Anth 461/761

Ceramic Analysis

Glossary of Ceramic Terms

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All glossary terms are from published sources.

Key:

  • G&W = Alex Gibson and Ann Woods, Prehistoric Pottery for the Archaeologist (second edition), 1997, Leicester University Press, London

    S= Carla Sinopoli, Approaches to Archaeological Ceramics, 1991, Plenum Press, New York.

    Sh= Anna O. Shepard, Ceramics for the Archaeologist, 1956, Carnegie Institution of Washington, Publication 609, Washington, D.C.

    Other references are taken from published sources cited in the term header.

  • a-b inversion (G&W)

    The change that occurs in quartz at 573° C and which is often thought to be a problem in pot firing and in the subsequent use of the vessel for cooking. At this temperature, quartz changes during heating from the low temperature a form to the high temperature b form, and reverts back during cooling. The coefficient of thermal expansion of quartz is high in comparison with that of most other minerals but is particularly pronounced at this temperature, jumping form 3.76% volume expansion at 570° to 4.55% at 580° and of course, undergoing a corresponding decrease in size during cooling. Changes in the size of quartz inclusions during the cooling stage after firing are the main causing of dunting.

    Adsorption (Sh)

    The adherence of one material on the surface of another, usually in monomolecular layers. A phenomenon to be distinguished form absorption. Water absorbed by clay is held on the surface of particles by loose bonding force; when absorbed, it penetrates the pores between particles.

    Anvil (G&W)

    A tool used primarily in secondary methods of manufacture and pressed against the inside surface of the wall of a vessel to support it while the exterior is worked with a beater or paddle. Ethnographic evidence indicates that various materials including stone, ceramic and even potter's hand can be used. Indentations left by the anvil can frequently be felt in the vessel wall and may be detected by X-radiography.

    Appliqué (S). Decorative technique that involves the addition of molded clay to vessel surface.

    Attribute

    A property, characteristic, feature, or variable of an entity. Attributes have "attribute states" which are scores or values assigned to attributes.

    Bevel (G&W)

    A flat, sloping area inside the rim (or lip) of a vessel frequently, though not universally, used as a platform for decoration. It does not appear to have been functional.

    Black coring (G&W)

    The dark zone sometimes found in the middle of sherds. It is caused by localized reduction during firing and really only occurs in kiln-fired vessels. Care must be taken to distinguish between real black coring, which is caused by the reduction of iron oxides within the pot walls, and the black zone that can be found in many open-fired pots and which is the result of incomplete oxidation of the carbonaceous matter present in the clay; the latter is an indicator of short firing (as there has been insufficient time to burn out this material) and therefore, frequently, of open firing.

    Bloating (G&W)

    Distortion of the body caused by the evolution of gases during firing particularly when the firing has been too rapid and sintering has occurred before organic matter has been completely burned out.

    Body (G&W)

    The term used to describe clay and any inclusions present in it, particularly prior firing, though it may also be used to describe the finished product. Synonymous with fabric and sometimes, but not always, paste.

    Body (Stoltman 1991)

    The bulk composition of a ceramic vessel, including clays, larger natural mineral inclusions in the silt, sand, and gravel size ranges, and temper. Synonymous with fabric

    Burnish (G&W)

    The smooth, sometimes faceted effect on the surface of a vessel produced rubbing leather-hard clay with a rounded tool. It can be done with a circular motion but more commonly was done with short, linear strokes; some Romano-British vessels were burnished on the wheel, producing long facets running all the way round the pot. The process compacts the surface, slightly reducing permeability, and often imparts a high shine; it is thus both functional and decorative. On occasions, mineral particles (for example, graphite or hematite) were applied to the surfaces of leather-hard vessels and burnished, the burnishing providing the most effective method of keeping the pigment in position. It is sometimes called polish (U.S.), but this term is more correctly used to describe a post-firing treatment of glazed wares. Water-worn pebbles are common burnishing tools used by modern traditional potters but other materials, such as bone, may also have been used.

    Burnishing (S)

    Finishing technique, rubbing leather-hard vessel with hard tool, such as a stone or potsherd, to produce glossy surface, with irregular luster and polishing marks visible.

    Calcareous inclusions (G&W)

    Inclusions composed of calcium carbonate limestone or shell. Such materials frequently occur naturally in sedimentary clays but were also often added by potters. Problems are incurred in the firing of these inclusions at temperatures between 650° and 900° C because of lime blowing. Rye (1976) has suggested that calcareous inclusions may be good for thermal shock resistance.

    Calcination (G&W)

    Strong heating of a substance. Some materials (for example, flint, bone and especially shell) can be crushed more easily after they have been heated. In the case of shell (especially brackish or marine shell), heating it to around 400° C drives off water that is found within the crystal lattice, resulting in macro and micro cracks within the calcium carbonate matrix. These cracks make the shell easier to crush and yield chunky pieces rather than the sharp, angular fragments that are obtained by breaking unheated shell. Calcination often turns shell and bone gray or bluish-gray.

    Calcite (G&W)

    Crystalline calcium carbonate, readily recognized by its rhombic cleavage and form. It can be scratched easily by metal (hardness 3 on the Mohs’ Scale) and reacts with hydrochloric acid. A commonly used opening material (or temper) throughout antiquity.

    Carbonaceous matter (G&W)

    Finely, divided organic matter present in nearly all clays and, in particular, in those sedimentary origin, when it is frequently responsible for gray coloring. During firing, it is burned out at temperatures above 250° C, although most effective combustion is achieved between 700° and 800° C. If not removed during firing, it may result in the black centers exhibited by sherds of open-fired vessels. If sintering occurs, particularly in kiln-fired wares, before it has been completely removed, black coring and bloating may result.

    Carination (G&W)

    A sharp change in direction or ridge in the profile of a pot, often forming a shoulder. In hand-made vessels, such sharp changes of direction are often the result of the method of manufacture, in particular, at the junction between two rings of clay. As such, they provide weak spots in the vessel wall that are likely positions for breakage to occur.

    Carination (S)

    Sharp angular turn in a vessel profile.

    Ceramic change (G&W)

    The point at which clay becomes ceramic. It is brought about by the removal of the hydroxyl groups from the chemically combined water in the clay molecules. The point at which this occurs varies according to the type of clay mineral involved but is generally considered to take place at 550°-600° C; afterwards, the clay is fired and will not regain plasticity when in contact with water.

    Clay (G&W)

    The American Ceramic Society has defined clay as "a fine-grained rock which, when suitably crushed and pulverized, becomes plastic when wet, leather-hard when dried and on firing is converted to a permanent rock-like mass."

    In general, clay is considered to be hydrated aluminum silicate and can be represented by the general formula Al2O32SiO22H2O.

