Joan & Tom Woodward on wed 1 sep 99
------------------
Hi All,
I just read a post from Tony saying that to avoid crazing, we should let the
kiln cool to room temperature before opening. I've not heard that advice
before. Generally I stick a wedge in around 165 degrees and open the lid =
all
the way at 150 or so. And I have removed pieces (as in yesterday) when they
were barely cool enough to carry. It did occur to me that this might not be=
the
smartest practice. Do people agree with Tony's advice? (People with the
proverbial patience of Job.)
Happily anticipating more learning and down to my last glazing tomorrow (I =
hope,
I hope) for my last firing for my first show/sale. Guests lured me from my
studio on Saturday when the sun was out and so was Mt. McKinley in all its
glory. Definitely the right choice ro spend the day outside. But I wish =
the
rest of my life ( which I generally like and which actually produces some
income) would have the courtesy to slow down when potting needs to take
priority=21
Ray Aldridge on thu 2 sep 99
At 01:53 PM 9/1/99 EDT, you wrote:
>----------------------------Original message----------------------------
>------------------
>Hi All,
>
>I just read a post from Tony saying that to avoid crazing, we should let the
>kiln cool to room temperature before opening. I've not heard that advice
>before. Generally I stick a wedge in around 165 degrees and open the lid all
>the way at 150 or so. And I have removed pieces (as in yesterday) when they
>were barely cool enough to carry. It did occur to me that this might not
be the
>smartest practice. Do people agree with Tony's advice? (People with the
>proverbial patience of Job.)
I don't agree with this advice, but perhaps that's because I advertise my
ware as being oven-safe. Most folks don't let their casseroles cool to
room temp before dinner.
I believe the gurus will tell you that if you unload with gloves and the
pots craze, they were going to craze anyway.
Ray
Ron Roy on fri 3 sep 99
Well yes and no - perhaps - it depends - sometimes.
Strictly speaking crazing is caused by glaze contracting on cooling - more
than the clay. Glaze is very weak when stretched so it has to break up or
fall off - it breaks up. The first breaks are call primary crazing and the
spaces between the cracks are quite wide - compared to the later developing
ones. If your crazing is of the large spaces between lines type when it
comes out of the kiln (it will craze more later to relieve the tension) -
but it means you are closer to solving the problem when you alter that
glaze - than if the spaces between the cracks are small to start off with.
It is not necessary to wait till the kiln cools to room temperature to
avoid crazing - otherwise any vessel that has boiling water poured into it
will promote crazing - if you need to cool your pots that slowly they are
going to craze eventually anyway.
So why does opening a kiln too soon result in crazing with glazes that
would not otherwise craze. I can't give you exact temperatures here but -
picture this - The clay and glaze are cooling and the kiln is at 400C -
everything is fine - the body is contracting a little more than the glaze
so it is in compression (bigger than the clay) as it has been ever since
the glaze has set (frozen became rigid) so it had not had any reason to
craze. Some damn fool comes along and lifts the lid enough to suddenly
lower the temperature in the kiln about 50 degrees - all the glazes that
aren't under enough compression to resist the sudden cooling - contract
more than the clay - because they get the cooling before the clay - and
they craze when ordinarily they might not. There might be other glazes in
the kiln which will still not craze with this kind of treatment - because
they are under enough compression to not.
This a bit harder to explain than I thought - if you crash cool the pot -
if the pot is hot enough - the glaze will cool faster - cause its getting
cooled faster - has to contract - clay is hotter and does not contract as
fast as the glaze - glaze crazes.
RR - feeling a bit crazed myself these days it seems.
There now - do I have to do it again?
RR - feeling a bit crazed myself these days it seems.
