Glossary N-Z

				Natron					A mineral found in the Near East containing sodium oxide (Na20) which acts as a flux in a glaze or in faience.


				Neutron Activation Analysis					A method of trace element analysis in which a small powdered sample of ceramic is bombarded with neutrons in a nuclear reactor and the resulting emitted gamma radiation measured to determine the concentrations of a range of elements. Long favoured as a precise, accurate and minimally destructive method of analysis of archaeological materials, the closure of civil nuclear reactors means that there is now a movement towards other techniques (see box p.161).
				(NAA)


				Ochre					A naturally occurring iron oxide-rich earth, used as a yellow, brown or red pigment.


				Opacifier					A substance, usually a metal oxide such as tin oxide, which, when added to clear glazes, suspends itself and
									renders the glaze opaque and white.


				Open firing					Firing without a kiln superstructure (q.v.), in a bonfire or similar device. Open firings are typically short and economical on fuel; they have the disadvantages that the pot is in contact with the fuel and may discolour, they are very vulnerable to the vagaries of the weather, and the type of body that can be successfully fired is limited by the initial very rapid rise in temperature (box, p.32).




				Oxidation,					Firing inside a kiln in conditions when a clean bright atmosphere flame burns with plenty of oxygen available, so
				oxidising firing					that the iron oxides tend to form red hematite rather than black magnetite, and carbon is burnt out of the clay body. The resulting pots are often bright coral red and iron-glazes become a yellow or brown colour.


				Paddle and anvil					Shaping a pot between a hard, smooth ‘anvil’ (often in the form of a stone), which is held against the inside wall of the vessel, and a ‘paddle’ with which the outside is beaten. Can be used to thin the walls and enlarge a pot formed by some other method, such as coiling (q.v.). Used in Indus Valley and America.



				Petrography, petrographic analysis					A method of identifying the inclusions (q.v.) in a ceramic. A thin slice cut from a pot is stuck to a glass slide and ground down until it is a thin section, only 0.03 mm thick. Many minerals are translucent in such a section, and may be examined using a transmitted light microscope in the same way that biologists examine cross-sections of wood or plant material. A technique derived from the discipline of petrology, the study of rocks, petrography may allow the identification of the provenance of a pot from the rock fragments it contains (box, p.161).



				Pigment					A colourant. Before the modern period, ceramic colours were based mainly upon metallic oxides of copper, iron, cobalt, lead, tin, antimony and manganese (q.v.).



				Pinching					Hand-building a pot by opening out a ball of clay between the thumb and fingers.


				Plastic					Clay which can be moulded without breaking is described as being plastic.


				Porcelain					A white, vitrified, high-temperature ware which is translucent and rings when struck. The body is made by mixing china clay, china stone and quartz together. In China, the term ‘porcelain’ is more widely applied to include non-translucent fine stonewares. To see the translucent effect, hold a porcelain cup up to the light and observe the shape of the fingers through the wall (boxes, pp.196, 203).




				Porcelain stone					See china stone.


				Porcelaneous					Get definition from Hallett.


				Potash					Potassium oxide (K₂O). An alkali which is an important flux (q.v.) in many clays and glazes. Occurs in clay materials, such as illite, in feldspar and mica, and is also an important constituent of many plant ashes.



				Potter’s wheel					Rapidly rotating device employing centrifugal force for the production of pottery. Q.v., kickwheel (box, p.16).


				Provenance					The place where a pot was made.


				Quartz					Mineral of silica (SiO₂), an important constituent of many clays and sands. When mixed with a suitable flux it forms a glaze.



				Radiography					See box p.73.


				Raku					A method of firing pots quickly using an open body. Used extensively since the eighteenth century in Japan and more recently in Britain and the U.S.A.



				Reduction, reduction firing					The opposite to ‘oxidation firing’ (q.v.): the oxygen content in the kiln atmosphere is kept to a minimum by burning damp fuel or closing air inlets. An excess of fuel and a smoky fire lead to reducing conditions. In high-temperature firings, iron oxides may be converted to black magnetite, and in low-temperature firings carbon may not be burnt out of the body. The resulting pots are often dark brown, black or grey, and iron-glazes become green or blue in colour. (Freestone boxes pp.42, 89)



				Resist					Decoration where one area is painted in a substance such as wax which resists colouring pigment or glaze when applied to the pot and therefore fires a contrasting colour.



