Adding Colour
There are a number
of ways in which you may add colour to your work. It is important that
you know which method to use at the various stages through which your work
passes on its way to becoming pottery.
The main ways of adding
colour are by using:- decorating slips, underglaze colours and oxides or
coloured glazes.
There are other ways
but these are outside the scope of this article.
Decorating Slips
These are
made from clay which has been mixed with sufficient water to make it liquid.
To this liquid mix has been added metallic oxides, typically cobalt oxides
to give blues, copper or chrome oxides to give greens, iron oxides to give
browns and tans and vanadium oxides to give yellows. Most of these decorating
slips look very similar in colour in their liquid state, usually a grey.
The colour does not appear until the work has been fired and the full colour
is not developed until the fired work has been covered with a clear glaze
and fired for a second time. These decorating slips are opaque and stable
and the colour of the clay used to make your work will not affect the final
fired colour. They will not run into each other on firing. Decorating slips
should not be used on work which has dried beyond the leatherhard stage.
A useful rule of thumb to use is, if your fingernail will still easily
make a clear indentation in your work you can apply decorating slips.
Underglaze Colours
These are
commercially produced ceramic colours which are available either as dry
powders, which will need thorough mixing with a small amount of water or,
preferably, a special medium, or as pre-mixed, ready-to-use 'paints'. These
underglaze colours are usually very close to the final colour they will
reach after firing and covering with clear glaze. They may be opaque or
semi transparent and because of their expensive nature, are more appropriate
for small areas and details rather than large areas where it is difficult
anyway to get an even colour. They may be used on leatherhard or dry clay
and also on work which has had its first firing. They can be used over
themselves and over decorating slips. Avoid using on wet clay as the clay
surface will tend to mix with the underglaze producing muddy colours.
Oxides
These are
used in much the same ways as Underglaze Colours. They do not however give
any indication of their final colour which only develops when fired under
a glaze. Applying a weak mixture of oxide and water onto a textured surface
and then sponging off the surplus will leave a residue of oxide in the
texture and often produces very attractive results under a transparent
or white glaze
Coloured Glazes
These are
a mixture of various ceramic materials which when fired to a certain temperature
will melt and fuse together to produce a glass-like surface on your work.
Because you are covering the work with what is to all intents and purposes
a layer of coloured glass, the colour of the clay which is used will usually
affect the final appearance of your work. Glaze may be applied to work
by spraying, pouring, dipping or brushing. Different coloured glazes may
be used next to each other but because the glazes tend to fuse on melting
a very clear edge to the different colours is not possible. Different glazes
can be used over each other but the final result is often not that which
you would expect. Glazes must only be used on work which has already had
its first firing unless they are suitable for a 'once firing'.