After the basic vessel shape has been formed, a variety of methods may be employed to smooth its surface and modify its size or shape: these include beating the clay surface with a paddle while it is still damp or when it is leather-hard, or scraping with a tool. This technique leaves linear scars or ridges on the surface of the pot where particles were dragged along it with the scraping tool. Pots may also have excess clay trimmed from them while they are still wet.
These forms of decoration are fairly self-explanatory, in that objects such as shells, textiles, bits of basket or mat, grooved or carved sticks or paddles, the ends of hollow tubes such as reeds, were used to create a simple decoration on the surface of the clay while it was still wet. A potter might use his own finger or nail to impress a decoration on a pot. A sharp pencil-like tool can be used in a more free-hand technique by incising decoration.
These forms of decoration are probably the most common on all types of pottery from all over the world in all periods, and adorn crudely-potted cooking vessels as well as higher-status wares that may also be glazed. "Sgraffiato" (Italian for "scratched") was a very popular decorative technique on all types of Islamic pottery, and was frequently combined with other more complex techniques of decoration like slips and transparent glazes. The decoration formed by incising into the clay can sometimes be very complex, and shows the craftsmans skill and effort as much as a finely-potted vessel.
The above forms of decoration all involve a displacement of the clay in some way, even if it is the tiniest piece that is removed. These techniques involve joining new bits of clay to the surface of the pot to form a relief decoration. Small pieces of clay are shaped into the desired decorative motif and applied to the surface of the vessel by "luting", in which a small amount of fluid clay slurry or slip is used as a glue. These applied pieces may not necessarily be ornamental, as feet, bases, necks and handles may also be applied to the pot in this way.
A slip (sometimes called an engobe) is an addition to the vessel which covers its entire surface, much like a glaze but with different composition and firing properties. It is a fluid suspension of fine clay in water which is applied to the vessel as a thin coating before firing. If coloured (by the addition of natural pigments) the slip may be applied as a decorative element in itself, but frequently it provides the base for subsequent painted decoration.
The surface of the vessel may be decorated by stamping/incising etc before or after the pot is slipped: sgraffiato tends to be decoration incised through the slip into the surface of the vessel. "Silhouette wares" of C12th Iran are dipped into a thick black slip, then the slip is carved away to leave the design in black against the pale body of the vessel; the whole vessel is then covered in a clear or, most commonly, a transparent, turquoise-coloured glaze, which gives the vessel its characteristic silhouetted design of black-on-turquoise/white. The term "self-slip" refers to vessels with a chalky white finish on the surface, which results from salts and other minerals leaching out of the slip during firing. This can be deliberately caused by the potter to enhance the effect of his finished product.
To apply the slip, the vessel may be dipped in a large vat while the potter holds on to its foot. The slip thus fills all the holes and irregularities in the vessel surface, and assures an even coating. For vessels too large to be dipped, the slip may be poured in and swilled smoothly and rapidly so that it coats the pot evenly. The slip can be applied by hand but tends to be uneven when it dries. Water from the slip is absorbed into the porous walls of the pot.
Painted decoration means that a pigment, formed from a mixture of a natural colourant, fine clay, water and a binding agent, has been applied to the vessel surface with some sort of brush or painting tool. The colourant is usually a metallic oxide if the vessel is to be glazed and high-fired, as organic pigments can burn off under high-temperature firing conditions. The pigments are painted onto leather-hard or biscuit-fired vessels, either over a slip or directly onto the clay body, then they are covered in a transparent clear or coloured glaze. As he paints, the pigment appears to the potter to be nothing more than grey sludge, so he must imagine the polychromy and designs that he is applying to his pot. Therefore, when we look at the beauty and sophistication of some of the colours and designs, the potters skill is to be praised even more.
The most popular metallic oxides used are naturally those that occur most frequently on the earths crust: these are usually copper (gives a turquoise or green colour depending on whether the glaze is alkaline or lead-based, and can even seem black when applied thickly); cobalt (most commonly used by Kashan and Iznik potters for deep blues); manganese oxide (gives a purple colour, but also black if applied thickly enough); antimony (gives a yellow colour in a lead glaze, though this is not used frequently by Islamic potters); iron (for reddish colours). It was not until the successful experiments of the Iznik potters in the second half of the C16th that a bright red colour was adopted onto Islamic pottery, and this colour characterises this phase of the industry at Iznik.
"Lustre" has already been referred to as an example of over-glaze painting, and for more detailed description of the manufacturing process involved in this type of pottery see section 7.a. Briefly, over-glaze painting is a technique of decorating pottery after the glazed vessel has already been high-fired. Minai is another type of over-glaze decorated pottery. However, another innovation of Islamic potters was underglaze-painting, which came to be the most common style of pottery decoration not only in the Islamic world, but also in Europe and China.
Underglaze painting probably developed out of the technique of "silhouette wares". Carving a design through a slip does not let the potter execute looser, free-hand designs, such as those that were becoming increasingly popular on manuscript illuminations of circa 1200, when underglaze seems first to have been developed. Initially, underglaze painted vessels continued the combination of blue and black, the blue (derived from cobalt) used as a decorative wash, and the black (a mixture of iron, manganese and chrome) used for details of the drawing. The important discovery of the Islamic potters of this era was that these elements remained stable under an alkaline glaze, and so the new type could be adopted into their increasingly complex repertory of pottery-producing techniques. Underglaze painting continued to develop in sophistication, to culminate with the beautiful polychrome painted Iznik pots.
In-glaze painting was used on the very earliest Islamic ceramics: the opaque white glaze was applied to the leather-hard vessel which, being porous, absorbs the liquid from the glaze leaving a powdery raw glaze on the surface. The pigment (usually cobalt blue) was then added on top of this to form the decoration: liquid was likewise absorbed from the pigment into the vessel, and the pigment takes on the properties of the glaze, ie. if the glaze is opaque then the pigment becomes opaque. The design is therefore neither under or over the glaze, and is thus described as "in-glaze".