Ron Roy on sun 14 sep 97
> Zinc is an oxide that has one of the
>highest "coefficient of expansion" factors. Calcining removes the water
>molecule from the emperical formula of your material and thus cuts down on
>shrinkage.
On the contrary - with the exception of MgO, ZnO has the lowest expansion
of the common fluxes. Li2O has a similar low expansion under some
circumstances. If you count B2O3 as a flux (some do) it has the lowest
expansion of all.
I think what the poster meant in this case was - zinc has the capability of
high shrinkage during drying and sintering and can lead to crawling. The
calcining of zinc is an attempt to minimize this early cracking of the raw
glaze.
One possible way of dealing with this is to make sure your glazed ware is
dry before firing it.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Brad Sondahl on sat 28 jun 03
Though there should be little difference between calcined and uncalcined
zinc, my own experience with one particular brand of calcined zinc
oxide was that it increased pinholing, so I've stuck to uncalcined.
When I mentioned this a few years ago on Clayart someone pointed out
that calcined zinc is slightly more concentrated, having had the water
removed, so it might change the proportion of zinc in the glaze.
I now work regularly with crystalline glazes (which have a large amount
of zinc) and although they sometimes pinhole, I attribute that to rutile
rather than the quantity of zinc in the glazes (of course I could be
wrong on that--it's just that rutile in other glazes seems to cause
pinholing as well, probably from off gassing some impurities).
The causes of pinholes are elusive and varied--but you might try both
kinds of zinc and see which yields better results
Brad Sondahl
--
For original art, music, pottery, and literature, visit my homepage
http://sondahl.com
Pottery sales page http://sondahl.freeyellow.com
My music site at mp3.com http://www.mp3.com/stations/sondahl
clayart@DIGITALFIRE.AB.CA on sun 29 jun 03
Check here to trouble shoot pinholing.
http://www.digitalfire.ab.ca/cermat/education/129.php
-------8<--------
The causes of pinholes are elusive and varied--but you might try both
kinds of zinc and see which yields better results
Brad Sondahl
--------
Tony Hansen, Digitalfire Corp.
Paulette Carr on mon 30 jun 03
>>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? >>
Yes, Paul, something like that may work, if the problem is merely the=20
presence of water in the compound.=20
In my days as a graduate student, I was a TA (translation- the laboratory=20
instructor) for Quant (Quantitative Analytical Chemistry). We would first d=
ry=20
our compounds in the oven (similar to calcining Zn, but at lower temperature=
s),=20
allowed to cool somewhat, and then placed in a desiccator. The lid was left=
=20
ajar for about 10 minutes, so that a partial vacuum would not form in the=20
desiccator after the air inside had cooled. To insure a tight seal and effi=
cient=20
drying (or keeping the ZnCO3 from reabsorbing water from the atmosphere) the=
=20
surfaces of the desiccator and the lid were lightly greased with petroleum=20
jelly. The desiccant, or drying agent (kept at the bottom of the desiccator=
,=20
physically isolated from mixing with the anhydrous compounds had to be fresh=
ly=20
charged (dried itself in an oven at 110=CB=9AC for a couple of hours, and c=
ooled as=20
above), and was usually calcium chloride, drierite (CaSO4.1/2H2O --a colored=
=20
form was available which could be used to tell when the desiccant needed to=20=
be=20
recharged, or magnesium perchlorate, the most efficient water absorber. The=
=20
packets that you find in pill bottles, and even shoes, may already contain a=
=20
great deal of water, and would not be efficient in removing the water from t=
he=20
atmosphere around the zinc.
However, the problem with ZnO is, according to Hamer and Hamer,=20
"raw zinc oxide has a high initial shrinkage in the early stages of firing.=20=
=20
This is before fusion takes place. This shrinkage can cause cracks in the=20
glaze which are the source of scar when the glaze melts. Zinc oxide glazes=20=
have=20
fairly high surface tensions and are therefore prone to crawling. To=20
eradicate the initial shrinkage, the zinc oxide should be calcined before us=
e in a=20
glaze batch. Zinc oxide is unusual in that it does little to lower surface=20
tension yet much to lower viscosity in the molten glaze."
