ARMCHAIR POTTING.
NO OPACIFIER OPAQUE EARTHENWARE GLAZES.
Post war years (1945-55) were very difficult times for the ceramic manufacturers
due to shortages of a many raw materials- mainly ceramic stains and glaze
opacifiers. Scores of manufacturers frantically worked to produce their own
stains by blending metallic oxides with fluxes, extenders, opacifiers ect
and calcining the mixtures to high temperature with subsequent pulverising.
The chief opacifier in those days was tin oxide ,which was and still is a
very important ingredient in ceramic stains to produce burgundy and maroon
with chromium oxide and also to render clear earthenware glazes opaque. It
was almost impossible to get it and thus a race for very different type of
opaque glazes begun. A score of ceramic researchers reported milkiness in
glazes containing boric oxide as far back as the beginning of last century.
Dozens of papers were published on this subject, mainly up to the middle of
the century, which concentrated on ternary (triaxial) systems of base oxides
of lead ,calcium, barium and zinc and others with boric oxide and silica.
In many of the systems strong opacity developed due to liquid immiscibility
also named as emulsion glazes. It is interesting to note that in most of the
experimentations alumina was not present.
When one reads fritt and glaze information supplied by fritt manufacturers
(copies are available at ceramic supply shops) one can not escape repeated
suggestion like "fritt 4124 will produce milky glossy glaze when used
alone" or "fritt 4113 under some condition produces chun effect
especially when used with fritt 4193" and further "fritt 4l08 will
produce milky chun effect well defined on red bodies". All these fritts
have one thing in common-they are low in alumina. Reading more information
we find that many fitted glaze formulas contain the following suggestion "fritt
4113 needs a judicious addition of alumina or china clay to produce a clear
glossy glaze". It is now clear that the alumina content in fritted glazes
containing boric oxide is of paramount importance - low alumina = milky glaze
and conversely judicious addition of alumina or china clay = clear glaze.
It may sound very simple but that the way it works with very few exceptions.
If you have a fritted glaze which does not fire clear, add to it the right
amount of china clay or hydrated alumina to clear it. Of course the right
additions you find by trial and error on a line blend principle. One can now
assume that no alumina in fritted glazes (containing boric oxide) will increase
the incidence of milkiness. This assumption is correct and to achieve complete
opacity we have to do bit more i.e. make sure the glaze has above average
calcium oxide, zinc oxide or barium oxide as well as boric oxide. We cannot
make fritts to the specifications just described and therefore we look for
a fritt which is already available on the market and try to explore it. The
fritt in question is Ferro 4112 marketed as synthetic colemenite and the recipes
is as follows:
In late fifties the race for the unusual opaque glazes were indeed numbered the supplies of tin oxide peaked and a new opacifier micronised zircon - came on the marked. The ceramic industry prefers the more stable traditional opaque glazes where the opacity is due to the glaze saturation by tin oxide or zirconium silicate. The ceramic craftsperson or artist has a tremendous potential now to use the unusual opaque glazes and create a new set of standards for colour variation for ornamental ceramics. Please do not use these glazes for utility kitchenware.
Cheerio for now
Mike Kusnik
This glaze has no clay to keep it in suspension - you have
to use 5% of bentonite or glaze hardener that you can get from ceramic supply
shops.
The glaze should fire to cone 06-02 with good gloss and opacity - with additions
of metallic oxides you will achieves interesting colour variations.
|
Glaze
|
C |
| Fritt 4112 |
57%
|
| Fritt 4108 |
9%
|
| Barium carb |
15%
|
| Silica |
19%
|
|
100%
|