaze decoration prepared in frit form (a metal oxide pigment + a glass flux ) which melts at a low temperature. It is painted over a pre-fired glaze and refired in a reducing kiln (q.v.). It allows a wide range of colours and is also called 'on-glaze decoration'. (see box, p. 198)




				Engobe					Another name for slip, now largely obsolete.


				Fabric					The combination of clay and inclusions that makes the ceramic. Each pottery-making centre, assuming it uses different clays and inclusions (q.v.) from others, will produce pots in a characteristic fabric. Potters may use more than one fabric if they produce different types of ware, eg. cooking pots and tablewares. A fabric may be characterised by its visual appearance, by microscopy, by chemical composition and by physical properties. Fabric analysis is a key element in the investigation of provenance (see boxes, p.42, 64).





				Faience					French term for tin-glazed earthenware of post-medieval Europe (ie., maiolica), derived from the Italian manufacturing centre of Faenza. Also a ceramic composed of a body of crushed quartz and an overlying glaze, common in the ancient Near East and Mediterranean. It was capable of being moulded into small objects or pots, and the surface became glassy when fired; if dusted with copper oxide it takes on a turquoise colour. Such products were initially confused with earthenware which had a white tin-opacified glaze, and the term faience incorrectly applied, has remained in use (see box, p.104).






				Feldspar					A basic ingredient in many clay bodies as well as glazes. A group of silicate minerals formed from igneous rock material, and an important constituent of china stone. The combination of alkali (potassium or sodium), alumina and silica makes feldspars valuable fluxes for both porcelain and stoneware. Melts at 1250ºC to form a simple glaze.




				Firing					The transformation of a soft clay into a hard and durable ceramic by heating to a high temperature. Temperatures in the region of 600ºC or more are needed to produce a ceramic which will not readily break down in water.



				Firing temperature					In archaeology, this refers to the highest temperature attained by a ceramic during firing, and it can be estimated using a range of analytical techniques.


				Flocculation					The clustering or coagulation of suspended particles.


				Flux					A substance that lowers the melting point of some other substance and encourages fusion. Essential glaze ingredient which causes other ingredients to melt and fuse to form a glaze: lead or alkalis were important fluxes in the formation of glazes, causing it to melt and coat the surface of the vessel. In porcelain bodies, a china stone flux was added to make the body vitrify. In bodies made from calcareous clay, the lime (calcium oxide) is an important naturally occurring flux which vitrifies the body. Different fluxes cause different colouring results from the mineral oxide pigments, to give a wide range of colours.






				Frit					A partially fused glassy material, usually an intermediate stage in the production of a glaze or glass, or an additive for a ceramic body. It's made by heating together raw materials to the point of fusion. The mixture is shattered by being poured into cold water and subsequently ground to powder. ‘Fritting’ is an important process in rendering lead harmless for use in glaze, but is most widely used in the preparation of glazes.





				Fritware					A ceramic ware made by combining a ground glass frit with a small percentage of plastic ball clay; popular in Near Eastern ceramics on account of its whiteness.



				Glaze					A thin glassy layer on the surface of a ceramic, discovered in the Near East about 1500 B.C. Typically applied by dipping the ceramic in a watery suspension of glaze components, but can be brushed on, effloresced from the body, or deposited from a vapour.




				Glaze fit					The stability of a glaze on the fired body. A glaze that fits well displays no crackle or blistering, and it does not peel or flake off easily. A glaze that does any of these things and so fails to adhere is wrongly constituted for the type of body on which it is used.




				Glost					An adjective used by potters to describe a glaze; hence, a glost firing is one in which glazed pots are fired (as opposed to a firing with only unglazed pots).



				Green-fired					A ceramic body that is allowed to dry naturally in air, and is then glazed and fired in a single process.


				Grog					Crushed pottery, used to temper (q.v.) the clay.


				Hand-building					To archaeologists, this term implies the fabrication of a vessel without the potter’s wheel, using techniques such as coiling, slab-building and pinching. Modern ceramists, however, may include wheel-thrown pots in the ‘hand-made’ category (ie., they were produced by the hands of a potter) as opposed to machine-formed, factory-produced wares.





				Hard-paste porcelain					See box p.203.
				Hard-paste porcelain


				Hematite (haematite)					An oxide of iron, ferric oxide (Fe₂O₃). It is formed from iron compunds in clay when fired in oxidation (q.v.) and is responsible for the orange-red colouration of many ceramics. Naturally occurring hematite was sometimes crushed and burnished into the surface of the pottery to give a shiny red appearance (see boxes, p.42, 89).



				Imbibing method					One way in which reducing conditions can be produced to alter the appearance of a ceramic ware. When the critical maturing temperature is reached, earth, damp straw, and other vegetation are shovelled on to the top of the kiln or bonfire, and water is dribbled over it. This causes a rapid drop in temperature as the damp penetrates the kiln roof or the casing of the bonfire, and the oxygen is sucked out.