				Rocker-stamp decoration					Decoration produced by rocking a stamp on a curved soft clay surface.


				Roller-die decoration					Decoration produced by rolling an engraved cylinder across a soft clay surface.


				Rouletted decoration					Decoration produced by rolling a small toothed wheel across a soft clay surface. In antiquity, this decoration was formed by allowing a tool to 'chatter' against the side of a leather-hard pot as it was revolving on the wheel. It can also be formed by holding a relief-decorated revolving wheel held against the turning pot.



				Rustication					Roughening the surface of a vessel, for example by pinching the clay or covering with a slurry containing coarse inclusions (q.v.).



				Saggar					Protective ceramic container in which glazed or decorated pottery is placed during firing, to avoid damage from kiln gases and contact with other pots.



				Salt					Sodium chloride (NaCl). It can be mined, but in traditional societies was usually obtained by evaporation of sea water or saline springs (boxes pp.124, 136).



				Salt glaze					A thin glaze produced by introducing salt into the kiln at a high temperature. A chemical reaction forms between the ceramic body and sodium vapour in the kiln atmosphere, and a glaze forms on the surface of the pots. (boxes pp.124, 127).




				Samian ware					See terra sigillata.


				Sand					A sediment consisting of particles between 0.06 mm and 2 mm in diameter. Widely used as a temper (q.v.) in early pottery, sand is usually thought of as consisting mainly of quartz, but its composition is in fact closely related to the regional geology. In volcanic regions, for example, sand consists of particles of volcanic rock.




				Scanning electron microscope (SEM)					A microscope based upon the interaction of a beam of electrons with the surface of a material. Gives a black and white television-type image. Very useful in the examination of ceramics because of its high magnification, the ability to focus on ‘hills’ and ‘valleys’ in a smaple at the same time, and because it provides information on the chemical elements present.



				Sgraffito decoration					Also known as sgraffiato. Decoration which is scratched through the slip (q.v.) to reveal the colour of the underlying clay.


				Silica					Silicon dioxide (SiO₂), a major chemical component of clay (q.v.) and the rocks of the earth’s crust. Occurs naturally as quartz, flint. etc.



				Silt					A dust-sized sediment, finer than sand but coarser than clay, with grains between 0.004 mm and 0.06 mm.


				Slab-building					Hand-building a pot using slabs of clay.


				Slip					A fine clay surface coating applies as a suspension of fine clay particles in water. Slips can be prepared by levigating a clay and decanting the solution after the coasre particles have settled. Slips usually has consistency of cream and may be applied by dipping the pot into the suspension, by trailing or piping, or brushing (boxes, pp.89, 191).




				Slip-casting					Forming an object by casting liquid slip into a poroous mould, usually of plaster.


				Slip-trailing					Slip decoration applied through a nozzle.


				Smudging					Firing a pot in a smoky, carbon-rich atmosphere so that the surface is impregnated with fine particles of carbon and becomes black. Also, known as smoking or smoke-firing.



				Soda					Sodium oxide (Na₂O). An alkali which occurs in the mineral feldspar and in some plant ashes, which were used to make glazes.



				Soft-paste porcelain					A European imitation of Chinese porcelain made by mixing white clay with a frit or a flux such as bone ash or talc, which vitrifies at earthenware temperature to give a white translucent body. See box p.203.


				Spalling					Flakes of clay breaking off from the surface of a pot, one cause of which is lime-blowing (box, p.136).


				Sprigging					Application of pre-formed relief-moulded decorative elements (made of clay) to the surface of the leather-hard pot. The added pieces may be moulded or modelled and are usually luted to the surface with slip. This technique was used extensively by Wedgwood in the production of his ‘jasper’ wares.




				Stamped decoration					Use of a pre-formed stamp, modelled in wood, bone etc, to impress a repeated motif into a soft clay surface.


				Stonepaste					A body made chiefly of crushed quartz (box p.114).


				Stoneware					A dense ceramic body, typically fired at temperatures in excess of 1100^oC (upto 1350^oC), so that the body vitrifies, and the glaze and body become partially fused. In the European tradition, stonewares were made from naturally occurring refractory clays, whereas in the Far East, fluxes were commonly added (boxes pp.100, 123, 184).





				Stoneware glaze					A glaze which is suitable for stoneware and porcelain bodies, fired at temperatures between about 1150 and 1350^oC. Typically has a high alumina content, and is fluxed by lime with some alkalis (boxes pp.103, 187).