    Clays are formed by the breakdown of feldspathic rocks, particularly granites, diorites and basalts. Two principal processes are concerned. When hypogenic or pneumatolytic action is involved, the feldspars are subjected to chemical processes from within the earth and decompose to form kaolinite. With epigenic action, the breakdown is primarily a physical, rather than chemical, processes, involving the action of agencies such as air, water, wind and glaciers, and occurs on the earth's surface.

    Two main clay types are generally recognized, residual (also called primary) and sedimentary (also called secondary). These terms are the geological classifications and are more accurate for archaeological purposes than the potter's terminology of earthenware, stoneware and terracotta.

    Clay (S)

    Fine-grained sediment, with particle-size less than two-thousandths of a millimeter, and which becomes plastic when wet.

    Clay minerals (Sh)

    A group of minerals forming extremely small crystalline particles and consisting essentially of hydrous aluminum silicate, although magnesium or iron partially or fully replaces alumina in some, and alkalies or alkaline earths are essential constituents of others. The principal groups of clay minerals are kaolinite, montmorillonite, and illite. Clay may contain a single clay mineral or be composed of a mixture of several, and in addition, most clays contain varying amounts of nonclay minerals.

    Clay minerals (G&W)

    There are seven main families of clay minerals, but of these only the kaolinites, smectites and illites are of importance to archaeologists. Many clay minerals are hexagonal plates. They are extremely small, frequently measuring only Angström units in size, and usually cannot be identified using an optical microscope. Their size and platey nature allow the adsorption of water between the particles to create plasticity.

    Cleavage (G&W)

    Cleavage is the ability of a mineral to break along certain planes and is determined by its lattice structure and crystal form. Macroscopically, it may allow a mineral to be identified: crystalline calcite cleaves readily into small, angular rhombs which can frequently be recognized in sherds. In thin section, cleavage may be a diagnostic optical property allowing the identification of minerals. For example, cross-sections of crystals of calcite reveal the same rhombohedral cleavage, albeit on a microscopic scale, as do hand specimens, and minerals of the amphibole family show cleavage traces that intersect at angles of 124° and 56°; this cleavage habit is not exhibited by any other mineral family.

    Closed vessel (G&W)

    A vessel in which parts of the body are of greater diameter than the rim. Amphorae are typical examples. The term is synonymous with restricted vessel.

    Coiling or coil-building (G&W)

    Method of pot manufacture utilizing long ropes of clay. The potter winds the ropes of clay round to form the desired vessel shape or part thereof and then joins them together by smoothing the internal and external surfaces. This basic shape is then usually further refined by thinning and expansion of the walls. It is generally almost impossible to distinguish it from ring-building.

    Coiling (S)

    Hand-building technique, involves forming and joining narrow coils of clay to build up vessel walls.

    Colloid (Sh)

    This term does not distinguish material of a certain chemical composition but rather material whose particle size falls in the range between molecules and microscopic particles. There is no break between these three size ranges, but for convenience the limits of the colloidal range are defined as 500 and 1 millimicron (five thousandths and one millionth of a millimeter). The colloidal range is singled out because the reduction of coarser material to this state greatly increases surface area, in consequence of which special properties are developed. Clay not infrequently includes particles in the colloidal range, the effect of which is to increase plasticity and volume of water required to bring the clay to a plastic state, and also to lower fusion point. Colloid have the property of remaining in suspension in water for long periods. They therefore increase the body of a slip mixture and aid in its application, but high colloidal content will cause excessive shrinkage.

    Compacted (Sh)

    A condition of surface, resulting from rubbing with a hard, smooth tool. The term polished is reserved for that surface quality which reflects light specularly and hence is characterized by glossiness or luster. The compacted surface is not necessarily glossy since tactual smoothness is retained after firing shrinkage has destroyed luster.

    Cord Marking (G&W)

    Cord decoration is perhaps the most commonly used decorative technique other than incised decoration on pottery. Cord is a generic term used to describe any collection of twisted fibers but, in reality, impressions labeled twisted cord vary greatly from soft and blurred, as if wool has been used, to very thin, sharp and well-defined, suggesting the use of sinew. Sometimes impressions of individual fibers can be seen.

    Simple twisted cord impressions can take the form of short lengths or encircling lines, or the whole vessel may be covered with encircling lines of twisted-cord impressions and the style is known as All Over Cord. Twisted cord wrapped around itself or another cord is termed whipped cord and similarly appears in a variety of thicknesses, lengths and degrees of definition. Short lengths of whipped cord are usually called "maggots" after their short, segmented appearance. Double cord is found on some pottery and is simply, as the name suggests, two lines of cord placed close together to form a double line. Plaited cord impressions are also found, and appear almost as a narrow herring-bone motif.

    Core (S)

    Interior portion of vessel wall, often different in color than interior or exterior surface.

    Deflocculation (Sh)

    The separation of aggregate particles into the individual particles of which they are composed. This can be accomplished with deflocculating agents such as ammonia and salts of alkali metals. Defloccuation improves the quality of slip by increasing the proportion of particles in suspension through reduction in particle size.

    Downdraft kiln (S)

    Kiln in which fuel chamber is adjacent to vessel chamber and heat passes into vessel chamber over a wall or barrier from above.

    Drying shrinkage (G&W)

    All clays shrink as they dry. After a pot has been made, it is left to dry before firing; the water of plasticity evaporates from the surfaces of the vessel and the clay particles are gradually brought into contact with one another. The finer the clay, the greater will be the shrinkage on drying. Thin parts of a vessel obviously dry more quickly than thick parts, creating stresses that cause cracks. The drying rate can be made more even and drying shrinkage reduced by the addition of opening materials.

    Dunting (G&W)

    Cracking that occurs when a fired body is cooled too quickly. It is thought to be caused by the change in volume of free silica as it cools through the a-b inversion, and therefore vessels that contain abundant quartz are most at risk. Typical dunting cracks run vertically down from the rim and horizontally around the vessel.

    Earthenware (S)

    Ceramic vessels fired to temperatures of 900-1200° centigrade. Earthenware is generally porous, and is often brown or red in color.

    Fabric (G&W)

    The term used to describe the clay and inclusions of a vessel. Synonymous with body and sometimes paste in the U.S.

    Filler (G&W)

    Inclusions deliberately added to the clay by the potter. See also opening materials and temper.

    Fire clouds (G&W)

    Black patches on the surfaces of vessels produced in open firings. They are characteristic of this type of firing and are the result of the deposition of carbon on the pot; they frequently occur where the pot has been in direct contact with the smoky part of the flame or with incompletely burnt fuel.

