Ron Roy on mon 16 jun 97
There are 5 mid range fluxes, CaO, ZnO, MgO, SrO and BaO. This means they
are of most use as fluxes at the higher temperatures starting about 1100C
depending on their individual characteristics. Each will "become" a flux at
a certain temperature depending on a number of circumstances.
All of these fluxes will produce a matte glaze if oversupplied and again
depending on certain circumstances.
CaO is the only flux capable of producing a transparent matte.
In fact each of the first three CaO, ZnO, and MgO produce their own
distinctive type of matte surface.
BaO is well know for it's ability to produce a very fine, silky smooth type
of matte glaze which most potters find very attractive. We have to
understand that in oversuppling a glaze with any oxide durability will be
affected. This is true with these middle range fluxes as well. In fact it
is predictable - the more the oversupply the more chance durability will be
affected. It is true that some oxides will affect durability more than
others and I cannot say I know which ones and how much. Testing is the only
way to find that out.
I can say however that matte glazes are not functional as liner glazes.
They scratch, mark and stain easily. They are more susceptible to chemical
attack than shiny, well formulated and balanced glazes.
The other mid range flux that produces a matte surface most similar to BaO
is SrO. The same silky, satin smooth texture. The difference between the
two is in the colour response to coloring oxides, most commonly copper and
cobalt. I have worked on some of these glazes. I did find the response to
iron to be more attractive when SrO was substituted for BaO.
The other advantage SrO would have over BaO. Many matte glazes craze - it
so happens that SrO has a lower expansion/contraction rate than BaO and you
need less of it to get the same fluxing power. To put this another way - if
you use the same amount of strontium you get more melting which means you
can add more of the low expansion fluxes like silica and alumina. If the
problem is a low expanding glaze the substitution of strontium for barium
will probably make the glaze have even less expansion - make sure you test
for fit problems whenever you are adjusting a glaze.
I think it is the right time to start at least trying to use strontium so
we will be ready when the controls are extended to it's use. Yes I do think
it will happen and I am not saying it is justified either. I do know there
are irresponsible people out there who will cause it to be controlled
though.
I can remember seeing a blue matte barium glaze on a mug - it had been
glazed inside and out with the same cone 8 barium matte glaze. Trouble was
the glaze was pure white on the inside because some one had left some
neo-citrin in it.
It's just a matter of time folks. The more potters who use these kinds of
glazes on functional work will only make it happen faster.
I think we must realize that when a customer experiences a problem with our
pots some of them will bring them back. More will simply decide not to buy
from us again. There will even be some who will never buy a hand made pot
again from any potter.
Choosing glazes by colour and texture only is simply not in our own best
interest. Testing for fit and using glazes in a responsible way is simply
in our own best interest.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/ronroy.htm
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