Material: Recycled
Glass
Issue: Many of the elements making
up soda lime container glass are the same
as those used in glaze formulations, although
in somewhat different proportions. This
Best Practice describes some of the issues
involved with using recycled glass as
a constituent of ceramic glaze formulation.
In order to make effective use
of this Best Practice, some knowledge
of glaze chemistry is helpful.
Best
Practice: Although
the exact content of soda lime container
glasses vary somewhat by manufacturer,
color, and container application, a typical
average chemical content of the oxides
in container glass is given by the following: Oxide Percentage by weight Molecular weight SiO2 73.3 60 Na2O 14.1 62 CaO 9.4 56.1 MgO 1.9 40.3 Al2O3 1.3 101.9
The ceramic industry has developed a method for analyzing
the chemical constituents of any material
as a possible raw material for glaze formulation.
The method, often called the “Unity
formulation,” uses the molecular weight
of the oxides and the percentage by weight
in the chemical make-up to derive a chart
showing the relative numbers of molecules
of each oxide contained in the raw material.
The oxides are expressed in the
“RnOm” format, where
R stands for any element, n stands for
the number of atoms of the element R in
one molecule of the oxide form of the
molecule, and m stands for the number
of atoms of oxygen in one molecule of
the oxide.
The analytical method is called the unity formulation
because it normalizes the chemistry of
the oxide for one molecule of the RO,
or flux column of the chart.
The physical characteristics of
glasses result in the fact that for most
chemicals found in glazes, the elements
that have one molecule of oxygen for each
molecule of material are “fluxes.”
That is, they are materials that
promote the melting of other materials
in the glaze at elevated temperatures.
The material in the middle, or R2O3
column, usually alumina (Al2O3),
affects the final characteristics of the
glaze by reducing devitrification and
by making the glaze harder and more chemical
resistant.
The material in the third column,
almost always silica (SiO2),
is often called the “glass former.”
It forms the basic structure for
the glaze.
Glazes formulated for ceramic applications
are designed to become glasses at elevated
kiln temperatures.
Then, depending upon the desired
aesthetic, the glazes may remain glass
upon cooling or may be devitrified to
form crystalline glazes.
Using this convention, the following chart shows the
breakdown of the oxide molecules in an
average formulation of soda lime container
glass:
By this method, the effective molecular weight of soda
lime glass is 226.
The ratios of the oxides determine the firing characteristics
of the glaze.
For example, glazes that fire at
1650 Fahrenheit are expected to have a
silica ratio in the range of 1.0 to 1.5
in the unity formulation. Higher temperature glazes have higher silica
ratios.
Higher temperature glazes also
tend to have higher alumina ratios, and
different combinations of flux than are
present in container glass.
This means that other materials
must be added to glass to make an effective
ceramic glaze.
In addition, the coefficient of thermal expansion,
which must match the clay body under the
glaze for proper “fit,” may need to be
adjusted to use glass as a glaze.
Many excellent books have been
written on glaze formulation. The reference below is one good one for introductory
information.
Glass used in glazes must be ground very fine in order
to be reactive with the other materials
in the mix.
Implementation: These instructions can be used
as an introduction to those interested
in formulating ceramic glazes using recycled
soda lime container glass.
Benefits:
Glass
can be an effective and inexpensive constituent
of ceramic glazes, if properly processed
and combined with other appropriate raw
materials.
Application
Sites: Glaze
formulators.
Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org.
References: Rhodes, Daniel, Clay and Glazes for the Potter, Chilton
Book Company, 1973. Issue
Date / Update: October, 1998
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