    Fire spalls (G&W)

    The round flakes of clay that are blown out of the walls of clay vessels when the temperature is raised too quickly during the early stages of firing. Corresponding scars are left on the surfaces of vessels. Such wasters occur in all types of firings.

    Firing (G&W)

    The process whereby clay is converted to ceramic. A temperature in excess of 550° C is required to drive off chemically combined water from the clay molecules and make them ceramic; when this process has occurred, the clay will be hard and will not become plastic when in contact with water.

    Flocculation (Sh)

    Clustering or coagulation of suspended particles. If a suspension is maintained by the repulsion of charged particles, neutralization of the charges will cause particles to gather into groups of flocks and precipitate.

    Flux (Sh)

    Any substance that promotes fusion.

    Glaze (G&W)

    Glazes are vitreous coatings consisting of a glass former (usually silica) with the addition of a glass modifier, or flux, to lower its melting point. Typical modifiers are lead, sodium and potassium. Color is achieved by the addition of metallic oxides, such as copper and iron.

    For example, early glazes on pottery in western Europe were lead-based and arrived with the Romans. This type of glaze, which was again used in the medieval period, is easily prepared by applying crushed lead sulphide or oxide to the surface of a leather-hard vessel; during firing, the lead reacts with the silica present in the clay to form a glaze. In thin section, glazes are transparent in plane polarized light, inactive under crossed polars, and frequently exhibit highly rounded voids, a characteristic of molten material.

    Glaze (S)

    Composed of silica, fluxes, and metallic oxides, glaze becomes vitrified or glasslike when fired at high temperatures.

    Grainy (Sh)

    A texture characterized by coarse temper particles. The term is proposed as a substitute for granular, which in common usage refers to the texture of material composed entirely of grains of approximately the same size range and is therefore inapplicable to tempered pottery.

    Grog (G&W)

    Crushed, previously fired ceramic used as an opening material. Macroscopically, a difference in color between grog and the surrounding matrix often serves to facilitate its identification. In thin section, grog is usually angular and is often surrounded by voids created when the wet and plastic clay shrinks away from the inert grog during the drying of the vessel. Grog may be of clay similar to or different from the matrix, and this can also be seen in thin section. Frequently, grog within grog, indicating recycling and continuity of ceramic tradition, can also be observed.

    Grog (S)

    Fragments of fired ceramics ground to small size and added to clays as temper.

    Hematite (G&W)

    Red iron (ferric) oxide, Fe2O3. It is the iron oxide that is most commonly responsible for the yellow, red or brown color of clays and of fired pottery. The characteristic liver-red color of the mineral is easily recognized and it was extensively used in antiquity to color the surface of pottery. Most frequently, the pulverized pigment was applied to the surface of a leather-hard pot and then burnished, but was also sometimes applied as a slip.

    Hearth Firing (S)

    Open-air firing technique, fuels and vessel placed together, sometimes in shallow depression. (See also open firing.)

    Illite (S)

    Three-layer clay mineral, often used for paints or slips.

    Incised decoration (G&W)

    A very common surface treatment used throughout prehistory and indeed later. It involves dragging a sharp instrument through leather-hard clay. The technique can vary considerably in sophistication and in the type of incisions made.

    Inclusions (G&W)

    The term used to describe all non-clay and/or non-plastic materials present in a clay body or fired fabric. They may be naturally occurring or added by the potter. See also filler, opening materials, or temper.

    Inflection (S)

    Point on vessel profile where vessel wall changes direction.

    Intuitive Typology (S)

    Classification of artifacts on basis of perceived similarities and differences; the variables are often defined post facto.

    Iron oxides (G&W)

    These are responsible for the color of most clays, fired vessels, and, on occasions, of glazes. They are usually derived from the breakdown of ferruginous minerals, such as biotite, amphiboles or pyroxenes in the parent rock. Ferric oxide (hematite, Fe2O3) is the most usual, but others, such as ferrous oxide (FeO) or magnetite (Fe3O4), are also important. If firing is carried out under oxidizing conditions, the iron oxides will be brought to their highest state of oxidation and the fired product will be yellow, red or brown; if, however, a reduction firing is carried out, the wares will be dark brown, black or gray. The actual color obtained depends on the original iron content of the clay and the degree of oxidation or reduction achieved.

    Haematite was used extensively for slips and for other surface coatings; it was either powdered and burnished into the surface or applied as a coating after being mixed with water. Iron was also a frequent glaze colorant in antiquity; under oxidizing conditions, it yields yellow or brown, but green under reducing conditions. Refiring experiments indicate that, for many vessels, it was not added tot he glaze mixture and the potters instead relied on reducing the iron in the clay to obtain a green glaze.

    Jar (G&W)

    Upright vessel usually with a shoulder and sometimes an everted rim. The rim diameter in invariably narrower than the maximum diameter, which occurs at the shoulder of the vessel (see also closed or restricted vessel).

    Join void (G&W)

    The gap between two coils or rings of clay that have not been thoroughly joined in manufacturing process. During drying any gaps left will increase in size as the clay shrinks. They can frequently be detected in thin section and can sometimes be seen with the naked eye in the walls of vessels that have been ring- or coil-built. Vessels tend to break along these planes of weakness, leaving rounded edges exposed, frequently referred to in the past as false rims.

    Kaolinite (S)

    Common clay mineral, three-layer clay, used in manufacture of porcelain vessels.

    Kiln (G&W)

    A structure for firing ceramics. Kiln are almost invariably associated with wheel-thrown pottery, as the finer clay bodies required for that process contain few opening materials and must therefore be fired slowly at first, so that the water of plasticity can be removed gradually and thus prevent fire spalls and explosions. Kilns consist of a firebox, a flue, a firing chamber, a dome (which may or may not be permanent) and an exhaust vent. The raw clay vessels are placed in the firing chamber, thereby being separated from direct contact with the flames from the fire.

    In contrast to open firing, kiln firing utilizes hot gases rather than direct contact with the flames to fire the pots. It is a much slower and less economical process, as much time and fuel are expended in heating the kiln structure itself. However, the end products are generally of higher quality, because finer bodies are used and higher temperatures may (but were not always in antiquity) be reached.

    Kiln (S)

    Firing facility; fuel and vessels are placed in separate chambers linked by flues.

    Leather-hard (G&W)

    The stage reached in the drying of clay when most of the water of plasticity has evaporated off, maximum shrinkage has occurred and the clay particles have come into contact with each other. It is at this stage that most surface treatments and some secondary manufacturing techniques, such as paddling, are executed, because the clay is still moist enough to allow alteration. Also known as green-hard.

