Stephani Stephenson on wed 19 jul 06
Annealing, slow controlled rate of cooling
is used by glass artists and glass blowers.
when glass cools, the molecules are under such tension
that if it cools too quickly, the glass will later 'check', crack, even
shatter.
is annealing ever used or considered in the cooling of glazes?
to improve glaze fit or glaze strength or bond?
would this affect crazing or reduce the chances of delayed crazing
(though delayed crazing usually happens because the claybody not the
glaze
absorbs water, expands...)
just a question popped into my head.
Stephani Stephenson
steph@revivaltileworks.com
http://www.revivaltileworks.com
Wayne Seidl on wed 19 jul 06
Stephani dear:
I'm sure others have differing opinions about the annealing process as
applied to glaze, but I "believe" that is precisely what is happening =
when
we "fire down" as the Mayor has mentioned a few times.
Best,
Wayne Seidl
watching the rain come down hard enough to wash week old paint off walls
(sigh)
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Stephani
Stephenson
Sent: Wednesday, July 19, 2006 10:20 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: crazed glaze craze...annealing
Annealing, slow controlled rate of cooling
is used by glass artists and glass blowers.
when glass cools, the molecules are under such tension
that if it cools too quickly, the glass will later 'check', crack, even
shatter.
is annealing ever used or considered in the cooling of glazes?
to improve glaze fit or glaze strength or bond?
would this affect crazing or reduce the chances of delayed crazing
(though delayed crazing usually happens because the claybody not the
glaze
absorbs water, expands...)
just a question popped into my head.
Stephani Stephenson
steph@revivaltileworks.com
http://www.revivaltileworks.com
_________________________________________________________________________=
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Paul Herman on wed 19 jul 06
Hi Stephani,
I think it's a considerably different set of circumstances in play
with slow cooling of clay or glass.
When a glassblower anneals a big fat paper weight or something like
that, they are trying to cool the piece slowly enough that the
surface of the glass and the core cool together. But it all one
material. If the surface is chilled too rapidly, it causes a strain
and the piece cracks or shatters. I've seen blown glass pieces that
crack days after annealing, similarly to the delayed dunting that
happens to pots sometimes. With clay we are working with a glass
coating on the body, two different materials.
I've never heard the expression "annealing" used to describe slow
cooling of a ceramic kiln. My impression is that it is done to
encourage crystalization in the glaze only, and doesn't affect the
body that much. Thinking about it, I don't believe it would affect a
crazing glaze very much. If someone knows more though, I'd like to
hear about it.
Wishing everyone good glaze fits,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
> -----Original Message-----
> From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Stephani
> Stephenson
> Sent: Wednesday, July 19, 2006 10:20 AM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Re: crazed glaze craze...annealing
>
> Annealing, slow controlled rate of cooling
> is used by glass artists and glass blowers.
>
> when glass cools, the molecules are under such tension
> that if it cools too quickly, the glass will later 'check', crack,
> even
> shatter.
>
> is annealing ever used or considered in the cooling of glazes?
> to improve glaze fit or glaze strength or bond?
> would this affect crazing or reduce the chances of delayed crazing
> (though delayed crazing usually happens because the claybody not the
> glaze
> absorbs water, expands...)
>
> just a question popped into my head.
>
> Stephani Stephenson
> steph@revivaltileworks.com
> http://www.revivaltileworks.com
Ivor and Olive Lewis on thu 20 jul 06
Most forms of Glass, are annealed, that is it is held at a temperature =
above their Transition Points for a period of time then cooled in stages =
at a predetermined rates which are related to the composition and =
thickness of the glass.
The important stages are from 5 degrees above then to 5 degrees below =
the "Annealing temperature when a very slow cooling rates are applied =
then down to 55 degrees below the "Annealing point" when the rate of =
cooling is doubled. Below that the rate of cooling is 3 deg C per min.
As you may imagine, Glazes are compositionally and structurally more =
complex than glasses. They have Linear Expansion Coefficients that are =
quite different and their "Glass Transition Temperatures" are much =
higher. These value would be unknown to most of us but they can be =
determined using the apparatus that Ron Roy uses
I do not see a relationship between arrested cooling for the first two =
to three hundred degrees for a glaze and the annealing process. When a =
potters kiln is allowed to revert to natural cooling the temperature for =
most glazes, including Cone 6 specifications, would be above the glass =
transition temperature.
I do see advantages with some complex designs and combinations of clay =
and glaze for interposing a arrest and dwell period when it is suspected =
that the mature ceramic has potential for a Quartz or Cristobalite Phase =
Change. This would be useful to anyone who is persistently experiencing =
episodes of Dunting.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on thu 20 jul 06
Dear Wayne Seidl,=20
The purpose of "Firing Down" for two or three hundred degrees is to =
allow reconstructive chemistry to take place.
If insufficient time is allowed then Crystallites do not form and act as =
nuclei for crystal growth.This prevents those substances that lead, via =
translucency to opacity and surface texture, to form in the glaze =
envelope or from the clay body.
