Sandy Henderson on fri 27 jun 03
I am curious what glaze problems calcined zinc would solve. Would it affect
pinholing?
Thanks,
Sandy Henderson
> Date: Thu, 26 Jun 2003 10:16:52 -0500
> From: "Longtin, Jeff"
> Subject: Re: Reviving old plaster
> [snip]
> Years ago I thought I needed to calcine some zinc to solve some glaze
> defects. It went in powder it came out lumpy gravel. (Not the intended
> result!) Luckily I was able to find some calcined zinc at A.R.T in
> Chicago in powder form
Jeff Longtin on sat 28 jun 03
Craig wrote:
"So calcining the zinc will get rid of the physically absorbed water
and will reduce the bulk of the zinc a little."
While I appreciate Craig's desire to set the record straight I do want to
acknowledge that this was a situation I confronted sixteen years ago and any
error in the explanation is solely due to my failed memory.
On the "error" of suggesting zinc shrinkage is the cause of crazing I would
just kindly suggest that "reduce" and "shrink" are somewhat synonymous in most
dictionaries.
Jeff Longtin
Paul Lewing on sat 28 jun 03
on 6/28/03 1:26 PM, Craig Martell at ashglaze@WVI.COM wrote:
> So calcining the zinc will get rid of the physically absorbed water
> and will reduce the bulk of the zinc a little. This is a reversible
> process though and over time the zinc will again suck up atmospheric
> moisture.
I wonder if anyone has ever tried, besides keeping your zinc oxide in an
airtight container, also keeping in with it one or two of those little
packets of chemical that come in prescription bottles? I just opened a new
bottle of MoveFree (glucosamine and chondroitin- you can tell I'm getting
old) and it had one in it. I believe they're often in there to absorb
moisture from the air and keep the pills fresh.
Paul Lewing, Seattle
WHC228@AOL.COM on sat 28 jun 03
Yes! It can help with pinholing.
Calcine some yourself and try it. Load some into a pot and fire it to 1256F.
Bill Campbell
Jeff Longtin on sat 28 jun 03
Sandy,
If I'm not mistaken the problem I was having at the time was crazing in a
clear glaze over a porcelain body (cone 6 slip cast).
It was explained to me, by my good friends at Minnesota Clay (now at
Continental Clay) that crazing is mostly a "fit" issue. (Zinc, apparently, shrinks
alot.) By calcining it I decreased the amount it shrank prior to melting, i.e. I
"pre-shrank" it. (As I explained, calcining it on my own was unsuccessful, but
luckily, I found a source for it anyway, A.R.T. in Chicago.)
Calcined zinc didn't solve the problem actually. Instead I just bailed on the
clay body and now use a commercial slip casting stoneware body.
Years ago, while in high school, I worked with some potters here in town,
Bill Kaufman and Cynthia Mosedale, and I bought some porcelain mugs for my
parents. Those mugs have wonderful brushwork with a clear glaze on top. (I think?)
The glaze has crazed wonderfully over the years. Thanks to those mugs I
realized not all crazing is such a bad thing and now just accept it.
(I have yet to find a clear glaze that does not craze after a few years
though)
take care
jeff longtin
in, very wet, minneapolis
Craig Martell on sat 28 jun 03
Jeff sez:
>It was explained to me, by my good friends at Minnesota Clay (now at
>Continental Clay) that crazing is mostly a "fit" issue. (Zinc, apparently,
>shrinks
>alot.) By calcining it I decreased the amount it shrank prior to melting,
>i.e. I
>"pre-shrank" it. (As I explained, calcining it on my own was unsuccessful, but
>luckily, I found a source for it anyway, A.R.T. in Chicago.)
Hi:
When we're trying to get at the causes of crazing, it's important to make a
distinction between "shrinking", and "expansion". Shrinking as it applies
to clays and any other minerals that have water in the lattice, is a
physical action resulting from the evaporation of water. In clays, the
water we're talking about is called the "water of plasticity" and isn't the
chemically bound water that departs under heat during a bisque fire, or in
the early part of the glaze firing.
Shrinkage doesn't have anything to do with crazing and glaze
fit. Expansion does. When a fired claybody has a glaze fusion bonded to
the surface the glaze will craze if it has a high expansion. Sometimes the
claybody is at fault too. Some clays are not very balanced in terms of
added quartz that is there to counteract the expansion of the fluxing agents.
Zinc doesn't have a "loss on ignition" and it's not changed chemically by a
calcine fire. The main problem with zinc is that it's a real water magnet
and will absorb atmosheric moisture. If there is enough zinc in a raw
glaze, the "wet" zinc has a high probability of causing the glaze to
crawl. So calcining the zinc will get rid of the physically absorbed water
and will reduce the bulk of the zinc a little. This is a reversible
process though and over time the zinc will again suck up atmospheric
moisture. I don't use zinc any longer because I reduction fire but I used
to put zinc in the electric kiln a few days prior to mixing glazes and let
it sit at about 500 degrees for 4 to 5 hours and this removed the water. I
saw no need to run it up to 1300 to 1500 degrees F.
regards, Craig Martell Hopewell, Oregon
clayart@DIGITALFIRE.AB.CA on sun 29 jun 03
Making a transparent glaze that does not craze is easy.