    Leather-hard (Sh)

    The condition of a clay body or paste when it has become firm but not dry. It is not a state that is strictly defined, but clay workers judge it confidently from experience. A vessel in the leather-hard state can be handled without risk of deformation because the clay is no longer plastic and it can be carved of incised without chipping because it still retains considerable moisture.

    Leather-hard (S)

    Clay that has dried to the point that it has lost most of its plasticity but is still soft enough to be carved or altered.

    Lime blowing (G&W)

    Post-firing defect normally taking the form of small spalls pushed out of the walls of vessels containing calcareous inclusions. When fired between 650° and 890° C, and in particular at temperatures in excess of 750° C, calcium carbonate decomposes to lime:

    CaCo3 &endash;> Ca+Co2

    After firing, the lime so formed will absorb water from the atmosphere to form slaked lime:

    CaO+H2O &endash;> Ca(OH)2

    In doing so, it undergoes an increase in volume that forces flakes out of the surface of the fired clay. In extreme cases, for example, high temperature or excess calcareous material, complete disintegration of the vessel may result. Lime blowing is readily recognized by the presence of pits in the surface of a vessel, each containing a soft, white or yellowish inclusion. Lime blowing can be reduced or prevented by firing at a low temperature (below 750° C) or in a reducing atmosphere or by the addition of common salt.

    Lip (S)

    The edge of the vessel orifice.

    Lug (G&W)

    Projections, either raised or applied, protruding from the sides of vessels and which may or may not be functional. Some lugs may be either vertically or horizontally perforated and it has frequently been suggested that the perforations are to allow the suspension of the pots. Other lugs may be solid and their function is more difficult to speculate upon; a cord could still have been tied beneath the lugs to allow suspension. Equally, they may have been to aid the lifting of the vessel or have been purely decorative.

    Mode

    "Any standard, concept, or custom which governs the behavior of artisans..." (Rouse 1960). This definition is problematic and is superceded in most archaeological analyses by the addition of a clause which substitutes "governs" with "which reflects the behavior of the artisans".

    Molds (S). Hand-building techniques using permanent forms into or over which clay is impressed to shape vessels.

    Neck (S)

    Part of jar or restricted vessel between body and rim, marked by constriction and change in orientation of vessel walls.

    Open firing (G&W)

    Method of firing vessels without a proper kiln structure. The most common methods are bonfires or pits. Open firings are rapid and economical of fuel. They are characterized by rapid rise in temperature and frequently last less than one hour, though the duration of the firing is largely determined by the type of fuel used. Pit firing normally last longer than surface bonfires: firstly, they are usually ignited form the top or side and the fire takes longer to burn to the bottom of the stack of fuel and, secondly, the walls of the pit retain heat and so ensure a lengthier cooling period than occurs in a surface firing. Temperatures in the range 650°-900° C are commonly attained, quite sufficient to ensure that the clay minerals have been through the ceramic change. Open-fired vessels can usually be recognized by the blotched, multicolored appearance of the surface, the presence of fire clouds, and of unburned carbonaceous matter, indicative of a short firing time, in the center of sherds.

    Opening materials (G&W)

    Any inclusions whether naturally occurring or added by the potter, that occur in a clay body. When added by the potter, they may also be call filler or temper. A great variety of materials, both mineral and organic, have been used. When a vessel is being constructed and the clay is in the wet, plastic state, such materials strengthen the clay and prevent the walls from collapsing in on themselves. However, in the fired product, they interrupt the bonding of the clay particles with one another and therefore have a weakening effect: a very coarse pot is much more easily broken than a fine one.

    They also serve to facilitate drying by allowing water to evaporate more easily from the clay vessel and reduce drying shrinkage. However, the main function of opening materials in open-fired pottery is to open the body, allowing remnant water of plasticity to escape in the form of steam during the early stages of firing.

    Over firing (Sh)

    Fired to or above the point at which defects such as warping, bloating, and blistering occur. Either excessive temperature or too rapid firing can cause these defects. The term has at times been loosely applied to pottery that was assumed to have been fired higher than the average of its type, although the color difference on which judgment was based may have been due to more complete oxidation rather than higher temperatures. Ambiguity will be avoided if the expression is limited to pottery that shows firing defects.

    Oxidized Pottery (Sh)

    Pottery in which the constituents in the paste have taken up as much oxygen as they can. The color of such pottery will depend on the amount, particle size, distribution, state and combination of impurities, of which iron compounds are by far the most important. The color will be white, buff, orange or red. Pottery which has had only a short firing in direct contact with the gases formed in burning fuel is often incompletely oxidized. The interior of the vessel wall often contains unburned carbonaceous matter, even when the surface appears oxidized, or carbonaceous matter was burned out but iron compounds are not brought to their highest state of oxidation. The surface itself frequently shows different degrees of oxidation from the effect on limited areas of jets of gas from smoking fuel. Oxidation can therefore be uniform but incomplete, or uneven. The unoxidized matter may be an original component of the clay or carbon from a smoky fire. The original condition of clay, and the temperature, atmosphere, and length of firing are all factors that effect the extent of oxidation. Adjectives can be used to indicate degree of oxidation, incompletely or partially when surface is oxidized and cores unoxidized, unevenly when there is surface variation. The expression fired in oxidizing atmosphere has been used to describe oxidized pottery. In so far as it suggests greater uniformity of firing conditions that are obtained by simple methods and tends to create an oversimplified concept of the factors that affect color, it is open to criticism. For pottery description, the term oxidized is preferable.

    Oxidation (G&W)

    Under oxidizing conditions, iron oxides present in clay will be brought to there highest state of oxidation and, depending on the amount present, will give a yellow or, more commonly, a red or reddish-brown color to the fired clay. Open-fired vessels are usually not completely oxidized, because the atmosphere fluctuates during firing and the firing time involved is too short to allow for the total combustion of the organic matter present in the clay and for the iron oxides to be affected.

    Oxidizing Atmosphere (S)

    Oxygen-rich firing atmosphere.

    Oxidizing conditions (G&W)

    Firing atmosphere characterized by the presence of excess oxygen. Under such conditions, the fire burns cleanly and total combustion results:

    C + O2&endash;>CO2.

    Pottery fired in such an atmosphere will be subject to oxidation.

    Paddle (G&W)

    A tool used in conjunction with an anvil, usually as a secondary method of manufacture. It can also be called a beater. Signs of the method may be detected by X-radiography because it can impart characteristic particle orientation. Macroscopically, laminar fracture may indicate the use of a paddle.

    Paddle and Anvil Technique (S). Finishing technique; involves beating exterior of vessel with wooden paddle against stone or ceramic anvil held on vessel interior; thins and shapes vessels.

