Glaze & Clay Tutorial - 6

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glz2c.txt         Clay and Glaze Formulation    Robert Fromme


At this point in our course of study, we need to turn back to
the first lesson and the discussion of the three ways of looking at
a glaze.  You will remember that most batch material
sources contribute two or more oxides to the melt.  We decided that
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 powerless to look deeper than 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 - Tin Oxide, Stannous Oxide
SrO - Strontium Oxide, Strontia
ZnO - 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.  If we want to understand the glaze we must turn our
attention to the oxides in the fired melt. 

I would like you to look over this list of oxides and notice
that they come in several forms. If we make (R) stand for the
chemical symbol and O stand for Oxygen, some of these will match
the form of RO or R2O.  Some will match the form of R2O3.
Others will match the form RO2.

When we group the oxides in our list according to this
relationship, we get SiO2 (Silicon Dioxide, Silica), SnO2 (Tin
Oxide), TiO2 (Titanium Dioxide) and ZrO2 (Zirconium Oxide) in
the RO2 group. We get Al2O3 (Aluminum Oxide, Alumina) and B2O3
(Boric Oxide) in the R2O3 group. The remainder of our list will
fit in the RO, R2O group.

In reality, glaze-forming oxides can be divided into these three
major categories.  The majority of our list, those that fall into
the RO, R2O group are called the base or fluxing oxides.   Among
other things, all of these oxides do help to lower the melting
point of silica, alumina and other refractories in the glaze.
The oxides that fit in the R2O3 group are called the neutrals
(refractory) oxides which help to control the viscosity (flow) of
the glaze. On occasion, you will also hear these R2O3 oxides
referred to as the viscous agents and the crystal retardants
because crystals do not form well with the viscosity which these
ingredients contribute.

Silicon, or our RO2 oxide is called the acid oxide or the
glass former. (For those of you who need to know what the R in
the formulae stands for, I have been told that it represents the
root element (radical). 'O', of course, represents oxygen.  You will
also remember that the number behind (usually below when possible)
the element tells the number of atoms of the element that are
present in the molecule. If there is no number, that means only one
atom is present. When the number is on line and ahead of the
oxide symbol,it means that there are that many of the particular
oxide relationships in the formula.)

Thus, the base (flux) symbol RO indicates that one atom of an
element is combined with one atom of oxygen to form one RO
molecule.  R2O means that two atoms of the root ingredient have
linked with a single atom of oxygen to pattern one molecule of
R2O.  Still, the base group contains both RO and R20 oxides, it
is commonly referred to as the RO group. The neutral (refractory
or viscous) classification R2O3 includes all oxides that have two
root atoms in association with three oxygen atoms. The acid
(glass-former) group.

RO2  contains those oxides having one root atom united with two
oxygen atoms.

                RO/R2O =  The Fluxing or Base Oxides
The fluxes or base oxides stimulate a glaze to liquefy, melt
or flow. Each one of this oxide group is somewhat active as a
flux ingredient. Some of these bases will melt early and burn out
as the temperature increases, others will melt early and last
throughout the firing.  Some will not get involved until the firing
is well underway.  Yet, each one of these fluxes or bases will
bring its own characteristics or qualities of texture, color
response, viscosity, and other unique constitution to the glaze.

If we take a minute to think about the previous paragraph, we
will come to the conclusion that, while in theory, it is needless
to include more than one base oxide in a glaze, doing so generally
gives a more ample firing range and a elevated potential
for diversity in the texture, color qualities and glaze effects.

   R2O3 = Neutral, Viscous, Crystal Retardant or Refractory Oxides

The neutral oxides give the glaze strength, durability or body. 
As you have discovered, the principal neutral oxide is
alumina (Al2O3).  In the melt, alumina enhances the glaze
viscosity. In turn, this inhibits the tendency of the glaze to
flow off the surface of the  clay, and it slows devitrification
or the retards the formation of crystals in the cooling mixture.

Although  boron (B2O3) is also considered neutral, it cannot
replace the full amount of alumina needed except at very low
temperatures.  In some ways, boron is like the class clown, it
refuses to fit and act like it should according to its formula.
As an example, it can form a kind of soft glaze without the
presences of silicon at very low temperatures. We usually
associate the RO2 oxide or silica as the glass former in most
glazes.  Boron is also a very active flux throughout the full
temperature range of glazes so it acts like a RO or R2O oxide,
as well.  We will talk more about boron later.

               RO2 = The Acid or Glass-Forming Oxides 

The RO2 or Acid oxides are the genuine glass formers for
most glazes.  Of course, the principal acid or glass-forming
(RO2) oxide in a glaze is (SiO2) silica, which usually can not
be replaced with any other oxide.

Zirconium (ZrO2), tin (SnO2). and titanium (TiO2) also are acids,
however, they behave as opacifiers in a  glaze and are usually
added later,  as are colorants.  Opacifiers are refractory
ingredients which are added to make the glaze opaque.  White
glazes are created through the use of these elements.  We will
discuss their use in a later lecture.

Here are ceramic oxides as they function in a glaze: 
RO, R2O Base or Fluxing Oxides

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                               
SrO - Strontium Oxide, Strontia
ZnO - Zinc Oxide
R2O3 Neutral, Viscous or Refractory oxides

Al2O3 - Alumina 
B2O3 - Boron (acts at times like a base, neutral, and/or acid)
RO2 - Acid or Glass former

SiO2 - Silicon
RO2 Acid Oxides that serve as Opacifiers

SnO - Tin Oxide, Stannous Oxide
TiO2 - Titanium Dioxide, Titania
ZrO2 - Zirconium Oxide
(c) 1994 Robert Fromme            For educational use, only.

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