A GLAZE is a glass that has been modified to melt onto a clay. The chemical name for
glass is
SILICON DIOXIDE (Si02). Common names for glass are silica, sand, quartz and flint. Silicon dioxide is one
of the most common minerals composing the earth and can be easily found in most parts
of the world. Glass cannot be used alone to make glazes however, because its melting
point is 3100° F, making it unusable as a pure component. This is simply because the
melting point of our clays, even our highest firing clays such as stoneware and porcelain,
is much below this. Thus the clay would melt in the kiln long before the glaze itself
would. Therefore, modifications need to be made to the SILICA to lower its melting
temperature.
FLUXES are chemicals added to the SILICA to lower its melting point. The fluxes comprise
a large group of minerals, and one of the few things they have in common is their
lower melting points, relative to SILICA. Since low fire glazes require more melting,
they generally contain a higher percentage of FLUXES relative to SILICA than do the
high fire glazes used on stoneware and porcelain. Simply stated, the more FLUX added
to a glaze, the lower the melting point will be.
Additionally, there is a third class of ingredient that must be added to a glaze to
produce an acceptable result. Since glaze is to be applied to bisque ware that will
undergo additional shrinkage during the glaze firing, the glaze itself must 'shrink-to-fit'
the bisque to which it is applied. The ceramic material we already know that will
shrink is CLAY itself. So, an amount of
CLAY, called the
ALUMINA portion is added to the glaze to achieve this essential shrinkage. Remember, a generalized
formula for clay is SILICA, ALUMINA, water and various impurities. Thus, the addition
of ALUMINA to a glaze in the form of a clay actually introduces two of the three essential
parts of a functional glaze: SILICA AND ALUMINA.
In summary: All
GLAZES are composed of three components:
SILICA - the glass former
ALUMINA - the portion that 'shrinks-to-fit' the clay
FLUX - to lower the melting point of the higher temperature ingredients
REFRACTORY INGREDIENTS are those with high melting points, such as the SILICAS
and ALUMINAS.
SILICA SOURCES IN GLAZE FORMULATION
SILICA is added to glazes by the addition of compounds such as: SILICA, BALL CLAY,
KAOLIN, and FELDSPAR. BALL CLAY is a commonly used clay in glazes and in clay bodies,
and is a source of SILICA and ALUMINA. KAOLIN is another commonly used clay, very
white in color, used in glazes and in the formulation of Porcelain clays in particular.
FELDSPAR is probably the single most commonly used glaze ingredient. It is a natural
mineral, derived for granite. It is a source of SILICA, ALUMINA, and a relatively
smaller
amount of FLUX.
ALUMINA SOURCES IN GLAZE FORMULATION
ALUMINA is added to glazes by the addition of BALL CLAY, KAOLIN, and FELDSPAR
(discussed above).
FLUX SOURCES IN GLAZE FORMULATION
FLUX is added to glaze by a wide variety of compounds, a few of which are listed he
re:
- LOW FIRE GLAZE FLUXES - lead oxide or carbonate, borax, gerstley borate, commercially prepared frits,
and many others.
- HIGH FIRE GLAZE FLUXES - whiting (calcium carbonate), dolomite (calcium/magnesium carbonate), bone ash (calcium
oxide), barium carbonate, organic ashes, most coloring oxides, feldspar (contains
some flux), and many others.
A
BASE GLAZE is a mixture of these three basic groups: SILICA, FLUX AND ALUMINA.
A base glaze is clear or white in color. All colored glazes derive from additions
of colorants
to a base glaze.
Here is a typical base glaze formula:
HIGH ALUMINA MAT Cone 10 (This is a shorthand for 2350° F)
Feldspar 48.9% (SI, AL,FLUX)
Kaolin 25.1% (SI, AL)
Dolomite 22.4% (FLUX)
Whiting
3.6% (FLUX)
100.0%
NOTE: All three categories of essential glaze ingredients are present. We do not know
from this analysis whether they are present in the proper proportions or not. This
would involve a quantitative analysis that is beyond the scope of this class. If you
are interested in further study, read the text, The Craft and Art of Clay by Susan
Peterson. GLAZE CALCULATION is a course offered here at Glendale College that provides
in depth study in this field.
The
BASE GLAZE will be white or clear unless colorants are added.
For a discussion of color in glaze go to
GLAZE COLOR.
Ingredients have several characteristics that make them suitable for use in glazes
besides whether they are sources of SILICA, FLUX AND ALUMINA. Obviously, we need to
find sources that are
INEXPENSIVE. Also, we are looking for the ingredients that are
NON TOXIC. Lead, for example, is inexpensive and an excellent flux, but is so toxic that it
is not used much anymore. A third requirement is that the potential glaze ingredient
must be
INSOLUBLE in water. This is because glaze is applied to a piece of bisque as a liquid SUSPENSION
of materials in water. After dipping, the water in the glaze absorbs into the porous
bisque, depositing the insoluble glaze materials on the surface of the pot. Thus the
water serves only as the VEHICLE to get the glaze onto the bisque. Were a glaze ingredient
SOLUBLE in water, it would travel with the water into the bisque, and not be deposited
on the surface. Suppose this soluble ingredient were a FLUX -- then the FLUX would
travel with the water into the bisque, leaving the glaze. Thus the bisque would melt,
and the glaze would not. You would have, in a sense, turned the pot inside out. Therefore,
all glaze materials must be INSOLUBLE in water.
If a chemical in INEXPENSIVE, NON-TOXIC, INSOLUBLE, and a source of SILICA,
FLUX OR ALUMINA, you can bet we use it in glaze composition.
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