Additives can be worked into moist clay, prior to forming, to produce desired characteristics to the finished ware. Various coarse additives, such as sand and grog (fired clay which has been finely ground) give the final product strength and texture, and contrasting colored clays and grogs result in patterns.
Colorants, usually metal oxides and carbonates, are added singly or in combinations to achieve a desired colour. Combustible particles can be mixed with clay or pressed into the surface to produce texture.
Throughout history, potters have used a mixture of coloured clays as a distinctive decorating technique. In traditional studio pottery in Great Britain, these techniques were known as agateware. The name is derived from the agate stone, which shows bands of colours. In Japan, various techniques for combining coloured clay on the potter’s wheel are jointly known as “neriage.” An analogue of marquetry can also be made, by pressing small blocks of coloured clays together, and using the resulting mosaic to create distinctive patterns. The Japanese term for this technique is nerikome.
Agateware and the other varieties of ‘mottled’ ware are made by combining two or more colours or varieties of clay into one completed piece. Different colours of clay are lightly kneaded or slapped together before being formed into a vessel or decorative item. This method is most commonly used for handbuilt pieces. Coloured clay can also be added to a base clay after it is centered on the wheel. Although in principle any clays can be combined, differing rates of drying/shrinkage and expansion in firing create structural difficulties. It is best to select a light neutral clay body, and then add a colourant to separate portions of the same body. The different coloured clays can then be joined without significant structural problems. Members of commercial clay ‘families’ often have a similar chemical composition and a similar shrinkage rate, and can be used together.
Burnishing, like the metalwork technique of the same name, involves rubbing the surface of the piece with a polished surface (typically wood, steel, or stone), to smooth and polish the clay. Finer clays give a smoother and shinier surface than coarser clays, as will allowing the pot to dry more before burnishing, although that risks breakage. To give a finer surface, or a coloured surface, slip can be coated onto the leather-dry clay. Slip produced to a specific recipe is sometimes called an engobe. Slips or engobes can be applied by painting techniques, or the piece can be dipped for a uniform coating. Many pre-historic and historic cultures used slip as the primary decorating material on their ware.
Sgraffito involves scratching through a layer of coloured slip to reveal a different colour or the base clay underneath. If done carefully, one colour of slip can be fired before a second is applied prior to the scratching or incising decoration. Often slips/engobes used in this process have a higher silica content, sometimes approaching a glaze recipe. This is particularly useful if the base clay is not of the desired colour or texture.