Marianne Lombardo on thu 15 feb 01
>From reading some past posts, I seem to have picked up that one is not =
supposed to fire a pot with chrome oxide in the glaze in the same =
kiln-load as another pot with tin oxide in its glaze. The reason being =
that the pot with chrome in the glaze will turn the pot with tin oxide =
in the glaze pink? Have I got this right, or have I misinterpreted =
something?
I'm going to do a firing in a few days, and I've used a white glaze with =
tin oxide inside a few pots, but I also have another pot that I =
experimented on with chrome in the glaze. If I vent the kiln by keeping =
the lid cracked open for a few hours will that solve the problem?
Marianne Lombardo
Omemee, Ontario, Canada
email: mlombardo@nexicom.net
John Hesselberth on fri 16 feb 01
Marianne Lombardo wrote:
>From reading some past posts, I seem to have picked up that one is not
>supposed to fire a pot with chrome oxide in the glaze in the same
>kiln-load as another pot with tin oxide in its glaze. The reason being
>that the pot with chrome in the glaze will turn the pot with tin oxide in
>the glaze pink? Have I got this right, or have I misinterpreted something?
You have it right.
>
>I'm going to do a firing in a few days, and I've used a white glaze with
>tin oxide inside a few pots, but I also have another pot that I
>experimented on with chrome in the glaze. If I vent the kiln by keeping
>the lid cracked open for a few hours will that solve the problem?
Doing this probably won't solve the problem. The chromium doesn't
migrate at low temperatures. If you are determined to try it, place the
pots as fare from each other as you can. If you have a kiln vent system
with the blower underneath your kiln place the white pots on a top shelf
and the chromium containing pots on the bottom. If you don't have a kiln
venting system reverse the above. Good luck.
Regards, John
"The life so short, the craft so long to learn." Hippocrates, 5th cent.
B.C.
Rick Shanks on tue 20 feb 01
Hello Marianne,
Almost daily, we fire glazes containing tin oxide
beside glazes containing up to 3% green chrome
oxide. Some tin glazes blush pink, some don't.
The ones that turn pink from fumes in the kiln
will often turn pink if very small amounts of
chrome oxide are added directly to the glaze. The
ones that don't blush pink will give little or no
pink color when chrome is added to the glaze. In
other words, if a glaze that contains tin oxide
is not chrome/tin pink compatible, the glaze will
not blush pink from fumes. It will remain white
or whatever the uncontaminated color is supposed
to be.
The key to avoiding chrome/tin pink fume effects,
or on the other hand, to developing rich pink or
maroon colors, is the amount of calcium oxide in
the glaze. Excessive amounts of zinc, magnesium,
boron and/or aluminum oxides tend to fade or
eliminate Cr/Sn pink. By increasing calcium oxide
their effect can be overcome, yielding stable
pink or maroon colors. To fade or eliminate pink
in-glaze color or pink fume effects, reduce or
eliminate calcium oxide from the formula or
increase the offending oxides. Common sources of
calcium oxide in glaze recipes are whiting
(calcium carbonate) and wollastonite.
Tin glazes that are not Cr/Sn compatible will
sometimes blush green if placed very close to
glazes with substantial amounts of chrome oxide,
however. Chrome bearing glazes with lots of
volatile constituents, such as high alkali
content, are more likely to be troublesome
(example: lots of lithium carbonate). Copper is
as likely to fume tin glazes green or blue as
chrome.
For glazes that are not Cr/Sn compatible, giving
a little room between pieces will usually be
adequate to avoid problems. We do not use
downdraft kiln vents or leave lids cracked open
during glaze firings.
Rick Shanks
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