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NOTES ON SIMPLE GLAZE CHEMISTRY
by Robert Fromme
The chemical compositions which make up clays, glazes and glass are
the result of atomic and molecular combinations. In fact, all
materials in our universe are made up of atomic elements, the
building blocks which can be built or coupled together to form each
of the various materials around us. There are at least 109
different elements but only 40 are commonly encountered in
ceramics. These are chemically expressed in the form of symbols
(Al for alumina, Si for silicon, O for oxygen, Pb for lead, H for
hydrogen, etc.). The clay artist will discover that these symbols
keep showing up in the written material about clays and glazes. It
is a good idea to try to put the most common symbols to memory.
The atom consists of parts (electrons) in orbit about a central
core (nucleus) forming from one to seven "layers." The structure of
the nucleus and the electron layers surrounding it combine to
determine the atom's atomic number and atomic weight. The nucleus
is made up of protons and neutrons. Within the atom, the
electrons are very light compared to the protons and neutrons.
The nucleus makes up most of the atomic weight, however, the total
number of protons, neutrons, and electrons in orbit determine what
the complete atomic weight of the atom will be.
Lets look at a few examples. The atom which has an atomic weight
of 1 is hydrogen. Here we find one proton and no neutrons.
Now, compare the simple atom of hydrogen with something which is a
little more complicated. Oxygen, with eight protons and eight
neutrons in orbit, has an atomic weight of 16.
The number of electron layers in a particular atom helps to
determine the precise relationship between it and other atoms in a
chemical compound. A compound is a chemical combination of two or
more atoms in some definite proportion by weight, forming a molecule.
The molecule is the smallest particle of matter that can exist with
all of the same chemical properties of a particular compound. It is
made up of two or more atoms united by their unique electron
When we think in terms of clays, glazes and glass, we are primarily
concerned with oxide molecules, which are derived from the
combination of various elements with oxygen.
As we mentioned, when atoms of different elements combine together,
a new material, a 'compound' is formed. Again, the smallest possible
portion of a compound is a molecule, this molecule being made up of a
combination of one or more atoms of one or more elements.
The molecular weight of a compound is the total of its component
atomic weights. Each atom combines with oxygen in a manner
agreeable with its electron structure.
For example, two atoms of hydrogen plus one atom of oxygen can
combine to form the compound water. This can be expressed
chemically as H + H + O = H2O (water). The grouping together of
the chemical element symbols to denote the compound is referred to
as the formula. All compounds can be chemically expressed in this
Here, an example can be the molecular formula for alumina (aluminum
oxide), Al2O3. (We should point out that the numbers which follow
the symbols are traditionally dropped below the normal line of text
and we will have to overlook the limitations of the ascii text
editors and accept the formulae with the number on the same line as
the symbols). The symbol of alumina, Al2O3, means that two atoms of
aluminum are combined with three oxygen atoms. The atomic weight
of aluminum is 27.0 and the atomic weight of oxygen is 16.0. The
molecular weight of the ceramic material, alumina, is 102 :
(2 X 27.0 + 3 X 16.0 = 102).
Water (H20) has a molecular weight of 1 +1 + 16 = 18, if we use
the system on our earlier example. Hydrogen has the atomic weight
of one and Oxygen has the atomic weight of 16.
Arrangement in glazes and glass depends upon the structure of
component atoms, in combination with oxygen. The combinations form
molecules of a specific oxides. The substance of our ceramic
materials are formed when the molecules are alone or in combination
with other molecules. When the materials are fired in the kiln,
the heat and atmosphere (oxidation vs. reduction) encourage the
combination of particular materials in specific relationships to
form glazes or glasses.
Solubility and Solutions
With just about any general consideration of the properties of
clays, glazes and glasses it is important to review the basic
principles of solubility and solutions.
One may remember that a 'solution' can be defined as a molecular
combination of two or more substances. So, if we go to the kitchen
and stir a glass of water and sugar until the sugar is totally
dissolved in water, the separate particles in the solution remain
molecules of sugar and water. This solution will remain
indefinitely unless the water is evaporated away. Then the sugar
will reform crystals from its molecules.
