iandol on mon 24 jul 00
I recall either reading in an article by Janet De Boos or being told at =
some time or other that the orange flash you are concerned with is due =
to the presence of Beta Alumina. This is one of the phases in the =
Na2O-Al2O3-SiO2 system. Though why this should have an orange coloration =
unless by contamination from a source of iron I would not know. I do =
know that at the Jane Hamlyn workshop the clay we used was a white =
stoneware which became remarkably discoloured through mobilisation of =
the iron oxide which was in the refractories of the kiln.
Regards,
Ivor.
Craig Martell on tue 25 jul 00
lvor remarked:
>I recall either reading in an article by Janet De Boos or being told at
>some time or other that the orange flash you are concerned with is due to
>the presence of Beta Alumina. This is one of the phases in the
>Na2O-Al2O3-SiO2 system. Though why this should have an orange coloration
>unless by contamination from a source of iron I would not know.
Hello Ivor:
Geez "beta alumina"!! It's always something else, isn't it? The alumina
is important in the development of these orange colors because when it is
in a higher ratio with silica it allows smaller amounts of iron to remain
crystalline. At least that's what I perceive to be the case. Since the
alumina will resist a heavier salt glaze, the smaller amounts of iron in
the clay or in slips cannot be dissolved by the salt glaze and you get a
nice orange blush.
regards, Craig Martell in Oregon
iandol on wed 26 jul 00
Dear Craig ,
You wrote
because when it is in a higher ratio with silica it allows smaller =
amounts of iron to remain crystalline. At least that's what I perceive =
to be the case. Since the alumina will resist a heavier salt glaze, the =
smaller amounts of iron in the clay or in slips cannot be dissolved by =
the salt glaze and you get a nice orange blush.>
Craig,
I understand what you are saying but this is conjecture unless =
substantiated with some analytical work, something I cannot do as I no =
longer have access to a lab. As I said in an earlier post, the chemistry =
of salt glaze is not yet clearly understood. I think it is much =
misunderstood as is the reaction with sodium carbonate.
However, if a vitreous phase is present in the surface of the clay body =
or a slip which has been applied to the body then salt, sodium chloride, =
will react with it. Yes, sodium chloride. Not Soda. Not Sodium ions. Not =
sodium oxide. From information I have found, some sort of base exchange =
occurs where sodium ions are left behind in the vitreous melt and other =
ions substitute, then migrate out of the system into the kiln atmosphere =
as gaseous chloride compounds. The following chlorides have been =
detected in kiln effluent. Potassium chloride, Aluminium chloride, =
Silicon tetra-chloride, Ferric chloride. Except for KCl all react with =
water to give basic oxides and hydrochloric acid.
Several people have found, as have I, that there are sometimes glittery =
crystals on the surface of a salt glaze piece. Under the microscope =
these look like haematite crystals. Finely divided haematite give a red =
streak. Experience also points to the notion that because reduced iron =
oxides assist earlier body vitrification they will be more susceptible =
to mobilisation and invade the kiln atmosphere before other reactions =
occur. Given that a high alumina/low silica body or slip will need a =
higher temperature to mature and to achieve a sufficient volume of =
vitreous phase to form a coherent glazed surface I can understand why =
iron oxide is redeposited into the open structure of an undeveloped or =
immature glaze surface. Hence, those superb orange flashes on semi mat =
surfaces.
I have fired test pieces and slides which show that pure alumina and =
silica do not react with NaCl at cone 8/9 so you are right to believe =
that excess free alumina resists the action of salt vapour. Perhaps you =
can accept my reasons for this phenomenon.
However, none of this explains why our friend Phil Rogers is getting a =
coarse dark texture on the side of his pots against which the flame =
impinge. And I am still thinking about that.
All the best,
Ivor Lewis.
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