Unfortunately, I do not use any glazes that require calcining ZnO, so I do=20
not have firsthand experience. I don't know if this is a problem is merely=20=
a=20
water absorption issue. My impression is that once you calcine ZnO it is=20
physically changed, but with the same chemical content. Is this not the cas=
e? Does=20
it reconvert over time, and in the presence of atmospheric water to the raw=20
form of ZnO?
Somehow my answers, always engender more questions...
Regards,
Paulette Carr
St. Louis, MO
Rider,Francis - Art Studio Technician on wed 29 oct 08
We are planning on a crystalline glaze firing and the recipe's we would lik=
e to try call for calcined zinc and I need to know if the temperature that =
I fire the zinc at is critical or can I put some in a pot and run it in a l=
oad of bisque at ^06 or does it need to be much lower? I have read that tha=
t usually calcination occurs around 1292 deg.F.
Thanks
Francis Rider
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of William & Susa=
n Schran User
Sent: Tuesday, October 28, 2008 12:51 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Damaged Kiln
On 10/28/08 2:04 PM, "Rider,Francis - Art Studio Technician"
wrote:
> I would like an opinion from clayarters on a problem that we are dealing =
with
> right now regarding the damage to a brand new 24 cf updraft gas kiln that=
was
> damaged during the shipment of the kiln. This company that I do not want =
to
> mention the name has a record of thirty years of business. When the kiln
> arrived the first red flag was that it was not crated, outside or no supp=
orts
> inside. The gentleman that delivered the kiln was the owners son and he s=
aid
> they deliver them all over the country just like that. We realize now tha=
t we
> probably should not have even unloaded the kiln but we had to make
> arrangements with the local lumber yard for the use of there forklift so =
time
> was an issue.
Were the damages listed on the delivery receipt?
If you signed for it, without notation of damage, then you accepted it as
being in good condition.
Having said that, I would suggest the director of your studio/department
chair send a written letter requesting the owner of the kiln company come t=
o
the kiln and everybody inspect the damage. It certainly would not be out of
line to ask that the kiln be replaced or be completely rebuilt onsite.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
John Hesselberth on thu 30 oct 08
On Oct 29, 2008, at 3:11 PM, Rider,Francis - Art Studio Technician
wrote:
> We are planning on a crystalline glaze firing and the recipe's we
> would like to try call for calcined zinc and I need to know if the
> temperature that I fire the zinc at is critical or can I put some
> in a pot and run it in a load of bisque at ^06 or does it need to
> be much lower? I have read that that usually calcination occurs
> around 1292 deg.F.
>
> Thanks
> Francis Rider
Hi Francis,
Apparently zinc oxide can gradually absorb CO2 in damp conditions and
become zinc carbonate. My handbook says the carbonate gives up that
CO2 at 300C (572F) so I would think 600F would be high enough. I've
never encountered this problem myself--I guess I keep my zinc oxide
dry enough--but enough people say it needs to be done that I believe
it must happen. From a chemistry standpoint I would think that
conversion back to the oxide could just as well happen during the
glaze firing, but I gather the material is lumpy and about impossible
to disperse in the glaze if it has deteriorated to the carbonate.
Also, there should be no problem in calcining higher that 600F if it
is convenient to do so. Zinc oxide doesn't melt until 1800C or higher.
I'd be interested in other people's take on this. Having never
encountered the phenomena makes it a little difficult to be sure of
what I have written above.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Snail Scott on thu 30 oct 08
On Oct 29, 2008, at 2:11 PM, Rider,Francis - Art Studio Technician
wrote:
> I need to know if the temperature that I fire the zinc at is critical
> or can I put some in a pot and run it in a load of bisque at ^06 or
> does it need to be much lower?
I just toss it in with my regular bisque at ^06 or so.
Best to use calcined zinc always - it's clumpy
otherwise.
-Snail
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