Unlike metals and glass, pottery can not be melted down and re-used - once broken, it is thrown away. Also unlike metals, it can not corrode, and unlike wood and other materials, pottery can not rot. Hence it is generally the most common find on a particular archaeological site, and in some cases it may be the only artefact found at a site.

Because clay is easy to shape and decorate, pottery always reflects contemporary tastes and cultural ideals and so it is an ideal subject of art-historical study. Different shapes and decorations were used at different times, by different people, and for different purposes. So by studying pottery, archaeologists can date their sites and say a great deal about ancient cultures. It provides information about technology, craft specialisation, trade, industry, art, diet and a host of other attributes of ancient cultures.

WHAT IS POTTERY?: Pottery is most often made of clay, a naturally occurring fine-grained sediment. To be useful for pottery, the clay must be mostly made up of clay minerals. Clay can be made very soft and plastic when it is wet, and may be formed into any shape. To turn clay into pottery it must be fired by being made very hot in a fire or kiln. Clay minerals include water in their atomic structure. When clay is fired the structure of the minerals is destroyed forever, and they become a glassy mass. This fixes the pottery in its shape permanently.

There are three main types of fired-clay ceramic. Earthenware is a relatively low-fired clay ceramic (about 700-1200 deg. C), stoneware is fired to a higher temperature (about 1200-1300 C) and porcelain is a highly fired ceramic (about 1300 deg. C). There are various types of clay mineral, and not all can be fired at higher temperatures. For instance porcelain contains a high proportion of the kaolin clay.

Another important type of ceramic body is primarily made up of crushed quartz. This includes ancient Egyptian and Mesopotamian faience, which was generally a mixture of quartz and glass; stonepaste from the Islamic world which is a mixture of quartz, glass and clay; and European softpaste porcelain which is technically derived from stonepaste. For these ceramic bodies the glass fuses the body together in the firing.

An important aspect of the firing of the vessel is the fugacity of the kiln. This refers to the amount of oxygen in the kiln. An oxidizing environment has oxygen while and anaerobic or reducing environment has no oxygen.

GLAZES: Ceramic glazes are a type of anthropogenic glass, and are predominantly made up of silica. As such it represents a material in which the atoms and molecules are floating around in an unstructured manner. Silica is obtained as a raw material in the form of the mineral quartz. Quartz, however, has a very high melting temperature, between 1610 and 1713 deg. C. So to enable the quartz to melt at a lower temperature a flux must be added, just as salt enables ice to melt at a lower temperature. Important fluxes for ceramic glazes are lead, the alkali elements sodium and potassium, and lime or calcium oxide.

Lead glazes generally contain 50-60% of lead, probably obtained from lead ores such as the lead-sulphide galena. These appeared in about the first century BC and formed a widespread technology, being used across the Silk Road from the Roman Mediterranean through Soghdian Central Asia to Tang China. They become the most widely produced glaze type across Europe and the Islamic world in the medieval period.

Alkali glazes may contain up to 20-30% alkali elements such as soda and potash. These were originally developed in the ancient Near East from the 4th millennium BC onwards, first on faience bodies and after about 1500 BC on clay bodies. In Egypt a mineral source of soda, natron, was commonly used, but across the rest of the Middle East plant ash was the source of alkalies. In China a mineral source of alkalies, the potash- or soda/lime-alumino silicate minerals called feldspars, were often used in combination with lime. In Europe after 1200 AD salt (sodium chloride) was used for high-temperature stoneware.

Occasionally pottery will have combinations of lead and alkali fluxes. An important example of this is the tin-opacified glaze, often called a tin-glaze in the literature. This technique was developed in southern Iraq in the eighth century AD, and spread to the rest of the Middle East and Europe. It is important to realize that while a lead glaze or alkali glaze refers to the flux, a tin-glaze refers to the opacifier, which is one of the reasons I call it tin- opacified.

There are many ways of applying a glaze. In the Islamic world the flux elements would be heated with the quartz to form a glass which was crushed to a powder then suspended in water. This glaze mixture was then commonly poured over the vessel, or the vessel was dipped in the glaze. It may have been necessary for the alkali-glazed wares to have a preliminary or bisque firing in which the vessel was fired without a glaze, then a secondary or glostfiring to fix the glaze. In medieval Europe the lead appears to have been dabbed onto the vessel surface as a powder. A Chinese technique was to throw plant ash onto the pots in the kiln, while for European salt-glazed stoneware the salt was just thrown into the kiln during the firing.

COLOURANTS: paints, slips and glazes themselves can be given a specific colour by inclusion of a particular mineral oxide. A dark blue is given by cobalt; manganese produces black or purple; chromium produces black or dark green; copper was one of the earliest colourants and commonly gives a turquoise colour in an alkali glaze and green in lead glazes; iron may produce black or red if oxidized, but in reducing conditions it turns pale green and is the basis of celadon glazes; antimony forms a bright yellow; and tin is white.

SLIPS: A slip is typically a clay suspended in water which can then be poured over or painted onto a vessel. These are generally used to give a pot a finer surface, or a different colour to that of the main body of the ceramic. The use of slips dates to the first development of ceramics. Vessels can be decorated by having designs painted on with slip, or by having designs incised or craved (excised) from the slip. The same effect was obtained in the Islamic world by using stonepaste as a slip, either as a white covering over a red clay body, or for a finer whiter surface over a coarser stonepaste body

UNDERGLAZE PAINT: Although slip-paints are also applied under a glaze, this term refers specifically to vessels in which a paint with no mounting medium, no clay or other material, is used. Oxide colourants are applied to the surface of the vessel, and then the vessel is covered with a clear alkali glaze. This technique was first developed in Syria at centres such as Damascus in the early 12th century AD.