If we take our discussion to the beach, however, and look at the
mixture of sand and water in the tide, we see a 'suspension' and
not a 'solution'. With the suspension of the water and sand at the
edge of the tide the individual grains of the material mixed with
the liquid are not reduced in size down to the constituent
molecules. They remain floating there in the liquid for a time,
without being changed, and then they gradually settle out because
their weight is more than that of the liquid.
Our instance suggests that a mixture of glaze and water in the
studio is, for the most part, a suspension. The bulk of the glaze
particles are insoluble in water. Most of them remain quite static
and do not break down into their constituent molecules or dissolve
while in the glaze bucket. (Think of the sand and water in the
Solubility and Temperature
In a solution the substance present in the larger amount, the means
for dissolving, is usually alluded to as the solvent and the other
medium which is dissolved in the solvent is designated as the
We soon determine that regardless of the substance, at any
temperature, there is a explicit ratio of solubility in the
solvent. With few exceptions, the more elevated the temperature of
the liquid or solvent, the more solid it will dissolve. With gas,
the contrary is true. When gas is dissolved in a liquid, the more
heated the liquid, the less the quantity of gas it will be able to
When a solvent at a fixed temperature will not dissolve any more
solute, the solution is expressed as being 'saturated'. In this
condition no further solute can be dissolved. Additional materials
will remain unaltered unless the temperature of the solvent is
increased. However, if a saturated solution of a solid in a liquid
is cooled to a lower temperature the solvent becomes incapable of
holding so much solute in solution. Then we get a result showing
that some of the solute will separate out of the solution in the
form of the crystals of the original solute.
Remembering our first lesson about the origins of ceramics
materials, we will realize that the process of crystallization
resulting from a saturated solution is responsible for the
formation of many of the earth's minerals. As the earth cooled,
these materials have crystallized out of molten rock-developing
materials. As we think of over saturation and cooling
temperatures, we realize that this is also the phenomenon causing
the devitrification (crystallization) of transparent pottery glazes
and the formation of crystals necessary in the formation of matt
and some opaque glazes and glasses.
We should note that the formation of crystals will only take place
as long as the solution is sufficiently fluid to allow the
molecules to orient themselves to form the crystal lattice. In the
kiln, glazes and glasses are normally comparatively viscous when
molten. Because of the viscosity of the melt, the formation of
crystals of the various minerals dissolved in the glaze takes place
Glazes and Glass
Glazes and glass are inorganic materials that develop in a molten
state and then solidify when cooled. One may think of water in its
liquid form at room temperature being changed to a solid by cooling
the temperature to below freezing. With both the cooled ceramic
glaze (or glass) and with ice we have a solid which has been
created by cooling materials which were once in a fluid state.
Ice is, however, crystalline in nature, but glass or glaze has a
random, non-crystalline arrangement of molecules.
Glazes are therefore commonly referred to as 'solid solutions'
since the random arrangement of the atoms and molecules is similar
at room temperature to the random arrangement when the glaze was a
molten liquid. Glazes and glasses can be defined as 'supercooled
liquids of great viscosity at our ordinary temperatures'. They are
not definite chemical compounds but are mixtures of complex
silicates and borates.
Because of high viscosity (resistance to flow) from the random
(amorphous or helter-skelter) arrangement of its atoms and because
it is not overly saturated, glass is not free to move into a
crystalline structure when it solidifies. Being non-crystalline,
glass remains in its most placid (unstructured) condition. Because
of this amorphous structure, glass has varied points of weakness
and it softens gradually rather than at a specific melting point,
as do crystalline structures. Glass will flow if heated enough. In
most cases the viscosity of glass is determined by the amount of
The principal factor distinguishing glaze from glass is the amount
of alumina in a glaze. Since glaze must be more viscous than glass
in order to stay on the clay surface and not flow with gravity onto
the kiln shelf, it requires much larger amounts of alumina to
provide the needed viscosity.
While there are exceptions, most glazes are a mixture of finely
ground silica, alumina, and various oxides for melting or
fluxing the more refractory ingredients. The mixture, when
subjected to intense heat will fuse to form a glass-like surface on
any ceramic object.
As your course of study continues, you will realize that the ceramist
must know every oxide's influence on a glaze in order to choose the
best one for a desired effect.
(c) 1994 Robert Fromme / For educational use, only.
You may contact Robert Fromme at <email@example.com>