The production of a single pottery vessel requires a number of complex and often costly procedures. This section aims to take you step by step through "the story of a pot" from the preparation of the body material to the final firing, with a brief look at the next and final stages in the life of the vessel, down to the archaeologist who finds it. The section also aims to explain briefly specific techniques used for the production of eg. lustre, and to point out the often-sidelined involvement of the craftsman at every stage in the process. It also discusses technical innovations made by Islamic potters: these are not considered chronologically but alongside the relevant procedural sections. Links will take you to historical case-studies, interesting documents or images, on other pages in the programme.
|D.1.||The body material|
The term "body material" refers to the combination of ingredients which constitute the vessel. It is usually a mixture of clay and a variety of other things which the potter adds to modify the clay, to improve its workability, drying or firing properties: this added material is called the "temper". "Body material" does not include the glaze.
All ceramics consist of clays. The proportion of clay in a vessel varies to a greater or lesser degree depending on the type of end-product that a potter has decided on. Factors influencing this decision are generally the vessel function and perhaps consequently the type of decoration.
A clay is a fine-grained earthy material which results from the weathering and decomposition of rocks.
Rocks are formed from minerals which are deposited through volcanic/glacial activity; the most common minerals in rocks are silicates, ie. minerals which have silicon oxides (= silicon + oxygen) as the major part of their composition; thus the most common minerals in clays are also silicates. Of these, the two elements that are most resistant to weathering, and which are thus the primary elements in a clays chemical composition, are silica and alumina.
After a rock weathers, these mineral-rich clays may be deposited in shallow lakes, slow-flowing streams, rivers or estuaries, or exposed in situ. Therefore the very first process in the production of a pot is the collection of the raw materials from river-beds or rocky outcrops: sometimes these sources may be nearby, for example at Kashan the "qamsari stone" which was an essential ingredient in producing the vessel body was, and still is, found as an outcrop about a mile out of the town. However, potters may have had to travel great distances to collect even the most basic raw ingredients, or it may have been necessary to import at great expense elements such as tin (from Europe or Malaysia) for opacifying glazes, or cobalt (from Iran) for making a dark blue pigment for decoration.
The principle properties of a clay are i) plasticity, resulting from its very small particle size, which allows the clay to be formed into shapes by the application of pressure and to retain that form when the pressure is removed; and ii) the alteration of its physical and chemical characteristics with the application of heat, meaning that the clay hardens into an artificial stone.
The collected raw clay by itself would not have all the necessary properties to make a good pot. The potter needs to take certain preparatory steps by which he modifies his raw materials to make them suitable for the type of pot he wants to produce. These modifications may begin with levigation, ie. the potter separates out the fine from coarser material in his collected ingredients by mixing it with water and washing it down in a series of sieves, so that the coarser material settles as residue, and only the fine clayey material is left. This procedure is used if the clay is coarse or stiff, or if it contains a high degree of organic matter (eg. rootlets, leaves) or large fragments (eg. pebbles) that may damage the clay products during firing (eg. by causing cracks).
On the other hand, if the clay is particularly fine-textured or sticky, the potter may instead modify his clay by the addition of tempers. This term describes the coarse, non-plastic material in a clay, and usually refers to artificial inclusions deliberately added by the potter, such as ground quartz pebbles, or grains of sand, salt, crushed seashells, or even sherds of old pots ("grog"). The value of tempering the clay by adding coarse inclusions improves the working properties of the clay, and the strength of the unfired body; it also aids drying and firing properties as the larger particles in the clay enable water to travel more quickly from the inside to the surface of the pot through capillary action, and thus allows the pot to dry faster and fire more quickly. If the potter is making a high-fired body, the presence of glassy inclusions such as quartz encourage the body material to vitrify in the kiln.
These are two opposite ways of treating the clay before forming the vessel, but the potter will choose a method depending on the kind of vessel he is intending to produce. For example, stonepaste is so called because it includes a small proportion of clay and a high proportion of glassy frit made from quartz. The composition of the clay body can be examined by modern day scientific techniques (Technology Part 2) which can identify the inclusions or fine particles in a clay, and even sometimes identify provenances by comparing these with known areas whose chemical composition is very similar.
To make the clay as plastic as he needs it, the potter must knead it with his hands or tread it under foot on a sort of threshing-floor, to eliminate air pockets, and to assure an even distribution of moisture and inclusions, thereby enhancing the clays workability.
The final function of the vessel dictates the type of processes adopted by the potter in its formation. At the most basic level, this could be the distinction between an earthenware utilitarian vessel to be used for cooking, and an item of luxury table-ware off which the food is to be eaten at important dinner parties. The function itself may be dictated by the intended market, ie. prince or pauper, and another linked factor is the decoration that the potter intends for the end-product. For example, a Kashan potter working on a well-paid and one-off commission is going to choose all the best ingredients for his pot, and will probable choose to make it out of the finest stonepaste so that he can adorn it with a rich lustre decoration. The same potter may mass-produce a quantity of cooking pots for a lower-status market, and in this case he will select an earthenware body material high in clay content.