Glaze & Clay Tutorial - part 1

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              Notes on Glaze Formulation (glz1a.txt)
                          Robert Fromme 

We have three ways of looking at a glaze, (1.) the aesthetic and
functional contribution it makes to the design of the ceramic
form, (2.) its batch weight recipe of raw materials which will
provide the oxides for the chemical action which results in the
quality of its fired composition and (3.) the oxide composition
of the fired glaze.  The oxide composition is the primary tool to
analyze problems and positive results from a firing.  The batch
weight recipe of raw materials is of value while we are involved
with the cost, sources, weighing, physical behavior and use of the
glaze slurry materials. The aesthetic and functional value of the
glaze is the bottom line for the clay craftsman. The success of
the product depends upon its design the quality of surface is a
critical aspect of its formal composition.

Most of us are sensitive observers and can quickly sense if an
object is poorly designed and we usually share an appreciation
for those objects which are extremely successful in their
sensitivity and unity of execution.  Within the world of creative
expression and formal organization of ceramic objects, concerns
for the elements and principles of design usually boil down to the
nature of the clay bodies which are chosen, the specific three-
dimensional form of the object, and the characteristics of the
finished clay or glaze surface on the object. We do not need to
understand ceramic processes or the technical aspects of making
ceramics in order to gain a basic appreciation of these objects.
However, when we want to move from the experience of one who
simply appreciated the object to a person who is expressively
involved in the creation of these objects, we must look beyond
matters of fired clay surfaces, glaze color and other surface

Soon after we begin to discover some of the techniques for
creating ceramics, we are confronted with batch weight recipes
for glazes. In the clay studio classroom, we begin to think of
glazes in terms of the batch weight recipes, as well as, their
aesthetic and functional properties.  The glaze batch recipe is
created from powdered ceramic minerals measured to a specific
relationship in order to get the mixture to melt into a glassy
coating when the desired temperature in the kiln has been
achieved. In the melt, the  materials are reduced to their basic
chemistry in oxide form.  As the objects in the kiln cool, the
melted glaze materials begin to form a glass surface.  The glaze
coating results from a structure of oxide molecules provided by
the original glaze materials.  When the glaze materials have
melted they can never again be returned the recipe materials
which were used in the raw batch recipe.

Since most batch material sources contribute two or more oxides
to the melt, an understanding of the basic oxide chemistry of the
melt is the only reasonable source of useful information for the
control and evaluation of glazes.  We will be unable to move
beyond mystery and confusion if we are unable to look deeper
then the raw materials of the batch weight recipes of glazes.  In
other words, the fired (final) properties of the glaze need to be
managed and evaluated and we can begin to accomplish this
control when we learn to concentrate on the oxide makeup of the
fired glaze.

All kinds of fired properties, such as surface, color response,
expansion, and melting temperature, can be predicted when you
develop an understanding of the unique characteristics of the
oxides used in glaze compositions.  These oxide formulas
routinely draw from fifteen or fewer oxides.  The most common
oxides found in ceramic base glazes are:

SiO2 - Silicon Dioxide, Silica
Al2O3 - Aluminum Oxide, Alumina
B2O3 - Boric Oxide
BaO - Barium Oxide, Baria
CaO - Calcia, Calcium Oxide
K2O - Potassium Oxide
Li2O - Lithium Oxide, Lithia
MgO - Magnesium Oxide, Magnesia
Na2O - Sodium Oxide, Soda
PbO - Lead Oxide
SnO2 - Stannous Oxide, Tin
SrO - Strontium Oxide, Strontia
ZnO2 - Zinc Oxide
TiO2 - Titanium Dioxide, Titania
ZrO2 - Zirconium Oxide

Each of these oxides has its own "personality" in the melt and
provides unique contributions to the final fired glaze properties.

The batch weight recipe can come from hundreds of materials
and each of these raw materials can provide an assortment of
oxides for the chemistry in the melt. Thus, the picture takes on
an overwhelming complexity if we try to focus on the glaze in its
batch weight recipe of raw materials.  If we want to understand
the glaze we must turn our attention to the oxides the materials
contribute to the fired melt. 


Color in glazes results from the chemical interaction between the
base glaze and additional metal oxides which are usually added
or tested after a functional base glaze has proven to melt at the
desired temperature with the desired expansion, surface qualities,
etc.  Some of the most common coloring metals are:

Ag - Silver
Au - Gold
Cd - Cadmium 
Co - Cobalt
Cr - Chromium
CuO - Copper 
Fe - Iron 
Mn - Manganese
Mo - Molybdenum
Ni - Nickel
Pr - Praseodymium
Sb - Antimony
Se - Selenium 
U - Uranium
V - Vanadium 


1. In addition to considering the aesthetic and functional 
concerns for a glaze, we can look at a glaze in its batch 
weight recipe and its oxide form.

2. You mix and apply the glaze recipe (made from powdered
ceramic minerals) to the clay object before the firing.          

3. The kiln temperature decomposes the raw materials to liberate
their basic oxides. In the molten glaze almost all of the oxides
are emancipated and fluid, but the melt solidifies to a new oxide
glass structure as the kiln cools.        

4. The glaze has changed in the kiln and emerges as an object
composed of oxide molecules.        

5. Each of the glaze oxides has its own "personality" in the melt
and provides unique contributions to the final fired glaze

6. Color in glazes results from the inclusion additional metal
(c)1994 Robert Alexander Fromme  
NOTE: for educational purposes only.      
You may contact Robert Fromme at <>

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