| 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. Shredded fiberglass can
be used as an additive to improve tensile strength in the finished
piece. However, the resulting clay contains sharp fibers, is hard
to work with and must be carefully handled.
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 vessal 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.
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