COLUMBIA

Glaze, Continued from Page 9

melt on their own at normal kiln temperatures

but they are dissolved into the melt created

by others. Consider an inventory of some of
them (we won't look at colorants, opacifiers,
variegators):

* Pure mineral flux particles like dolomite,
whiting and talc are active melters at cone
10 but certainly not at cone 6. You could
increase them all you like, youll never get
a workable cone 6 glaze. By ‘workable’ I
mean that although you might be able to
get it to melt, so much of the recipe would
be taken up by these materials that a stable
low expansion glaze could not be created for
the lack of alumina and silica.

e

Kaolin and ball clays are refractory (melt
high) and they have to be dissolved by other
things. To reduce a glaze’s melting range the
percentage of these materials obviously must
be reduced. However their amounts can only
be taken down to about 15% or the glaze
slurry won't suspend or dry hard (unless
you employ organic binders that introduce
side effects not easily dealt with by potters
or small operations.) More important,
clays are the key alumina supplier, taking
them too low will detrimentally affect glaze
hardness and melt viscosity and increase
thermal expansion.

e

Silica is nowhere close to melting at
has

dissolved by other materials that create

stoneware temperatures. It to be
a fluid melt. Attempting to employ high
temperature fluxes at cone 6 is not going to

dissolve any badly needed silica into a melt.

e

Feldspars (and Nepheline Syenite) have
complex chemistries and soften over a range
of temperatures, they begin to melt at cone
6. However they don’t melt well. If you add
enough high-thermal-expansion feldspar to
a high fire glaze to melt it at cone 6 you will
have a guaranteed crazing glaze as well as
one that is likely to leach because sodium
and potassium will be oversupplied.

The bottom line is that we cannot just
reorganize a cone 10 recipe to melt at cone
6. We must add something new, a flux or
fluxes not normally found in cone 10 glazes.
We must also reduce the proportion of silica
and alumina in the glaze, since even with
added fluxes it is not possible to get a glaze to
dissolve the amounts of high melting alumina
and silica typically found in cone 10 glazes.
I am not saying it is impossible, you might
be the magician who has found a way. But
remember, the challenge here is to adjust

an existing cone 10 recipe to melt at cone
5- 6 and not craze or leach or scratch easily
and still have good application and working
properties. That is not easy.

‘The most obvious solution is to add powerful
fluxes like zinc oxide or lithium carbonate
which are not commonly used or needed. at
cone 10. They melt very early and vigorously
and can impart significant melting effects in
small amounts in some circumstances. The
only problem is that this is not one of those
circumstances. A little zinc is not going to
dissolve a lot of refractory particles. A lot of
zinc and/or lithium to make the glaze melt at
cone 5-6 will create a whole new animal and
a whole new set of problems, especially with
regard to colour response, surface character,
tendency to devitrify (crystallize), crawl,
bubble, etc.

I want to point out that when I say we
need to ‘add something new’, I did not mean
a material. I meant an oxide. Materials are
composed of oxides, and most materials
contribute more than one oxide, sometimes
six or eight different ones, e.g. feldspars.
When you talk about a fired glaze and the way
it melts and solidifies, you are talking mainly
about chemistry (oxides), whether you like
it or not. If we add or remove feldspar, for
example, we are making a fundamental change
in the balance of oxides in the glaze because
feldspar contributes so many oxides. This does
not jibe with the objective of making changes
that will give the greatest reduction in melting
behaviour accompanied by the least change in
overall glaze appearance and fired properties,
eg. colour response, thermal expansion,
surface character, working properties.

‘There are two main approaches we can take.
Method 1: Transplant Mechanisms

At the risk of negating most of what I have
just said, identify the “mechanisms” in the
glaze and transplant them into a good base
glaze for cone 6 that has similar surface texture

ge
BNWCF
North-West

Ceramics
Foundation

and degree of melting. Compare with a flow

tester if needed. Answer these questions:

* What gives it the color? Is it a stain, a
metal oxide, a stained clay or mineral, or
the growth of micro-crystals (surface or
entrained) that have a characteristic colour?
Put this ‘mechanism’ into a cone 6 base.
Note however that you need to think about
whether a color depends on a sympathetic
chemistry in the base glaze. Many stains,
for example, require that the base glaze be
zinc or magnesia free, for example. Others
require the presence of a certain amount of
CaO. Crystallization likewise depends on
the presence of specific oxides.

e

Is the glaze opaque or transparent? If it is a
deep and vibrant colour, especially one that
is darker where the glaze is thicker, it is likely
a transparent that contains a colorant that
can be seen deep into the glass layer. If it has
a pastel surface it likely contains a colorant
plus has an opacity mechanism (contains an
opacifier, is crystallizing, or is not completely
melting.) If it has a variation in coloration
(without variation in thickness) then there
may be some sort of crystallization going
on to produce the color. If it is just white
all over then it is opacified. If the white
turns to transparent at contour edges, then
crystallization is involved in the opacity
(e.g. from titanium dioxide). Actually, just
look at the recipe. Does it have an opacifier
like tin oxide, titanium or zircopax? Does
it have a colorant like cobalt or chrome
oxide? Non-crystallizing opacification and
color mechanisms will often transplant into
another base.

e

Is the glaze variegated or mottled? If the
effect is a result of the presence of rutile
or titanium then transplant either material
into the cone 6 base glaze.

* Does the glaze have speckles that are a
product of a granular or unground material?

Continued on Page 11, Glaze

Talk at Emily Carr: Pau! Mathieu

The Art of the Future: Fourteen Essays on Ceramics is the working
title of a new book by Paul Mathieu that is nearing completion.
Paul will give a well illustrated public lecture, sponsored by the
North West Ceramics Foundation, that will explore the themes

of the book at Emily Carr University room SB 301, October 4, at

3.30 pm. The lecture will consider the relationship of ceramics to culture in terms of its

meaning across time and history as a way to re-evaluate and define a contemporary role for

ceramics as an art of the future.

Paul Mathieu teaches ceramics at Emily Carr University. He is the recipient of the

Governor General Award in visual Arts 2007 and the Saidye Bronfman Award for

Excellence in Crafts 2007. He is the author of ‘Sexpots: Eroticism in Ceramics’.

Potters Guild of BC Newsletter : October 2008