    Particle orientation (G&W)

    The methods employed in the manufacture of pottery push clay particles, voids and inclusions present in the clay into positions such that their long axes are parallel to the direction of the forces employed. Elongated particles (for example, shell fragments, mica laths, etc.) and voids only will be affected: equidimensional particles (for example, rounded sand grains) will not be subject to the forces in the same way. This orientation can be detected in a tangential thin section and by X-radiography and is characteristic of the manufacturing method employed. In wheel-thrown vessels, the characteristic orientation is diagonal; the faster the wheel is rotating, the steeper will be the angle of orientation away form the horizontal. In ring or coil-built pots, the particles become aligned with their long axes running horizontally and parallel to the rim and base of the vessel.

    Paste (S)

    Clay or mixture of clay and added materials.

    Paste (Stoltman 1991)

    The aggregate of natural materials, i.e., clays and larger natural mineral inclusions, to which temper was later added to produce the body from which a vessel was made.

    Permeability (Sh)

    A property which permits the flow of liquids or gases through a body. It differs from porosity in that not all of the pores enter into the system of connected capillaries that extends from one surface to the other. It is measured in terms of volume of fluid that will pass through a given area of unit thickness under specified head pressure.

    Pinching (S). Hand-building technique, involves forming vessel by opening clay ball and pulling vessel walls up between fingers.

    Pinch pottery (G&W)

    The simplest method of pottery manufacture, involving the opening out and expanding of a ball or cone of clay by squeezing the clay between the fingers, while the shape is supported by and turned in the potter's hand. It tends to result in small, round-based, open shapes (such as bowls), in which the method of manufacture can be recognized by the indentations in the vessel walls left by the pressure of the potter's hands. It can, however, be used as a preliminary method of manufacture, the shape so formed being added to later by the addition of coils or rings of clay: present-day Gciriku potters in the Kavango region of Namibia start all their vessels by forming the base in this way, adding flattened rings, or straps, of clay and finishing the vessels by using a tool known as a rib. It has generally been thought that pinch pottery was probably not used in antiquity or, if the technique were employed, it was purely for the manufacture of small vessels, but this is to deny the possible use of the technique as a preliminary method of manufacture, to be supplemented by other methods, as in the African example give above; as with other less immediately obvious methods of manufacture, it may have been missed because such researchers have not known what to look for.

    Pit firing (S). Firing technique in which fuel and vessel are placed together in an excavated pit, sometimes covered with stones or earth.

    Plastic decoration (G&W)

    Decoration that is modeled on the wall of a vessel (see also appliqué).

    Plasticity (G&W)

    The property of clay that allows it to be deformed by pressure and to retain the shape created once the pressure has been removed. It is the result of water being present between the clay particles, which allows them to glide over each other. The smaller the clay minerals involved, the more water can be absorbed between the particles in a give volume of clay; therefore, sedimentary clays, the particles of which have been physically broken down by transport, are more plastic than residual clays.

    Plasticity (S)

    The ability of clay to be molded and maintain its shape.

    Pleochroism (G&W)

    An optical property shown by some minerals when examined in thin section with plane polarized light in the polarizing microscope. Pleochroic minerals are colored and change color when the microscope stage is rotated. A typical example of a pleochroic mineral is biotite; when its cleavage traces are parallel with the north-south crosswire in the microscope, it is typically yellow or light brown in color, but when the crystal is turned through 90°, so that the cleavage is parallel with the east-west crosswire, it usually exhibits a dark brown color.

    Porcelains (S). Ceramic vessels fired to temperatures above 1350° centigrade, with vitrified bodies, usually white and translucent.

    Porosity (Sh)

    Volume of pores expressed as a percentage of total volume. True porosity is calculated from total pore volume, that is, volume of both open and sealed pores. Apparent porosity is calculated form volume of open pores alone- those pores that will absorb water when the piece is soaked. Sealed pores are negligible in low-fired pottery, hence apparent porosity closely approximates true porosity for such wares. Porosity should not be confused with permeability.

    Preheating (G&W)

    A process sometimes used to warm clay vessels prior to open firing. It is aimed at heating them gently through the water smoking stage, thereby driving off remnant water of plasticity, before firing proper is commenced. Archaeologically, it is unlikely to leave any remains different from those of an ordinary hearth and will probably always be impossible to recognize. However, there is much ethnographic evidence for the use of this technique and archaeologist should be aware that it might have been used.

    Polish (Sh)

    That surface quality which reflects light specularly and hence is characterized by glossiness or luster.

    Polishing (S)

    Finishing technique, rubbing vessel with hard tool, such as stone or potsherd, to produce uniform and very glossy surface.

    Porosity (S)

    Number and size of pores or voids in a fired vessel.

    Primary Clay (S). Clay found in close proximity to its parent rock source (see also residual clay).

    Quantitative Typology (S). Ceramic typology based on statistical distribution of two or more variables.

    Reduced Pottery (Sh)

    Pottery which the iron oxide is present in the lower state of oxidation. The color is gray, but not all gray pottery is reduced. It may be colored by unoxidized carbonaceous matter or by carbon deposited in firing. Pottery cannot be positively identified as reduced without firing tests. The expression fired in reducing atmosphere has been used to describe reduced pottery and sometimes loosely extended as a blanket term for all gray pottery. It is open to the same criticisms as the expression fired in oxidizing atmosphere.

    Reducing Atmosphere (S)

    Oxygen-poor firing atmosphere.

    Reducing conditions (G&W)

    Firing atmosphere characterized by the shortage or absence of oxygen. Under such conditions, combustion will be incomplete and carbon monoxide will be created.

    2C + O2 &endash;> 2CO

    Reducing conditions are almost impossible to maintain in an open firing because the atmosphere is constantly changing, but if such conditions are sufficiently stringent in a kiln, reduction of the iron oxides in clay bodies will occur.

    Reduction (G&W)

    The carbon monoxide evolved under reducing conditions has an affinity for oxygen (i.e., it is oxygen hungry) and will attempt to obtain it from any available source; if none is present in the firing atmosphere, it will take it form the iron oxides in the clay bodies being fired, removing some of the oxygen (not all, as is the case in metal smelting) and reducing them to lower oxides which are dark (brown, gray, or black) in color

    Fe2O3 + CO &endash;> 2FeO + CO2

    or

    3Fe2O3 + CO &endash;> 2Fe3O4 + CO2

    For this reason, properly reduced wares usually exhibit one or more of these colors, depending on the amount of iron oxide present in the clay and the stringency of the reducing atmosphere. Other factors can intensify the colors. For instance, if the atmosphere in the kiln is reducing, some of the fuel and the carbonaceous matter present in clay will not be burned during the firing and a smoky atmosphere, with plenty of free carbon, will result; this carbon, in the form of soot, may be deposited on the pots or within the pores of the fabric, intensifying the dark color. (This is also what happens in smoking or smudging.)