Annealing is something else at much lower temperatures. Best way to =
think about it is as stress prevention or amelioration.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Snail Scott on thu 20 jul 06
At 07:19 AM 7/19/2006 -0700, you wrote:
>Annealing, slow controlled rate of cooling
>is used by glass artists and glass blowers.
>is annealing ever used or considered in the cooling of glazes?
I've done a little bit of glass, and the annealing
schedule is based on the thickness of the glass.
It's a HUGE increase as the piece gets thicker.
A piece 1/4" thick needs almost no special
annealing, just natural cooling in a kiln with IFB
walls, while glass three inches thick will have to
be annealed for over a week. There's a glass piece
by Roni Horn at the Pulitzer Foundation museum
here in St. Louis, and it's 20" thick - solid.
That's months and months of annealing. They do
that with those huge glass telescope mirrors, too.
The point is to let the entire piece cool evenly,
so that there is no temperature gradient from the
inside to the outside. It all shrinks during
cooling at the same rate and no stresses are
created. Glass/glaze is not a great conductor of
heat, so it takes a while for the heat to even
out through the thickness of a big piece of glass.
I've seldom seen a glaze even 1/4" thick, except
as drips. Most glaze on pottery is a small fraction
of that - a 1/16" or less. If that were 'regular'
glass, it wouldn't require any annealing to speak
of, just the sort of slow cool like you'd get when
you turn off a cheap electric kiln. So, actually,
glazes DO get just the same annealing that an
equivalent piece of glass would.
By the way, cooling glass too slowly through the
higher temperatures causes what glass people call
'devitrification': the glass gets opaque and
cloudy on the surface...ceramics people call that
'creating a microcrystalline matte glaze'.
-Snail
Wayne Seidl on thu 20 jul 06
Ivor:
Right you are...stress prevention, as in aiding in the reduction of =
crazing,
which is a stress between the body and the glaze.
Best,
Wayne Seidl
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ivor and =
Olive
Lewis
Sent: Thursday, July 20, 2006 3:58 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: crazed glaze craze...annealing
Dear Wayne Seidl,=20
The purpose of "Firing Down" for two or three hundred degrees is to =
allow
reconstructive chemistry to take place.
If insufficient time is allowed then Crystallites do not form and act as
nuclei for crystal growth.This prevents those substances that lead, via
translucency to opacity and surface texture, to form in the glaze =
envelope
or from the clay body.
Annealing is something else at much lower temperatures. Best way to =
think
about it is as stress prevention or amelioration.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
_________________________________________________________________________=
___
__
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Ron Roy on thu 20 jul 06
Hi Stephani,
I have done many measurements of glazes in my dilatometer - and the
expansion/contraction curves are like those of glass.
If you look at page 71 in our book - you will see the kinds of curves
typical of glass. The hook at the top end indicates where each glass
(glaze) is starting to remelt. This means the material is not only not
getting longer because of heating - but is actually getting shorter because
it is melting.
Just below that hook you will see a steep decline afer which the
contraction becomes more gradual. That steep decline is called the
"transition stage" and represents fast contraction during cooling. If glass
(or glaze) is cooled too fast one part contracts more than another and you
get cracking - so that is what glass anneling is all about. Cool glazes too
fast during this stage and you will probably get crazing.
I don't know how fast glass is cooled - it has more to do with even cooling
of course and there must be different rates depending on how thick the
glass is.
RR
>Annealing, slow controlled rate of cooling
>is used by glass artists and glass blowers.
>
>when glass cools, the molecules are under such tension
>that if it cools too quickly, the glass will later 'check', crack, even
>shatter.
>
>is annealing ever used or considered in the cooling of glazes?
>to improve glaze fit or glaze strength or bond?
>would this affect crazing or reduce the chances of delayed crazing
>(though delayed crazing usually happens because the claybody not the
>glaze
>absorbs water, expands...)
>
>just a question popped into my head.
>
>Stephani Stephenson
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ivor and Olive Lewis on fri 21 jul 06
Dear Wayne Seidl,=20
Not sure if I touched the "Cat's Whisker" on the right part of the =
crystal.
Crazing may be caused in a glaze when stress is set up by a Quartz =
Inversion in the Clay part of a pot. But my understanding is that it is =
differential contraction between glaze and clay as an object cools that =
induces Crazing and Shivering
Those variations are sometimes due to dissimilar Coefficients of Linear =
Thermal Motion. At other times crazing may be caused by chilling the =
skin of the pot.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on sat 22 jul 06
Adding silica in the form of quartz to a clay body is a cure for crazing as
it increases the contraction of the clay as it cools through 573C. It does
not cause crazing.
The lack of enough free quartz in a clay body can be a cause of crazing.
RR
>Crazing may be caused in a glaze when stress is set up by a Quartz
>Inversion in the Clay part of a pot. But my understanding is that it is
>differential contraction between glaze and clay as an object cools that
>induces Crazing and Shivering
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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