Roger Graham on tue 13 aug 02
No new information in this post, but an interesting observation to do with
zinc oxide in reduction-fired glazes. In my early adventures with copper
reds, I made up sample quantities of a dozen or so promising recipes to try.
Most of them contained barium carbonate, and zinc oxide.
Some time later, I encountered David Hendley's five-star article on "Simply
Red" (thanks David!) where he reminds us that zinc oxide reduces easily to
zinc, which in turn becomes zinc vapour in the hot kiln, and so evaporates
from the glaze. (Zinc melts at 420 degrees C, boils at 930 degrees).
Convinced by this logic, all the copper reds I've made since then contain no
zinc. But of course there's a half bucket of an earlier mix, with zinc and
barium. Probably not food-safe, but I'd not like to throw it out. Use it
sometimes on the outside of a pot.
I've been experimenting lately with an "exhaust sniffer" monitoring the
gases emerging from the flue of a little test kiln. And in two recent
firings, noticed a strange deposit of white fluffy stuff, like a miniature
snow drift, accumulating on a probe inserted in the kiln flue. Nothing seen
inside the chimney, only in a limited area at the mouth of the flue. And of
course, on instruments inserted in the flue. Puzzled all day over this
strange stuff, until a metallurgist friend looked at it and said "Looks like
zinc to me!".
Couldn't be, said I. No zinc in there. Just copper reds.... oops! I have
been using up the half bucket of older glaze, zinc and barium and all, on
the outside of various pots. And sure 'nuff, the zinc was volatilizing. Up
the chimney as zinc vapour. Combine with oxygen at the flue opening, and
turn instantly to zinc oxide. The powdery stuff is yellow when hot, white
when cold. Zinc oxide does that too, as I now remember from chemistry
classes over 50 years ago. It's rather humbling, to find that a once-quick
brain has slowed to the speed of a pregnant snail.
Interestingly, in a bisque firing in the same little kiln this morning, the
white fluffy deposit was still forming on the instruments in the flue. Only
a faint deposit this time, but definitely there. So it seems as if a deposit
of zinc or zinc oxide may have built up on the kiln furniture or lining, to
be slowly released in future firings. Hmm.
Thought you'd like to know. About the zinc oxide, that is. Not the pregnant
snail.
Roger Graham
http://members.optusnet.com.au/~rogergraham
David Hendley on tue 13 aug 02
Thanks for this report, Roger.
Since the "Simply Red" article, I have done some further experiments
with zinc oxide in cone 10 reduction glazes. The results have been far
from definitive.
I searched around for any glazes I could find that included substantial
amounts of zinc oxide (4% or more). I mixed up batches of these glazes
as written, and also batches without the zinc oxide, glazed sets of test
tiles, and fired them side-by-side.
In about half the glazes (4 tests), I could tell no difference in the glazes
with or without the zinc oxide, and in a couple of instances, there seemed
to be slight differences.
The remaining tests showed, without a doubt, a difference between the
batches made with and without zinc oxide. The differences showed
across several sets of test tiles. One glaze, a rutile blue sent to me by
Dave Cuzak from Clayart, really benefited from the inclusion of zinc
oxide. There is no doubt these glazes were reduced, as my firings always
have copper reds throughout the kiln, and I reduce early and heavily.
So, I am now reluctant, even knowing that zinc volatilizes at a low
temperature when reduced, to dismiss it as totally useless in reduction
glazes.
As I always like to say, I believe what I see before I believe what
should be.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
----- Original Message -----
> I've been experimenting lately with an "exhaust sniffer" monitoring the
> gases emerging from the flue of a little test kiln. And in two recent
> firings, noticed a strange deposit of white fluffy stuff, like a miniature
> snow drift, accumulating on a probe inserted in the kiln flue. Nothing
seen
> inside the chimney, only in a limited area at the mouth of the flue. And
of
> course, on instruments inserted in the flue. Puzzled all day over this
> strange stuff, until a metallurgist friend looked at it and said "Looks
like
> zinc to me!".
>
> Couldn't be, said I. No zinc in there. Just copper reds.... oops! I have
> been using up the half bucket of older glaze, zinc and barium and all, on
> the outside of various pots. And sure 'nuff, the zinc was volatilizing. Up
> the chimney as zinc vapour. Combine with oxygen at the flue opening, and
> turn instantly to zinc oxide. The powdery stuff is yellow when hot, white
> when cold. Zinc oxide does that too, as I now remember from chemistry
> classes over 50 years ago. It's rather humbling, to find that a once-quick
> brain has slowed to the speed of a pregnant snail.
> Roger Graham
>
John Hesselberth on tue 13 aug 02
Hi David,
I suspect the differences you are seeing are due to the differences in
seal-over temperature vs. where you start reduction. If you are
reducing before the glaze has sealed over the zinc will surely all (or
nearly all) be gone. But if you don't start reduction until the glaze
has sealed over most the zinc will remain as zinc oxide. Once seal-over
has occured about the only reduction you are really doing is to those
few molecules on or very near the surface.
Regards,
John
On Tuesday, August 13, 2002, at 09:20 AM, David Hendley wrote:
> The differences showed
> across several sets of test tiles. One glaze, a rutile blue sent to me
> by
> Dave Cuzak from Clayart, really benefited from the inclusion of zinc
> oxide. There is no doubt these glazes were reduced, as my firings always
> have copper reds throughout the kiln, and I reduce early and heavily.
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