william schran on tue 27 jul 04
For Ivor - Have been researching your question regards when & who
first stated/wrote about alumina inhibiting crystal formation in
glazes.
In a copy of the chapter on crystalline glazes in his book "Grand
Feu Ceramics", Taxile Doat lists formulas/recipes of frit compounds.
The constituents are composed of carbonate of potash (pearl ash),
zinc oxide & quartzy sand (silica). He stresses proper preparation of
the frit, but makes no mention of alumina or kaolin.
An abstract of an article W. G. Worcester wrote for Trans. Amer.
Ceram. Soc., X, 450-483 in 1908, titled: "The Function of Alumina in
a Crystalline Glaze":
"In groups where the alumina was fritted with the balance of the
glaze, spar is best source of alumina. The greater the fluidity of
glaze, the better the medium for crystals to grow in.
Alumina, fritted into the batch, is second best source for AL2O3.
The addition of .03 equivalent AL2O3 in the form of raw clay was
found to be sufficient to cause the glaze to float well, and to
adhere firmly to the body. (apparently the clay was added to suspend
the glaze and stick to the body during glaze application)
It may be said that while the use of alumina does not seem to
accelerate the growth of crystals, it does act as an agent to arrest
the action of the heat on the glazes, giving them a wider range of
usefulness. The use of AL2O3 from raw clay is to be recommended, not
only on account of its mechanical properties but it tends to hold the
glazes good over a long firing."
Since temperature is such a critical factor in crystalline glazes,
perhaps added alumina in the form of kaolin, sufficient to control
the viscosity of the glaze, may work but then, finding that exact
temperature would be the critical issue.
That said, I believe glaze movement is essential to creating large
crystals, thus I'm testing "normal" stoneware glazes on the bottom
half/two thirds of pots and the crystalline glaze on the upper part,
to be able to not use a glazing pedestal.
Bill
Ivor and Olive Lewis on wed 28 jul 04
Dear Bill,
Thank you for doing this work. Good reading. I appreciate your effort.
You say "Taxile Doat lists formulas/recipes of frit compounds. The
constituents are composed of carbonate of potash (pearl ash), zinc
oxide & quartzy sand (silica)."
This seems to be the root of some of the ideas presented by Sanders.
You quote from "An abstract of an article W. G. Worcester wrote for
Trans. Amer. Ceram. Soc., X, 450-483 in 1908, titled: "The Function of
Alumina in a Crystalline Glaze": Have you had a chance to read the
full text of this article. I would be interested in knowing about how
he came to these conclusions. Anything published by the ACS prior to
1920 is difficult to get hold of. There are no earlier holdings in
public libraries in Oz.
Sounds as though you are doing some interesting trials.
The idea of flow contributing to crystal growth is an interesting one.
If the glaze, as one might anticipate, is flowing downwards and the
crystal does not move with it, perhaps because of anchorage to the
clay, then the crystal growth should be upwards and laterally across
the pot face. But the examples we are presented with are usually
radially symmetrical which suggest that they flow downwards with the
glaze. Have you any ideas on this one?
Best regards.
Ivor Lewis.
Redhill,
S. Australia.
william schran on wed 28 jul 04
Ivor wrote:>The idea of flow contributing to crystal growth is an
interesting one.
If the glaze, as one might anticipate, is flowing downwards and the
crystal does not move with it, perhaps because of anchorage to the
clay, then the crystal growth should be upwards and laterally across
the pot face. But the examples we are presented with are usually
radially symmetrical which suggest that they flow downwards with the
glaze. Have you any ideas on this one?<
I think I can state, through my observations of firing results, that
crystals do flow down. Most of my best and largest crystals are found
in the catch basins!
The perfect process we folk doing crystalline glazes try to achieve
is: Get to the full melt of the glaze with movement of the glaze
quickly. Most critical is the last couple hundred degrees. Want to
get there at 350/500 =B0F per hour (I fire from start to cone 6 in 3-4
hours). Then crash cool to about 200=B0F below top temp (could be more
or less depending on glaze) and hold at that temp for desired length
of time.
Holding temp for each glaze is slightly different.
During glaze flow the bonds between zinc & silica are forming around
a nuclei. Once formed, then the flow needs to be stopped and
temperature held to allow continued bonding - or growing of the
crystals.
This is the most critical part of the firing - finding the optimum
holding temperature for each glaze. This can be changed by the
colorant used!
Bill
| |
|