				Impressed decoration					Decoration achieved by pressing a hard object such as cord, shell, fingertip etc into the surface of the soft clay body.


				Incised decoration					Use of a pointed tool to incise lines, by pressing or cutting into the unfired clay surface (see p.128, fig.1).
				Incised decoration


				Inclusions					Coarse, non-plastic particles in a clay body.


				In-glaze					Technique of decorating pottery in which colouring oxides are painted on to unfired opaque white tin-glaze. Brought to perfection by the Italians in the Middle Ages (majolica painting).



				Iron oxide					A component of pottery clay, present in the range 0.5-15 weight per cent. The main oxide responsible for the fired colour of the clay (q.v. carbon, hematite, magnetite, and boxes p.42, 89)



				Jiggering					The technique used to produce the Chinese whitewares which were imported into Iraq in the early C9th, and imitated by the Abbasid potters: i) throw a vessel of approximately hemispherical proportions; ii) invert it over a convex mould (throwing approximate shape first gives a better fit than a flat pancake of clay on would on a mould); iii) trim back the clay on the exterior to produce a thin-walled vessel; iv) cut the base at a 30° angle; v) incise a raised footring; vi) remove the finished vessel from the mould (this is eassier using a parting agent such as slip).






				Kaolin					China clay, rich in the mineral kaolinite and relatively aplastic. Kaolins are white-firing clays low in iron oxides and fluxes such as alkalis and lime. Primary kaolins are found in the position where they were formed from the original igneous rock; the kaolins of both Southern China (Jingdezhen) and Cornwall are of this type. Secondary kaolins, including the northern Chinese clays, are sedimentary clays which have been transported over long distances (see boxes p.184, 196, 203). Kaolins are essential for the production of fine porcelain.





				Kickwheel					A potter’s wheel which is turned by kicking with the foot.


				Kiln					Sometimes known as the potter's oven, kilns were developed originally in the Near East. They are the structures in which pottery is fired. Usually consists of a firebox in which the fire is set, one or more flues carrying the heat into the kiln, a firing chamber in which the pots are stacked, and a vent for the loss of waste gases. Important characteristics include the separation of the pots from direct contact with the fire and the high thermal capacity of the kiln, so that temperature rises much more slowly than in an open firing (see box, p.32). A kiln is thus more suitable for firing fine-textured clays. In an updraught kiln, the type most frequently used in pre-industrial times, the hot gases pass from the bottom of the chamber and exit at the top.







				Kwaart					Technique used by the Delft potters to imitate Chinese blue and white porcelain in tinglazed earthenware by using a clear transparent glaze over the white opaque glaze. This gave a greater gloss and depth to the colours.



				Lead glaze					A glaze in which the main flux is lead oxide (see boxes, p.94, 139).


				Leather-hard					Half-way stage between wet and dry clay. The clay is no longer plastic and can be handled easily without becoming deformed, yet is still soft enough to permit clean carving and incising. The stage at which turning is carried out on unfinished pots.



				Levigation					Process of refining the clay by settling in water, causing a separation of coarse from fine particles - the finer particles remain in suspension while the heavier particles sink to bottom.



				Lime					Calcium oxide (CaO). Present in many clays as calcite. Also a major constituent of plant and wood ashes (in some cases, 40% or more), in which form it was added to glazes.



				Limestone					A rock composed mainly of calcite.


				Loess					A form of wind-blown silt or dust.


				Lustre					A type of colouring decoration achieved by painting metallic pigment on to a fired glaze and refiring in a reducing atmosphere which gives an iridescent effect. (Freestone, ch.16; box, p.112).



				Luting					Joining 2 pieces of clay together, usually by adding a little water or slip to the join (see p.174, fig.5). Spouts and handles as well as some decorative elements are attached to vessels in this way.



				Magnetite					An oxide of iron (Fe₃O₄). Can be formed when clays rich in iron are fired in reduction (q.v.); for example, the black slips of ancient Greek pottery (box, p.89).



				Maiolica,					General term for tin-glazed earthenware decorated with oxides painted on to the unfired glaze (q.v., underglaze). Used specifically in the nineteenth century for moulded earthenware with relief pattern decorated with coloured glazes or majolica painting. (Freestone, see box, p.117)
				majolica


				Manganese oxide					A relatively common metal oxide, used as a black or brown pigment on many earthenwares (q.v.); can produce a purple or aubergine colour when dissolved in a glaze or enamel.


				Marbled ware					Covering a ceramic ware with coloured slips and then shaking or combing them on the surface to form a pattern.


				Marl					A calcareous clay.


				Mica					A potassium aluminium silicate mineral which occurs in clay, china stone and granite (q.v.). It has a thin sheet-like form which promotes plasticity.



				Micrograph					An image obtained under the microscope, for example, a scanning electron microscope.


				Modelling					Shaping a piece of clay with the fingers or with a tool.


				Moulding					Shaping soft clay by pressing into a mould.


				Muffle kiln					Protects pots from flames in a flame-burning kiln. Essential for production of enamels.