    Reduction is as yet not fully understood, even by those in the ceramics industry. Unfortunately, archaeologists have an even less comprehensive grasp of the problem, to the extent that the literature is full of statements to the effect that, if a pot is black, it has been reduced. This is particularly true of may reports on prehistoric pottery. However, although superficial, patchy surface reduction can sometimes be achieved; it is not possible to produce completely reduced wares in an open firing, as the atmosphere fluctuates rapidly and constantly within the fire and cannot be controlled for any length of time. Consequently, black patches and surfaces of pottery produced in such firings are usually the effects of smudging and not of reduction. Black coloration in the center of open-fired sherds is the result of incomplete oxidation, not reduction. Proper reduction only occurs when the iron oxides are affected, usually over quite length periods of time and at temperatures in excess of 850°C.

    Repair holes (G&W)

    These can sometimes be seen in the wall of a pot, usually high up in the profile, and consist of a pair of drilled holes made after the pot has been fired and has broken in use. Through these holes, cord or sinew could be passed and pulled tight to bind together a crack or fracture in the vessel wall. Where only a single hole exists, signs of wear can often be seen on the side of the repair hole to indicate the direction of binding. Care must be taken in the identification of these features, as they may also may be confused with holes designed to facilitate the fastening of a cover over the pot or to allow the suspension of the vessel.

    Residual clay (G&W)

    A clay that has not been transported from its site of formation. Also known as primary clay, such clays generally consist of relatively large particles and are accordingly less plastic than sedimentary clays. The china clay deposits around St. Austell in Cornwall provide an excellent example of a residual clay; they are very pure, consisting almost entirely of kaolinite, and contain little coloring matter and few organic impurities. As a result of this purity and large size, residual clays are not very plastic and require a very high firing temperature; they were not often utilized in prehistory for the manufacture of pottery.

    Residual clay (Sh)

    A clay occurring in the same position as the parent rock from which it was formed. Igneous rocks are the primary source of clay and residual deposits derived from them are classed as primary clays.

    Restricted vessel (S)

    Vessel form, or profile, characteristic by narrowing or constricting of vessel form in between base and rim. Often applied to vessels where the maximum body diameter exceeds the rim diameter (see closed vessel).

    Rilling (G&W)

    Grooves and ridges found particularly on the inside of wheel-thrown vessels, created by irregularities in the clay catching on the potter’s hands as the walls of the vessel are drawn up and expanded during manufacture. They tend to be more pronounced in closed vessels, as the shape of the pot prevents the potter from reaching inside to smooth them out or remove them. They can be an indicator of the direction in which the wheel was turning during manufacture: the vessel should be held and turned in a horizontal plane; if the rilling grooves are spiral from top right to bottom left, the wheel was rotating in a counterclockwise direction.

    Rim (S)

    Upper part of vessel, near orifice.

    Ring-building (G&W)

    A method of vessel construction involving the use of rings of clay. It is similar to coiling, except that a ring is only long enough to go once around the diameter of the clay vessel, while a coil is frequently applied as a longer roll of clay spiraling around the vessel. Vessels tend to break along the joins between the rings if they are not securely bonded because the junctions provide planes of weakness. A stronger bond should be achieved by applying the rings or coils on a diagonal or by using a tongue-and-groove technique, as these result in a greater surface area for binding.

    Scraping (S).

    Finishing technique which involves scraping leather-hard vessel with a tool held perpendicular to the vessel to thin or shape vessel.

    Secondary Clay (S).

    Clay that has been transported some distance from its parent rock source (see sedimentary clay).

    Sedimentary clay (G&W)

    A clay which has been transported from its site of formation. Movement of the clay (for example, by water, wind or glacial action) brings about a physical breakdown of the clay particles, with the result that sedimentary clays have greater plasticity than residual clays. They usually also contain more organic material and mineral and rock inclusions as a result of being moved. Although there are some exceptions, sedimentary clays are the types of clays that would normally have been used for the manufacture of prehistoric pottery: residual clays are usually too pure, contain too few inclusions and require too high a firing temperature to be used for open firing. See also secondary clay.

    Seriation (S).

    Relative dating technique, based on changes in relative frequencies of types or variables over time.

    Sherd (shard) (S)

    Fragment or piece of broken vessel.

    Shrinkage (G&W)

    Clay shrinks during drying and, at temperatures in excess of about 900° C, during firing. During drying, shrinkage is a direct result of the amount of free water present and fine clays will shrink more than coarse ones. The smaller the clay particles, the greater the amount of water that can be absorbed between the particles to make the clay plastic, and therefore the greater will be the shrinkage as the clay dries. Drying shrinkage can be reduced by the addition of opening materials, which effectively take the place of clay in a given volume. Less water will therefore be needed to produce a workable body and, as a consequence, less shrinkage will occur as the clay dries. Drying shrinkage frequently results in voids around opening materials.

    Firing shrinkage occurs as the clay particles begin to melt at high temperatures. It rarely occurs at temperatures below about 900° C; in contrast, most clays actually expand by small amounts, becoming increasing porous during the early stages of firing up to about 800° C. Few vessels produced during prehistory are likely to have been affected by firing shrinkage.

    Sintering (G&W)

    The stage in firing when the edges of the clay particles begin to soften and melt and stick together. It represents the initial stage in the vitrification process and results in a harder, denser and more rigid body.

    Sintering (Sh)

    Heating to the point of incipient vitrification, at which some cementing of particles occurs as a result of the softening of constituents of the body.

    Slab building (S).

    Hand-building technique which involves forming flat slabs of clay and connecting them to form a vessel. 

    Slip (G&W)

    Surface treatment involving the application of a suspension of clay in water to a vessel in the leather-hard stage. It can be both decorative and functional: it is most usually employed to change the color of a vessel but may also be used to reduce the permeability of food and drinking vessels by partially sealing the surface. Slips are applied in a viscous state by dipping, pouring or painting. They are not absorbed to any great extent by the clay body and in thin section are easily distinguished as an optically inactive, separate layer on the surface. Slip-trailing, a technique akin to icing a cake, was also used to build up plastic features on the surface of a vessel or to add different colored slips.

    Slip (S)

    Liquid mixture of clay and water applied over surface of vessel to affect color and texture.

    Smectite (S)

    Three-layer- clay mineral, often used for paints or slips.

    Smoking or smudging (G&W)

    A technique used to deposit carbon on and immediately below the surface of a vessel in order to turn it black. Although macroscopically similar to slip, in thin section it can often be seen to be a much thinner and less regular layer.