>Hi All,
>
>I just read a post from Tony saying that to avoid crazing, we should let the
>kiln cool to room temperature before opening. I've not heard that advice
>before. Generally I stick a wedge in around 165 degrees and open the lid all
>the way at 150 or so. And I have removed pieces (as in yesterday) when they
>were barely cool enough to carry. It did occur to me that this might not
>be the
>smartest practice. Do people agree with Tony's advice? (People with the
>proverbial patience of Job.)
>
>Happily anticipating more learning and down to my last glazing tomorrow (I
>hope,
>I hope) for my last firing for my first show/sale. Guests lured me from my
>studio on Saturday when the sun was out and so was Mt. McKinley in all its
>glory. Definitely the right choice ro spend the day outside. But I wish the
>rest of my life ( which I generally like and which actually produces some
>income) would have the courtesy to slow down when potting needs to take
>priority!
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
amy parker on fri 3 sep 99
At 01:53 PM 9/1/99 EDT, you wrote:
>----------------------------Original message----------------------------
>------------------
>Hi All,
>
>I just read a post from Tony saying that to avoid crazing, we should let the
>kiln cool to room temperature before opening. I've not heard that advice
>before. Generally I stick a wedge in around 165 degrees and open the lid all
>the way at 150 or so. And I have removed pieces (as in yesterday) when they
>were barely cool enough to carry. It did occur to me that this might not
be >the smartest practice. Do people agree with Tony's advice? (People
with the
>proverbial patience of Job.)
While this sounds like a good idea, the fact is that if you are making
pieces for oven use, they will be subjected to rapid cooling from about 350
F to room temp on a regular basis...might be good to go on & "shock" these
up front!
amy parker Lithonia, GA
amyp@sd-software.com
hasebe on fri 3 sep 99
Hi,
Thank you for help for once soaked Gerstley Borate question while ago. I
tested the several glaze and seems all right. I was trying to ease crazing
with the Gerstley Borate, and has promoted the crazing for one batch. I
mixed about 5% to the dolomite base glaze, and can hear cracking everyday
since I opened a kiln a week ago. Does Gerstley Borate have any effect for
crazing?
Anyway, I have crazing problem myself and fighting for three months since
switching to the white stoneware body. I have been adjusting glaze and
testing ever since. I am getting confused about crazing. I understand that
the thermal expansion is one reason, and by adjusting it with flint, etc.
for the glaze may ease the problem.
How does the pyrophyllite in clay body work compared to the flint during the
firing?
Does the firing shrinkage of clay body (not wet to dry) have any effects on
crazing? The firing shrinkage is not the same as the thermal expansion,
isn't it? Since I switch to the white stone ware and its firing shrinkage
has increased for 3-4%. Does grog help ease crazing by decreasing the
firing shrinkage?
Thanks,
Hodaka Hasebe
hasebeh@rochester.rr.com (Hodaka Hasebe)
Pittsford, NY
716-264-9698
----------
>From: Mike Bailey
>To: CLAYART@LSV.UKY.EDU
>Subject: Re: firing/crazing
>Date: Fri, Sep 3, 1999, 7:59 PM
>
> ----------------------------Original message----------------------------
> In message , Joan & Tom Woodward writes
>
> Dear Joan & Tom,
>
> Ref your enquiry about at what temperature to open a kiln. I've taken
> the view that if a glaze is going to craze - then its going to craze.
> Waiting for the kiln to get to room temperature might delay the on set
> of crazing but that is all it will do. Curing crazing is best done by
> getting the firing, clay body and glaze in harmony. I think you are very
> patient even waiting for 165 deg C.!!!!. Obviously you have to get
> through the 220 deg. C. cristobalite quartz inversion slowly but once
> that is over throughout the pack, why wait?
>
> Cheers,
>
> Mike
> --
> Mike Bailey. Bath. U.K.
Mike Bailey on fri 3 sep 99
In message , Joan & Tom Woodward writes
Dear Joan & Tom,
Ref your enquiry about at what temperature to open a kiln. I've taken
the view that if a glaze is going to craze - then its going to craze.
