Different Types Of Clay.

Slip.

Slip is any clay or body mixed with water to a smooth creamy consistency. Originally it was used to provide a thin coating of the then rare light - firing clays over a red body, it has been used world wide for decoration. The term slip is generally reserved for liquid 'natural' clays or bodies used for decorative pottery.

Porcelain.

In China the definition of porcelain (T'zu) is wider than is generally accepted in the west: it could be any fine, pale body or primary clay with a distinctive ring when struck fired to verification but often too thick too show the translucency which we expect today.

Different Types of Glaze.

A glaze is a special kind of glass, which is chemically formulated to adhere to the surface of the clay or fuse into the body when fired.

The two main types of glazes are Stoneware and earthenware glazes:

Stoneware glazes (High temp glazes)

Because of the composition of stoneware glazes and the higher temperature at which they are fired, they have a very different character to earthenware glazes. They are seldom glossy, but tend to be slightly shiny, silky or matt.

Stoneware glazes tend to be dark colours because they are fired at high temperatures.

Earthenware glazes (low temp glazes)

Also known as 'soft glazes.' They mature below 1150'c / 2102'f, and are used to cover a porous body. The typical Earthenware glaze is shiny and smooth; it can be transparent or white and fully opaque, the latter providing a background for bright clear colours. The formulas for Earthenware glazes differ from those of stoneware and porcelain mainly in there choose of bases, and in the ratio of base to silica.

Other popular glazes are;

Porcelain glazes.

The particular quality of a porcelain glaze is that both the body and glaze mature and fuse together to give a tough translucent body. Translucency depends on the composition of the body, and while it is possible to obtain a wide range of colours and effects on porcelain, the desired effect is often to enchance the particular quality of the body by using a transparentglaze. The glaze for porcelain should reflect the fine quality of the ware and therefore it should be finer than a stoneware glaze that suits a more robust body.

Tin Glazes.

A tin glaze is any glaze, which owes its opacity to tin oxide. An adequate glaze coating is needed to achieve opacity but not so thick as to crack and subsequently crawl. Calcined clay or the use of tin ash is reputed to help prevent crawling in a soft glaze.

Slip Glazing.

These glazes contain a high percentage of plastic clay (ball clay, red clay, or body clay.) it is essential that the body and glaze mature together because if the clay matures before the body the gases released from the body could impair the glaze.

Different Types Of Decoration.

Slip Decoration.

Slip may be used as a complete or partial covering on a pot or dish. A 'bib' can be dipped or contrasting colours pored over.

Slip decoration at its best is very direct and free. Essentially an earthenware technique, it can also be used on stoneware's, often using special angobes.

Sgraffito.

Decoration in which slip or other coating is scratched or cut away to reveal the clay body beneath. A variety of tools can be used: the fingers for very wet slip, a stiff brush or more traditionally a bamboo or cane tool cut to a flat end resembling a broad nib pen.

Wax Resist.

Wax designs are applied to pottery surfaces in order to repel liquid pigments, thin slips or glazes. The wax is usually applied as free brushwork but a design can also be drawn on with a wax crayon or piece of candle.

Impressing, Engraving.

A design cut into clay usually with a wooden or bamboo tool cut to an angle or to a rounded point. This is preferable to a sharp metal point, which will give a weak and furrowed line.

The decoration can be enhanced by an overlay of slip or a dry usually, ash glaze.

Firing Processes.

Firing is the heat treatment of ceramic materials at least to the sintering stage.

Types of firing:

Stoneware Firing.

High temperature firing. The generally accepted minimum is 1200'c / 2195'f and the range is 1200'c - 1350'c. Bodies with artificially lowered vitrification points have been marketed.

Biscuit Fire.

The greatest risk of breakage or explosion occurs in the 194'f - 302'f range. Thick pieces are obviously more at risk than thin walled ware.

Raku Kiln

A raku kiln must satisfy the following conditions it must reach 800'c / 1472'f and unless it is used for one special piece at a time, maintain that temperature for several hours, the chamber must be readily accessible and easily opened and closed with the kiln at full temperature; reasonably oxidizing conditions must prevel.