Check this web page:
http://www.digitalfire.ab.ca/cermat/education/4.php
The thermal expansion of a glaze, and therefore its fit to the
body is determined by its chemistry. Whether you are sourcing
zinc from a raw or calcined material is immaterial, it is still zinc
(unless of course you do not compensate for the loss of weight
curing calcination by putting less of the calcined material in).
You need to adjust the chemistry of your glaze.
It likely contains too much K2O and/or Na2O. Switch some
of their amount for MgO. Email me your recipe and I can
demonstrate. Send it to tony.at.digitalfire.com (put the @ symbol
instead of .at. in this address, I put it like this to avoid spam
engines harvesting this address.
-------8<--------
Sandy,
If I\'m not mistaken the problem I was having at the time was crazing in a
clear glaze over a porcelain body (cone 6 slip cast).
It was explained to me, by my good friends at Minnesota Clay (now at
Continental Clay) that crazing is mostly a \"fit\" issue. (Zinc, apparently, shrinks
alot.) By calcining it I decreased the amount it shrank prior to melting, i.e. I
\"pre-shrank\" it. (As I explained, calcining it on my own was unsuccessful, but
luckily, I found a source for it anyway, A.R.T. in Chicago.)
Calcined zinc didn\'t solve the problem actually. Instead I just bailed on the
clay body and now use a commercial slip casting stoneware body.
Years ago, while in high school, I worked with some potters here in town,
Bill Kaufman and Cynthia Mosedale, and I bought some porcelain mugs for my
parents. Those mugs have wonderful brushwork with a clear glaze on top. (I think?)
The glaze has crazed wonderfully over the years. Thanks to those mugs I
realized not all crazing is such a bad thing and now just accept it.
(I have yet to find a clear glaze that does not craze after a few years
though)
take care
jeff longtin
in, very wet, minneapolis
______________________________________________________________________________
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You may look at the archives for the list or change your subscription
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--------
Tony Hansen, Digitalfire Corp.
Ron Roy on mon 30 jun 03
Zinc oxide is easily reduced and that will lead to pinholing due to loss of
the oxide in an even slightly reduced atmosphere. ZnO is then reduced to
the metal which boils above 900C. High zinc glazes are well know to have
pinholing problems - make sure they are fired in a strictly oxidizing
atmosphere - have a vent system helps a lot.
RR
>Yes! It can help with pinholing.
>Calcine some yourself and try it. Load some into a pot and fire it to 1256F.
>Bill Campbell
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on mon 30 jun 03
Hi Jeff,
Actually zinc has a rather low expansion when compared with the other
fluxes and is know as an anticraze material in the business because of that
lower expansion rate.
You are right about shrinkage but as potters we reserve that term to
describe what happen when raw materials get smaller.
To avoid confusion we use the term contraction (when cooling) when we talk
about what happens to fired materials. Expansion is reserved to describe
what happens when fired materials are heated.
RR
>If I'm not mistaken the problem I was having at the time was crazing in a
>clear glaze over a porcelain body (cone 6 slip cast).
>It was explained to me, by my good friends at Minnesota Clay (now at
>Continental Clay) that crazing is mostly a "fit" issue. (Zinc, apparently,
>shrinks
>alot.) By calcining it I decreased the amount it shrank prior to melting,
>i.e. I
>"pre-shrank" it. (As I explained, calcining it on my own was unsuccessful, but
>luckily, I found a source for it anyway, A.R.T. in Chicago.)
>Calcined zinc didn't solve the problem actually. Instead I just bailed on the
>clay body and now use a commercial slip casting stoneware body.
>Years ago, while in high school, I worked with some potters here in town,
>Bill Kaufman and Cynthia Mosedale, and I bought some porcelain mugs for my
>parents. Those mugs have wonderful brushwork with a clear glaze on top. (I
>think?)
>The glaze has crazed wonderfully over the years. Thanks to those mugs I
>realized not all crazing is such a bad thing and now just accept it.
>(I have yet to find a clear glaze that does not craze after a few years
>though)
>
>take care
>jeff longtin
>in, very wet, minneapolis
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Hollister on sat 5 jul 03
If you calcine the zinc oxide what is the usage ratio compared to
un-calcined?
Ron Hollister
----- Original Message -----
From:
To:
Sent: Saturday, June 28, 2003 8:57 AM
Subject: Re: Calcined zinc?
> Yes! It can help with pinholing.
> Calcine some yourself and try it. Load some into a pot and fire it to
1256F.
> Bill Campbell
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
Craig Martell on sat 5 jul 03
Ron asked:
>If you calcine the zinc oxide what is the usage ratio compared to
>un-calcined?
Hello Ron:
Zinc has no LOI so the percentage you would add to a glaze is the same as
if you used raw ZnO.
regards, Craig Martell Hopewell, Oregon
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