iandol on fri 9 aug 02
Dear Geoff Walker,
I understand the reason for calcining Kaolin or other clays is to reduce =
the shrinkage potential due to plasticity or water adsorption and =
prevent shrinkage problems with high clay glazes.
I understand that many raw materials, especially natural minerals are =
calcined to cause them to disintegrate.
I understand that bones are calcined to eliminate the organic fractions, =
that some salts are calcined to remove water of crystallisation.
But I fail to understand why a refractory oxide such as Zinc Oxide must =
be calcined. Zinc Carbonate yes, to discharge the carbon dioxide. So why =
calcine Zinc Oxide?
Best regards,
Ivor Lewis=20
Stephani Stephenson on sat 10 aug 02
Ivor wrote:
.....But I fail to understand why a refractory oxide such as Zinc Oxide
must
be calcined. Zinc Carbonate yes, to discharge the carbon dioxide. So why
calcine Zinc Oxide?"
Ivor, I have always heard that one calcines zinc oxide to prevent glaze
defects associated with zinc oxide.
. For example, Hopper states "When used as a principal flux in amounts
over 25%, it may cause crawling, pitting and pinholing" later he states
that "faults associated with zinc may be improved by calcining up to
half the material to be used." I found similar statements in other
references.
Still, in all these references I could find no chemical explanation for
this.
I use zinc oxide regularly but never in the amounts described above. The
first time I used Zinc Oxide I did calcine because I had always read to
do it, but thereafter I didn't calcine and had no problems. But I
usually use less than 10%. sometimes a bit more.
ALso I wonder if the calcining affects the physical properties of the
zinc powder moreso than the chemical properties.
Since zinc is often used to promote opacity, a matte glaze, and dry
surfaces, perhaps too much of the un-calcined powder would tend to pull
the glaze coat away from the clay surface, even at room temperature?
Also I wonder if this happens in certain situations, primarily when
glaze is applied to greenware vs. bisqueware. Greenware is a dustier
surface and I have noticed some glazes which do not crawl at all when
applied to bisqueware will crawl on greenware. I do not think this is
due to shrinkage, but to the dusty, less absorbent surface of greenware.
Some glazes will bead up in the raw stage and this condition is
exacerbated , not healed, in the firing..
So I do wonder if this advice on calcining zinc has to do with certain
conditions that have to do with amounts, physical
batch properties, possibly even firing temp? perhaps wisdom from the
times when bristol glazes were favored?
One question I have is: What happens to the oxygen molecule in calcining
process? does this affect the way zinc will bond with other molecules
in the firing ? Also when firing, if some zinc molecules have been
calcined and others haven't, will the zinc in each of them melt at
different rates ?? I am wondering if this would matter if one is using
zinc at lower temps vs. stoneware temps.
It is interesting, and now you have me wondering too!
Sincerely
Stephani Stephenson
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