    Smooth (Sh)

    A quality of surface texture. Since there are degrees of smoothness, smooth is proposed for the texture obtained by rubbing a leather-hard surface with a hard, extremely smooth tool. The carefully wiped surface of a fine-textured, plastic clay may appear smooth, but the burnished surface is much smoother to the touch.

    Smoothing (S)

    Finishing technique, lightly rubbing leather-hard vessel with hard tool, such as a stone or potsherd, to produce smooth but non-glossy surface.

    Spalls (G&W)

    Flakes of clay pushed or blown out of the surface of a vessel. See fire spalls and lime blowing.

    Stonewares (S)

    Ceramic vessels fired to temperatures of 1200-1350° C. Stonewares have partially vitrified bodies, and most often are brown, gray or white.

    Surface treatment (G&W)

    Any decorative of functional method used to alter the surface of the vessel; examples include: burnish, slip, glaze, incision, and cord-marking.

    Suspension (Sh)

    Dispersion of a finely divided solid in a solid, liquid, or gas. Contrasted with solutions which are characterized by molecular dimension of the dispersed matter. Used with reference to clay, dispersion of fine clay particles in water.

    Temper (G&W)

    One of the terms used to describe opening materials added to clay by the potter. Synonymous with filler, the term has become subject to misuse in recent years.

    Examination of pottery fabrics in thin section is the best way to determine whether inclusions have been added or are naturally occurring in the clay: the nature, shape and size of the inclusions in relation to the clay matrix may yield results. Tempering can be distinguished, at least in most cases, by a bi- or multi-modal distribution of inclusions when considered from the perspective of inclusion size, mineral property, or angularity. Temper particles are often (but not always) polymineralic (comprised of more than one mineral). As a generalization, ceramics demonstrating a consistent unimodal composition would be classed as naturally occurring and being without temper.

    Unfortunately, interpretation of pottery fabrics in thin section is not always so simple and many confusing cases arise. Rounding of inclusions is sometimes thought to indicate that they are naturally occurring, the argument behind this being that transportation of clay (see sedimentary clay) not only breaks down the clay particles, but also abrades the rock and mineral fragments present. However, there are obvious problems with this argument, one of which is created by sand grains which, as they may occur in deposits formed by the action of wind and water, have been subjected to considerable rounding; they may subsequently be added to clays by potters. Comparison with the background matrix may, however, help to solve this problem. For example, large rounded inclusions in an otherwise homogeneous paste would also be classed as temper (or added opening materials) because, even though they are highly rounded, they occur in a similar size range, exhibit the same degree of rounding and are different in both size and shape from the smaller, more angular inclusions in the clay matrix (see G& W Figs. 215-216). This fabric would be described as bimodal. An obvious bimodal distribution of inclusions suggests that they were deliberately added, even if the rounding shown by some would often be used as an argument for their being natural inclusions.

    In contrast, the pronounced angularity and numerical frequency of inclusions is not a guarantee that they are additions (i.e., temper). If all of the particles are monocrystalline (i.e., all exhibit the same mineral or crystalline property) this is taken as an indication that the crystals have not been derived from crushed rock and their angularity is therefore probably natural. Consequently, it is difficult to determine whether these inclusions occur naturally in a clay which has been transported far from its site of formation or have been added from a sand deposit occurring close to the parent rock.

    The nature of pot fabrics and their interpretation through the use of thin sections can therefore be seen to be full of complications. In theory, prehistoric pottery, as a result of its coarse and open nature, should provide plentiful opportunities for provenience studies through the identification of rock fragments present. However, the nature of the most common inclusions (such as grog, chert, volcanic ash, and quartz sand), and the fact that they rarely exhibit any optical properties which allow them to be traced to a particular geological source, have limited the successful use of basic thin-section studies for specific provenience identification.

    Temper (S)

    Nonplastic inclusions deliberately added to clays to improve workability and affect other characteristics of fired vessels.

    Terracotta (S).

    Ceramic vessel fired to temperature of less than 900° centigrade, coarse and porous, usually red.

    Texture (Sh)

    A property determined primarily by the particle size, shape, grading and arrangement of particles. Surface texture is also influenced by finishing method. The expression apparent texture is proposed for paste descriptions based on unaided visual inspection since the finest grades cannot be seen and relative color contrast influences judgment. In defining particle size by microscopic methods, the Wentworth grade scale is used.

    Thermal shock (G&W)

    When vessels are repeatedly heated and cooled, they may become subject to thermal shock. In cooking pots placed directly on the fire, the side of the vessel in contact with the heat will expand more than the inside surface of the vessel, which is considerably colder, as it is in contact with the substance being heated. Small cracks develop and spread from the cooler surface into the body of the vessel, weakening it and eventually causing breakage. Thermal shock is a problem in modern high-fired fine wares which are oven &endash;but not flame &endash;proof: because they contain few non-clay inclusions and are fired to temperatures high enough to promote vitrification, their bodies are rigid and cannot accommodate rapid heating and cooling. Open firing usually results in very porous wares with little vitrification of the fabric and such bodies are able to expand and contract repeatedly without suffering damage.

    Thermal shock resistance (G&W)

    Resistance to thermal shock can be increased in several ways. Firstly, vessel shape is considered to be important and a round-based, globular pot with no sharp carinations should be able to expand and contract more evenly. Secondly, a porous body, with large voids, will ensure that crack propagation is limited. Thirdly, inclusions present in the body which have a low coefficient of thermal expansion, or one similar to that of fired clay, will provide good resistance. For this reason, grog, calcite and plagioclase feldspar are supposed to be good inclusion types for cooking vessels, while quartz, which has a high coefficient of thermal expansion, is not.

    Thermal Stress Resistance (S)

    Ability of vessel to withstand repeated expansion and contraction as a result of heating and cooling.

    Thin section (G&W)

    A slice of pottery used by ceramic petrologists for examining the mineral content and technology of pottery. The method has been borrowed from geology and is essentially the same as that employed in the examination of rocks. A slab of pottery about 4 mm thick and at least 2 cm long is consolidated and one side is polished to provide a smooth face. This is mounted on a microscope slide and the pottery is then ground away until it is 30 microns thick: at this thickness, quartz and feldspar inclusions will be pale yellow, white or gray when viewed under cross-polarized light (XPL) in the polarizing microscope, these colors indicating that the correct thickness has been obtained. The section is then trimmed and covered with a glass cover slip. Slices of pottery taken for thin sections are usually cut vertically down the pot, perpendicular to the rim and base, and this type of section is known as a normal or vertical section. On occasions, horizontal or tangential sections may also be used, the latter being essential for the determination of the method of manufacture.