LUSTRE-PAINT: Lustre-painting is a unique contribution of the Islamic world. It became an important paint for ceramics from about 800AD, but was used on glass in Egypt and Iraq before this date. Lustre pigment is a compound of silver, copper and iron oxide in a refractory earth kept together by gum. It is applied to the glazed surface of a previously fired vessel. The painted object is then refired to a red heat in a reducing atmosphere in the small specially constructed kiln used only for this purpose. In the second "firing" the metals are bonded to the glaze as a thin layer with a strongly metallic lustre, the refractory earths can then be brushed off. If any oxygen gets into the kiln it will hopefully join with the iron-oxide which it prefers. If too much oxygen gets in, the copper and silver will oxidize, volatilize, and be driven off. If the temperature is too low, the metals will not fuse to the surface; too high and the entire pigment will fuse, forming a blackened paint.

Basra Blue-painted ware: this type is often associated with the Abbasid dynasty or with the site of Samarra in Iraq. It was produced in Basra from about 700 to about 975 AD, and has a yellow fired clay body, a white tin-opacified glaze and a cobalt blue paint applied in a glaze medium over the vessel glaze.

Basra Lustre-painted ware: this is the "Abbasid" or "Samarran" lustre-ware of much of the earlier literature. It was made in Basra between about 800 and 975 AD, and has a yellow fired clay body, a white tin-opacified glaze and lustre-paint applied over the vessel glaze. Earlier examples may have different coloured lustre- pigments, but those of the tenth century are monochrome.

Slip-incised ware: this type is often referred to as "sgraffiato" in the literature. This term is apparently an Anglicised version of an Italian word for scratched, so essentially any scratched or incised material could and has been referred to as sgraffiato, although the term original came into the English language in specific reference to Italian slip-incised wares. The type will commonly have a red or pink clay body, and overall white or buff slip, and decoration executed by incision through the white slip to reveal the body. The vessel is then covered with a high-lead glaze. A related type is slip-excised ware, in which large areas of slip are carved or excised from the vessel. The type appears to have first developed in Iraq in the late 8th century, perhaps related to imports of Chinese Sancai wares. The simple technology became widely adapted across the Islamic world and various regional types are known.

Fustat Lustre-painted ware: this is the important Fatimid period elite ware of Egypt. It appears to have been produced from about 975 to 1175 AD, although the finer more art-historically significant wares are almost entirely dated to the first half of that range. These will have a buff or pink clay body derived from manipulation of Nile sediments, or a stonepaste body, or a body reflecting prototypical stonepaste technology; the glaze is always tin- opacified.

Incised ware: This type appears to have first developed in Egypt in the early eleventh century, perhaps provoked by imports of Chinese Song porcelains although the first Egyptian wares are in no way related to Chinese prototypes in their form or decoration. The type will become the numerically most dominant type among finer wares in Syria and Iran in the 12th century. These wares all have a white stonepaste body, decoration executed by incision into the body, and an overall transparent glaze which is a lead-alkali hybrid glaze in Fatimid Egypt and early Syrian wares, but is an alkali glaze in later Syrian and in Iranian wares.

Syrian Lustre-painted wares: Various styles of lustre-ware developed in Syria. The first of these was the "Tell Minis" style, of unknown production origin but probably made in western Syria in the last quarter of the eleventh century, and probably made by potters from Egypt. These wares have a stonepaste body and either a transparent alkali-lead glaze or less commonly a tin-opacified glaze. All later Syrian wares have an alkali glaze. Late 12th and early 13th century Syrian lustre-wares are often called the "Raqqa" style, and this site does appear to be the prominent production centre for the type. Damascus becomes the only production centre after 1250 AD, usually having a dark blue or turquoise glaze.

Underglaze-painted ware: Although slip-paints are also applied under the glaze, this term is uniformly used in reference to vessels which have been painted solely with a colourant with no clay or quartz medium. The paint is then covered with a transparent alkali glaze. The technology appears to have developed in Syria, probably Damascus, in the early 12th century, and from the beginning a wide range of colours was used. In Iran the 12th century "Silhouette" ware actually had the black pigment in a quartz-slip medium. The full technology reached Iran in the early 13th century, but the colours were not as diverse as the Syrian. An important Iranian style of the early 14th century is often called "Sultanabad" ware, and this has a grey overall slip and a white quartz slip-paint together with a black underglaze pigment. The technology was also applied in China to create Yuan blue-and- white in the 14th century. The Chinese porcelains provoked a blue-and-white phenomenon in the Islamic world, and the type became overwhelmingly predominant after the 14th century.

Kashan Lustre-painted ware: Lustre-wares were important in Iran between about 1100 and 1340, and during that time only one production centre appears to have existed: Kashan. Earlier wares are often associated with the Seljuk dynasty, and after 1250 are associated with the Ilkhanid dynasty or with the type-site of Sultanabad. These wares have stonepaste bodies and the glaze will commonly be tin-opacified, but alkali glazes are known throughout the period of production.

Minai ware: This type has a stonepaste body, a tin-opacified glaze with pigments and pigmented glasses applied over the glaze. It was made in Kashan mostly between about 1175 and 1200. A related type is called lajvardina (named after the colour of lapis lazuli), which generally has a dark-blue cobalt glaze and colours applied over the glaze, but whereas a multitude of colours were used for the often figural minai, the usually geometric lajvardina will only have overglaze white and red with gilding.

Robert B. Mason, January 2002

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