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zinc oxide and calcining

updated wed 11 aug 10

 

Alice DeLisle on tue 10 aug 10


I posted a question recently about some old zinc oxide that I have that app=
ears
light greenish and denser than some newer zinc oxide that I had bought. Th=
anks
to those who replied to my post and convinced me that the old stuff is inde=
ed
zinc oxide that needs to be calcined because it has absorbed water. I was =
also
directed to look at the Clayart Archives on how to do that. I have done so=
.
The consensus seems to be that zinc oxide is hydroscopic and absorbs water=
from
the air. Several people suggested putting the wet zinc oxide in a bisqued =
bowl
and firing it to bisque temperatures. One person said that she only fires =
to
1000 degrees (F?) because above that she gets some chunks forming. But the=
post
that intrigued me the most was a question from John Hesselberth. Here is t=
he
link:

http://lsv.ceramics.org/scripts/wa.exe?A2=3Dind0408B&L=3DClayart&P=3DR330&X=
=3D32169B5B53DD49F7DA&Y=3Dwanderland%40att.net


I reiterate John's question here because it doesn't seem to me that it was
answered. If the zinc oxide is just wet, why can't it be heated to 212oF f=
or a
period of time to evaporate the water? Should I just try it and report bac=
k or
does somebody know the answer?

Thanks!!

Alice DeLisle

wanderland@att.net
http://delisle.aftosawebhosting.com
http://www.etsy.com/shop/IslandTextures

Robert Harris on tue 10 aug 10


>
> I reiterate John's question here because it doesn't seem to me that it wa=
=3D
s
> answered. =3DA0If the zinc oxide is just wet, why can't it be heated to 2=
12=3D
oF for a
> period of time to evaporate the water? =3DA0Should I just try it and repo=
rt=3D
back or
> does somebody know the answer?
>


Just like with clay, when we talk about bone dry ware versus bisqued
ware in which "chemical" water has been driven off, many salts have
"water of hydration" associated with them. Essentially these minerals
and salts have water that is formally bonded to them, rather than just
absorbed into the matrix. In this case energy (heat) is required to
break the chemical bonds to drive the water off. The heat required for
this is often greater than 212F.

For those of you that work with Cobalt Chloride, when it has water
bonded to it, it is a pink colour, even though it is still a
crystalline solid. However if you heat it above a certain temperature
the water will be driven off and it will turn blue. (To get technical)
This is because water donates electrons in bonding to the Cobalt atom
which alters light absorption. I have often wondered how this can be
related to the observation that high magnesium and low alumina levels
will produce a cobalt pink glaze.


Hope this helps

Robert

Richard White on tue 10 aug 10


When calcining a material, you need to heat it hotter than just boiling
(212F) in the hopes of evaporating the moisture. Some of the water may be=
=3D

chemically bound to the material, and it will only release (and then
evaporate) at a higher temperature. Note that calcining the wet zinc oxid=
=3D
e
will bring you back to the original pure zinc oxide, but calcining some
other things will yield a slightly different beast. For example, calcined=
=3D

kaolin should not be indiscriminately substituted for regular kaolin (or
vice versa) in a glaze recipe because the calcined version (now missing t=
=3D
he
2 water molecules that were chemically bound) has a different contributio=
=3D
n
of alumina and silica for the same weight of the component and some of th=
=3D
e
physical properties of the glaze slurry may be different.=3D20

dw

William & Susan Schran User on tue 10 aug 10


On 8/10/10 12:27 PM, "Alice DeLisle" wrote:

> The consensus seems to be that zinc oxide is hydroscopic and absorbs wat=
er
> from the air. Several people suggested putting the wet zinc oxide in a
bisqued bowl and firing it to bisque temperatures. One person said that s=
he
only fires to 1000 degrees (F?) because above that she gets some chunks for=
ming.
But the post that intrigued me the most was a question from John Hesselber=
th.
Here is the link:
> http://lsv.ceramics.org/scripts/wa.exe?A2=3Dind0408B&L=3DClayart&P=3DR330=
&X=3D32169B5B
> 53DD49F7DA&Y=3Dwanderland%40att.net
> I reiterate John's question here because it doesn't seem to me that it wa=
s
> answered. If the zinc oxide is just wet, why can't it be heated to 212oF=
for
> a period of time to evaporate the water? Should I just try it and report=
back
>or does somebody know the answer?

Those of us who work with crystalline glazes deal with the issues of zinc
quite a bit. Calcining zinc to about red heat, 1200F will do the trick.
Firing up to bisque will begin to sinter the zinc, having it lump up and
needing to be ground.

Zinc will have water of absorption AND chemically combined water. You need
to fire hotter than 212F to remove both.
Beyond the higher coefficient of expansion when it contains water (more
shrinking), when you weigh the zinc that has chemically combined water,
you're weighing the water, so you're getting less zinc!

Bill

--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com