    At the correct thickness, most of the minerals present in a thin section will be transparent and will exhibit a range of optical properties that allow their identification. Some of the properties frequently used are shape, cleavage, pleochroism and twinning. Thin-section identification of mineral and rock fragments can provide evidence on the origin of pottery and they can also be used to determine whether opening materials are naturally occurring in or were added to the clay (see temper). It is, however, important to remember that the technique only provides information on the inclusions present and not about the clay: most clay minerals are too small to be identified with an optical microscope and, in addition, may be destroyed during the firing process. Provenience studies conducted using thin sections, therefore, are concerned only with inclusions present in the fabric and cannot provide concrete evidence of the origin of the clay itself. Problems are also encountered in the study of sandy fabrics, because quartz has no distinguishing features in thin section, regardless of origin; it is therefore not possible to determine the provenience of sandy wares in this way. Thin sections are also of use in the identification of methods of manufacture (see particle orientation) and can also be used to examine surface treatments and to obtain information on firing.

    Trace elements (S)

    Elements found in very small quantities in clays and vessels.

    Tradition (S).

    Consistent approach to vessel manufacture that persists over relatively long time period. The term tradition is generally applied in the context of a classification.

    Type

    a) "A number of things... sharing certain characteristics that cause them to be regarded as a group;" or b) "A thing regarded as a member of a class or category."

    Type-variety method (S).

    Method of ceramic typology in which vessels are grouped into successively finer categories on basis of some combination of variables, including: paste, decorative treatment, surface finish and location

    Unrestricted Vessel (S)

    Vessel form or profile characterized by open form with no narrowing or constrictions between base and rim.

    Updraft Kiln (S)

    Fuel chamber directly below chamber holding vessels.

    User Life (S).

    Length of time that vessel is used for primary function.

    Use wear (S).

    Traces on vessel formed as result of use, for example, charring on cooking vessels.

    Vitrification (G&W)

    The formation of a glass and the stage in firing at which the clay particles actually begin to form glassy melts. These flow into the pores between other particles and the fluxing process continues. On cooling, it produces a less porous, harder body. Extreme vitrification results in melting of the clay to such an extent that the pot collapses. Vitrification occurs at relatively high temperatures, which vary according to the types of clay minerals present. However, it rarely occurs below 900°C and consequently is usually not seen in prehistoric pottery.

    Vitrification (Sh)

    The formation of glassy material in a ceramic body. Vitrification is considered complete when pore spaces are filled and exterior volume is reduced to a minimum. Vitrification range, The time temperature period between incipient vitrification, when glass can first be detected, and over firing or deformation caused by increased fluidity of constituents of the body.

    Void (G&W)

    A hole or space in a fabric. Voids can be created in several ways. Firstly, they may be the result of drying shrinkage; if so, they may me visible in the thin section, surrounding opening materials in the clay. Many opening materials are inert and thus are not affected by the water added to make the clay plastic; however, the clay itself contracts as it dries, shrinking away from the inclusions and, as a consequence, voids occur around them. Other voids, also caused by shrinkage during drying, may be evidence of the method of manufacture. Others may be caused by the burning out of organic material, such as grass, during firing, or by the dissolution of inorganic material, such as calcite or gypsum during burial. Vitrification also creates voids, because the clay melts, bubbles and shrinks.

    Ware (S)

    Class of pottery characterized by similar technology, material, and surface treatment.

    Waster (S).

    Blistered or deformed vessel (or vessel fragments [sherd]), formed as result of firing errors.

    Waster (G&W)

    A vessel damaged during manufacture, in particular during firing. The most common causes of wasters are fire spalls, but others, such as bloating, dunting and overfiring, are also common. Wastage did not, however, necessarily render vessels unusable and the large number of spalled vessels excavated from sites of all periods indicates that many were in regular use.

    Water of plasticity (G&W)

    The water that is mixed with clay to enable it to become plastic. It acts as a lubricant, allowing the clay particles to slide over one another. Water is responsible for the greatest problems that occur during firing and accounts for most firing wasters: it cannot be completely removed by drying at room temperature, and a small amount, usually between 3 and 5 percent, always remains, regardless of how lengthy the drying period has been. It is this remnant water that causes fire spalls. Water of plasticity is removed during the water smoking stage of firing.

    Water smoking (G&W)

    The stage in firing around 100°C when remnant water of plasticity in the clay boils and turns to steam. This is the most dangerous stage in both kiln and open firings, when the likelihood of wastage through spalling and explosions is greatest. As the water boils and turns to steam, it increases in volume and, if it cannot escape through the pores in the clay, can exert sufficient pressure to blow flakes of clay (fire spalls) out of the walls of the clay vessels. The problem is overcome in kiln firings by raising the temperature of the kiln slowly, thereby gently heating the vessels within. In open firings, where temperature control is not possible, spalling does not occur if the fabric of the vessels is sufficiently coarse, i.e., contains relatively large amounts of opening materials.

    Water smoking (Sh)

    Removal of mechanically held water during the early stages of firing. The term is derived from the fact that moisture in the form of condensed steam appears as white smoke issuing from the kiln. If the temperature is raised too rapidly, the evolution of steam may cause spalling or cracking. In commercial work, a temperature of 120°-130° C is commonly used for water smoking, and the time required for it varies, depending partly on density of clay. Native potters often recognize the importance of water smoking and preheating their pottery or warming it gradually.

    Wheel-thrown (G&W)

    The term used to describe vessels that have been made on the potter's wheel. Such vessels are usually, though not always, flat-based. They can be recognized by the presence of rilling as revealed in thin section and by X-radiography, and frequently, by the presence of cheese-wire marks on the base. The term is not synonymous with wheel-turned, as turning is a secondary process used to tidy up vessels in the leather-hard state; the two processes are therefore quite different and require different terminology.

    Writers in Britain frequently refer to slow wheels and fast wheels, the inference being that the former is a simple, one-piece device, sometimes called a tournette or turntable, and the latter the more sophisticated wheel with two parts (a flywheel and a wheel-head) with which we, in the twentieth century, are all familiar. To make such a distinction in terminology is clearly nonsensical: any type of wheel can be made to rotate either quickly or slowly and, as long as sufficient speed has been obtained for centrifugal force to be created, vessels can be spoken of wheel-thrown. It is worth noting that the highly accomplished vessels of the Geometric, Archaic and Classical periods in Greece were all thrown on what many modern writers would describe as a slow wheel.

    Yield point (Sh)

    The shearing force that will just cause flow of a plastic mass. The more plastic clays have high yield points and long extension before fracture. Yield point is affected by the binding force of surface films and by extensibility which results from sliding on each other of platelike particles lubricated by water.

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