TechnoTip

Changing the Firing Temperature of a Glaze

conmiiued from page 7

important reason: Boron has alow thermal
expansion; additions of it will reduce glaze
crazing. This is so important because most
high temperature glazes contain far more
high expansion Na20, K20 than can be
tolerated at cone 6 without producing craz-
ing, In addition the bow expansion silica
and alumina they contain must be reduced
to achieve melting at cone 6; this also
moves the glare toward crazing.

The next question is; What source of be-
ron should you use? The best answer is in
the world of frits. There is a boron frit out
there to meet almost any fluxing chal-
lenge. However each frit contributes other
oxides besides boron; that means that to
reach the goal of only increasing the boron
in a glaze you need to be able to reduce
other materials in the recipe that contnib-
ute the same oxides as the frit. Thus to do
what I am about to describe, you need a
ceramic chemistry software program. The
kinds of material juggling I describe are

dealt with in the examples section of the
INSIGHT manual.

The first thing to consider is this: Does the
glaze need clay (does it settle or lack dry
hardness?) or does it have too much clay
already (does it shrink and crack on dry-
ing?) If the former, use something like
Ferro Frit 3134 (20% CaO 10% Na20O,
23% B203, 46% $102). Here is why:
Since it contributes Na2O and CaO, that
means you will be able to reduce the amount
of feldspar in the glaze (because it is nor-
mally the major contributor of Na2O).
That reduction will also mean a loss of
Al203, but since the fit contains none you
will be able to supply it using a kaolin.
Also, you will be able to reduce CaO-
contributing whiting or wollastomite if these
are present (since the frit contributes CaO).
If the glaze already has too much clay, then
use Frit 3 124. It has almost the samechem-
istry, but it sources alumina also (meaning
you canreduce alumina-contibuting clay).
If the amount of clay in the glaze is OK,

then try Frit 3195; it is balanced enough to
be a glaze in itself.

The second last question is: Wher level
should the Al203 and S102 be ar in the
newly adjusted glaze? My suggestion is
set them to about 2/3 of what they were.
However, analeof thumb is to always have
as much alumina and silica as you can in
the glaze, Since B203 (boron) performs
the same function as silica, you can afford
to have lower S102 in the interests of good
melting. Also remember that boron has
very good alumina dissolving abilities;
thus it is often possible to leave it at the
same levels as it was in the cone 10 glaze.

The last question is: How much boron de
1 add? | would start with about 0.3 molar
equivalents, Ifthe glaze is reactive and has
to be very runny to achieve the desired
effect, you might have to take the boron
high as 0.8.

comfinwed on page JO

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February 2003 Patters Guild of Bntish Columbia Newsletter