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| Check back here often to find some of
Jean’s favorite techniques. This month, she talks about Glazes
and the process of Glazing.
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Glaze and Glazing
A glaze is nothing more than a thin glass-like coating fused to
the surface of the pot by intense heat. Glazes vary in
composition from very simple (ones containing only 2 or 3
materials), to very complex (ones containing 15 or 20 ceramic
compounds). Understanding the formulation of glazes requires
knowledge of chemistry and math and can comprise an entire
course in itself. Here we are concerned with only a basic
understanding of glazes.
Every glaze must have the following three basic materials to
produce a transparent glaze.
- Silica
Silica is made
from quartz rock that is ground up into a very fine powder.
This is the glass-forming ingredient and has a very high
melting point (3,100 degree F).
- Flux
This lowers the melting point of the glaze and
combines with the silica to form a glass crystal. Lead is a
low-fire flux. Whiting is a high-fire flux.
- Kaolin
Kaolin, or clay, is the refractory element in glaze.
It makes the glaze less fluid during firing. Kaolin also
makes the finished glaze more resistant to scratching.
Note: silica and a flux will make a soft, runny glaze
by themselves.
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Color and opacity-Metal oxides Other ingredients must be added to the transparent glaze to
create color and opacity. Metal oxides provide the best source
of color in glazes.
Tin Oxide
Opacifies the glaze (makes it white) so that the clay body does
not show through as much.
Iron Oxide
Produces yellow, green brown, orange and black in stoneware,
depending on the percentage of it added. It is the most
important colorant.
Cobalt Oxide
Produces blue in glazes and is a very strong colorant. 1%
produces a bright blue, whereas 10% iron is needed to make a
brown blaze.
Copper Oxide
Produces green, and in some glazes will produce red.
Chrome Oxide
Produces a bright green.
Rutile
Produces a cream, tan or wheat color.
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Applying Glaze
When formulated, the glaze will be in powder form. When this
powder is mixed with water it can be applied to the pot in
various ways.
Pouring
The best means for glazing the inside of a pot. The outside can
be poured but it is difficult to achieve an even coating. Many
times the overlapping of pouring can be interesting but many
times it will leave the glaze too thick and can cause running in
the glaze firing.
Dipping
The simplest way to glaze and the best way to get an overall
even coating. Use tongs and make sure the glaze is thoroughly
mixed.
Spraying
Glaze is put into the spray gun and applied by air pressure.
Correct thickness is about as thick as a post card. Spraying
should be used only on pieces too large to dip or pour.
Brushing
Brushing is unsatisfactory on stoneware because of the uneven
surface that occurs. Sometimes small areas can be brushed off if
other methods are impractical.
Tip: If the glaze is too thin the surface will be
rough and too much clay will show through. If the glaze is too
thick it will appear as a coating like paint instead of
combining with the clay as a unified totality. The bottom of the
pot must be cleaned of glaze and at least ¼ inch up the side
also to avoid the pot sticking to the kiln shelf during firing. |
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Glaze firing
After the glaze has been applied, the pot is fired a second time
to melt the glaze into a glassy coating to give a better tactile
quality and a better appearance. Stoneware does not have to be
glazed to be watertight because the pores of the clay are sealed
by the melting of some of the components of the clay body at
high temperatures, (vitrification). There are two types of
firings:
Oxidation
Oxidation firing is used in a bisque firing. It is a normal
firing in which all of the fuel is burned completely in a clear
smoke-free atmosphere, resulting in a light colored clay body.
Reduction
In reduction closing the damper on top thus cutting down the
draft creates firing a smoky atmosphere. This causes a lack of
oxygen and an excess of gas. The oxygen, which exists in the
clay and glaze, is consumed, which darkens the clay body and
dulls the glaze colors, making them much richer and more earthy
in appearance.
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