				Temper					Coarse, non-plastic material added to the clay to improve working, drying and firing properties. Usually the use of this term implies material deliberately added by the potter, but it is also used to describe any coarse inclusions (q.v.) in the clay. Performs a rnage of functions including adding strength to the unfired body, modifying the working properties of the clay, even aiding drying, reducing shrinkage and, in open firings (q.v.), facilitating the escape of steam and preventing catastrophic failure of the pot.





				Terra sigillata					An orange-red pottery with a glossy red-slip surface, made in Italy and Gaul during the Roman period, and sometimes termed ‘Samian ware’ (box p.198).



				Terracotta					An unglazed clay-based earthenware body. Typically used to refer to hollow sculpted or moulded ornamental ceramics, but also used for bricks and low-fired unglazed pottery vessels.



				Thermal shock					Abrupt changes in temperature can cause minerals in a pottery body to expand or contract at different rates. This can cause cracks to develop and the pot to break catastrophically.



				Thermal stress					If there is a strong temperature gradient across a ceramic, for example when heated mainly on one side in a cooking fire, then stresses will develop which may cause it to crack and fail.



				Thermo-					Determination of the time elapsed since a ceramic (or stone) was fired and its internal ‘clock’ set to zero. When a sample of the ceramic is heated in the dating laboratory, it emits light, the amount of which is dependent upon the natural radiation to which the pot has been exposed, and thus indicates the time elapsed since firing. Thermoluminescence is a complex and demanding method of dating, which requires the accurate determination of a range of variables such as the environmental radiation dose and the internal radiation emitted by the object itself. It is widely used as an approximate technique for the identification of fakes.
				luminescence
				dating



				Thin section					See petrography.


				Throwing					The art of building up pots on the fast-spinning potter's wheel using centrifugal force. (Freestone box p.16).


				Tin glaze					More properly ‘tin-opacified glaze’, which is rendered opaque white by the presence of numerous particles of tin oxide. Tin-opacified glazes on traditional ceramics commonly contain substantial amounts of lead, due to the practice of heating a mixture of lead and tin metals to produce the oxide. Tin-opacified glazes do not appear to have been produced before the Islamic period (see boxes p.111, 117, 139).




				Tournette					Slowly rotating turntable device, used to aid the shaping of a pot formed using coiling or slab-building.


				Trace element analysis					Used to ‘fingerprint’ the clay products from different workshops by measuring the elements present at very low (trace) concentrations (boxes, pp.42, 161).


				Turning					Process of removing surplus clay from thrown pots by returning them, when leather-hard, to the potter's wheel or lathe and trimming with a sharp tool.



				Umber					A naturally occurring earth, rich in manganese and iron oxides, used as a black or brown pigment.


				Underglaze					A technique of painting colouring oxides on to painting unfired pottery which may or may not be subsequently glazed. The term is used occasionally to describe the technique of painting on to unfired glaze.



				Vegetal inclusions					Coarse material in a clay which is ultimately derived from plant or vegetable matter. Usually charred or burnt out during firing, and precise identification is not always straightforward. Commonly added as a deliberate temper and may include chopped grass or straw, rice hulls, dung, wheat chaff, etc.



				Vermillion					Bright red pigment based upon mercuric sulphiude (see cinnabar).


				Viscous					A glaze which when melted remains stiff and does not run down the pot is so described.


				Vitrification					When a clay is fired at temperatures of above about 800^oC, the body begins to melt or fuse. Very small amounts of melt form at first, which increase as the temperature is raised. When the fired pot is cooled, the melt becomes a glass. This process of conversion of the clay body to glass is known as vitrification. The glass bonds the body together and makes it less porous. Many low-fired pots have undergone very little vitrification: they are soft and porous. Stoneware is highly vitrified and is hard and impervious to fluids. Porcelain is very highly vitrified and its low iron content means that the glassy phase is colourless, producing the characteristic white translucent body. Over-firing can cause too much fusion, so that the body softens and collapses.






				Waster					A vessel or sherd that failed during firing. Commonly found in dumps on ancient pottery making sites.


				Water smoking					The removal of mechanically held water during the early stage of firing. In the firing cycle it is the period during which the kiln and wares are slowly and evenly warmed up, and it is indicated by steam being given off instead of smoke.



				X-ray					see box p.73.


				Xeroradiograph					A type of X-ray image, useful in ceramic studies because it emphasises cracks and joins.