I.Lewis on thu 3 jun 99
To: All Subscribers clayart=40lsv.uky.edu
From: Ivor Lewis. ivorredhill=40yahoo.com.au
Thursday, 3 June 1999
Subject: Kiln Volatiles. Zinc oxide.
We play with some pretty dangerous things don=92t we. A good question ask by=
Jeff
Lawrence, =22What other materials do we use which have a high volatility and=
are
toxic=22 . I=92m surprised that there are so few fatalities which are =
attributable
to the things we do with our kilns.
Not a lot is said about Zinc. After all, it seems to be part of or diets in =
some
way or other and it has been used in pharmacuticals for years so why should
potters worry.
Zinc gets out of glazes in a curious way. We tend to use the oxide in our =
glazes
as a supplementary flux to get glazes in the cone 4 to 6 range. This is =
great so
long as you fire electric or refrain from reducing. But a flame fire potter
cannot guarantee that there is always a uniform oxygen bearing atmosphere in=
any
kiln. This is the problem. Zinc oxide melts at 1975oCelsius and boils at =
913o
Celsius. This information is found in Tables of Physical and Chemical =
Constants
by Kaye and Laby, published by Longman. Zinc oxide is rapidly and readily
reduced to zinc metal in a reducing atmosphere. Therefore if a zinc bearing
glaze is reduced at least a proportion of the zinc must be lost. This will
change the nature of the glaze.
For a long time I used the first of Emanuell Coopers recipe books and a lot =
of
glazes with zinc in them. Cooper says they can be reduced. I was firing with=
oil
and wanted reduction. How did I recognise that I was loosing zinc? A yellow
frosting would collect around cracks in the wadding which turned white as =
the
kiln cooled.
One of the hazards faced by Welders comes from welding Galvanised Steel. =
They
get, and I have had, a condition known as =22Metal Fume Fever=22. It mimics
pneumonia or a viral chest infection. Lasts about three days. You feel like
hell, aching joints and head, tight chest raw cough etc. etc. It=92s caused =
by the
zinc which galvanises the steel and is boiled off by the arc. I wonder if
potters have experienced such symptoms after a firing when they have been =
using
high zinc glazes? Hot Whisky and Panadine seem to be the best cure for the
symptoms.
Notice that the Zinc Crystal instructions tell you not to reduce.
Keep the Good Oil coming.
All the best,
Ivor Lewis
Ron Roy on sat 5 jun 99
------------------
A small addition to Ivor's excelent note - there is often reduction in
electric kilns - and ZnO is easily reduced - perhaps this is why zinc
oxidation glazes are well known for pinhole problems.
Look for an article in CM by David Hendley on this subject - well
documented experiments that will confirm what Ivor just said.
=3ESubject: Kiln Volatiles. Zinc oxide.
=3E
=3EWe play with some pretty dangerous things don=92t we. A good question ask=
by Jeff
=3ELawrence, =22What other materials do we use which have a high volatility =
and are
=3Etoxic=22 . I=92m surprised that there are so few fatalities which are =
attributable
=3Eto the things we do with our kilns.
=3E
=3ENot a lot is said about Zinc. After all, it seems to be part of or diets
=3Ein some
=3Eway or other and it has been used in pharmacuticals for years so why =
should
=3Epotters worry.
=3E
=3EZinc gets out of glazes in a curious way. We tend to use the oxide in our
=3Eglazes
=3Eas a supplementary flux to get glazes in the cone 4 to 6 range. This is
=3Egreat so
=3Elong as you fire electric or refrain from reducing. But a flame fire =
potter
=3Ecannot guarantee that there is always a uniform oxygen bearing atmosphere
=3Ein any
=3Ekiln. This is the problem. Zinc oxide melts at 1975oCelsius and boils at =
913o
=3ECelsius. This information is found in Tables of Physical and Chemical =
Constants
=3Eby Kaye and Laby, published by Longman. Zinc oxide is rapidly and readily
=3Ereduced to zinc metal in a reducing atmosphere. Therefore if a zinc =
bearing
=3Eglaze is reduced at least a proportion of the zinc must be lost. This =
will
=3Echange the nature of the glaze.
=3E
=3EFor a long time I used the first of Emanuell Coopers recipe books and a =
lot of
=3Eglazes with zinc in them. Cooper says they can be reduced. I was firing
=3Ewith oil
=3Eand wanted reduction. How did I recognise that I was loosing zinc? A =
yellow
=3Efrosting would collect around cracks in the wadding which turned white as=
the
=3Ekiln cooled.
=3E
=3EOne of the hazards faced by Welders comes from welding Galvanised Steel. =
They
=3Eget, and I have had, a condition known as =22Metal Fume Fever=22. It =
mimics
=3Epneumonia or a viral chest infection. Lasts about three days. You feel =
like
=3Ehell, aching joints and head, tight chest raw cough etc. etc. It=92s =
caused
=3Eby the
=3Ezinc which galvanises the steel and is boiled off by the arc. I wonder if
=3Epotters have experienced such symptoms after a firing when they have been=
using
=3Ehigh zinc glazes? Hot Whisky and Panadine seem to be the best cure for =
the
=3Esymptoms.
=3E
=3ENotice that the Zinc Crystal instructions tell you not to reduce.
=3E
=3EKeep the Good Oil coming.
=3E
=3EAll the best,
=3E
=3EIvor Lewis
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Jim Brooks on sun 6 jun 99
how does it( Zinc Oxide) boil at a much lower temp than it melts........
AT.????.........Something is amiss here.!!!. .Please correct with the with
good information..thanks.....
Stephen Grimmer on mon 7 jun 99
Jim,
Zinc Oxide has a high melting point, indeed. The trouble is that the
stuff readily reduces to it's metal form, Zinc, which has a low melting (and
boiling) point, as ceramic temperatures go. This phenomenon occurs both in a
reducing kiln and in the heat of a welder's torch or arc.
steve grimmer
marion illinois
----------
>From: Jim Brooks
>To: CLAYART@LSV.UKY.EDU
>Subject: Re: Kiln Volatiles. Zinc oxide
>Date: Sun, Jun 6, 1999, 2:17 PM
>
>----------------------------Original message----------------------------
>how does it( Zinc Oxide) boil at a much lower temp than it melts........
>AT.????.........Something is amiss here.!!!. .Please correct with the with
>good information..thanks.....
>
Tom Buck on mon 7 jun 99
Jim:
Yeah, I noticed the ambiguity (confusion between Zn and ZnO
boiling points) but since I knew the answer I let it slide.
ZnO as such in a glaze mix starts its fluxing action around 1000
oC (bristol glaze) but by itself ZnO would not melt until 1975 oC is
reached.
However, ZnO is easily changed to Zn (metal) by the action of CO
and H2 in the reduction phase of a gas-fired kiln. AND pure Zn metal melts
at 419 oC and boils at 907 oC.
So, if the reduction of ZnO to Zn metal starts happening at 1000+
oC (commonly done in C10 firings), then ZnO goes to metal and boils
immediately so in effect the Zn metal is "formed" in the gas phase and
leaves the kiln.
Hence, putting zinc oxide in a highfire glaze that will undergo
reduction really is throwing money up the flue; the zinc doesn't stay
around long enough to do much fluxing.
David Henley ran some copper red tests, some with ZnO as a "flux"
and the same mixes without ZnO. He reported he very recently that he could
detect no difference in the fired pieces, both produced the red he
sought.
BFN. Tom.
Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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