Waiting for the kiln to get to room temperature might delay the on set
of crazing but that is all it will do. Curing crazing is best done by
getting the firing, clay body and glaze in harmony. I think you are very
patient even waiting for 165 deg C.!!!!. Obviously you have to get
through the 220 deg. C. cristobalite quartz inversion slowly but once
that is over throughout the pack, why wait?
Cheers,
Mike
--
Mike Bailey. Bath. U.K.
Ron Roy on wed 8 sep 99
Hi Hodaka,
Any material you add to a glaze will affect the expansion. If the material
has a higher expansion/contraction than the glaze it will raise the
expansion of that glaze. If the expansion of the material is lower than the
glaze then it will lower it. Silica has a very low expansion and therefore
is recommended when trying to stop crazing.
If you add Gerstley Borate to a glaze it will help to lower the
expansion/contraction - IF...
1. It's expansion is lower than the original glaze.
2. You don't get the % of boron in the glaze over 10%
3. The most important part - you use the addition of a low fluxing material
like GB or a frit to give yourself the opportunity to add more silica and
alumina - keeping the silica/ alumina ratio the same - so the glaze will
have a decent chance of looking the same.
Better still send me the recipe and I will send you back a few revised
recipes - RR
>----------------------------Original message----------------------------
>Hi,
>Thank you for help for once soaked Gerstley Borate question while ago. I
>tested the several glaze and seems all right. I was trying to ease crazing
>with the Gerstley Borate, and has promoted the crazing for one batch. I
>mixed about 5% to the dolomite base glaze, and can hear cracking everyday
>since I opened a kiln a week ago. Does Gerstley Borate have any effect for
>crazing?
>
>Anyway, I have crazing problem myself and fighting for three months since
>switching to the white stoneware body. I have been adjusting glaze and
>testing ever since. I am getting confused about crazing. I understand that
>the thermal expansion is one reason, and by adjusting it with flint, etc.
>for the glaze may ease the problem.
>
>How does the pyrophyllite in clay body work compared to the flint during the
>firing?
>
>Does the firing shrinkage of clay body (not wet to dry) have any effects on
>crazing? The firing shrinkage is not the same as the thermal expansion,
>isn't it? Since I switch to the white stone ware and its firing shrinkage
>has increased for 3-4%. Does grog help ease crazing by decreasing the
>firing shrinkage?
>
>
>Thanks,
>
>Hodaka Hasebe
>hasebeh@rochester.rr.com (Hodaka Hasebe)
>Pittsford, NY
>716-264-9698
>
>----------
>>From: Mike Bailey
>>To: CLAYART@LSV.UKY.EDU
>>Subject: Re: firing/crazing
>>Date: Fri, Sep 3, 1999, 7:59 PM
>>
>
>> ----------------------------Original message----------------------------
>> In message , Joan & Tom Woodward writes
>>
>> Dear Joan & Tom,
>>
>> Ref your enquiry about at what temperature to open a kiln. I've taken
>> the view that if a glaze is going to craze - then its going to craze.
>> Waiting for the kiln to get to room temperature might delay the on set
>> of crazing but that is all it will do. Curing crazing is best done by
>> getting the firing, clay body and glaze in harmony. I think you are very
>> patient even waiting for 165 deg C.!!!!. Obviously you have to get
>> through the 220 deg. C. cristobalite quartz inversion slowly but once
>> that is over throughout the pack, why wait?
>>
>> Cheers,
>>
>> Mike
>> --
>> Mike Bailey. Bath. U.K.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Ron Roy on tue 14 sep 99
Hi Hodaka,
Firing shrinkage has nothing to do with the expansionon heating (after
firing) and contraction on cooling of clay and glazes.
Everything -(well almost everything) expands when heated and contracts when
it cools and always comes back to the same size at any given temperature.
Shrinkage from drying is because water is leaving and firing shrinkage is
because of melting - it is not reversable - the way thermal expansion is.
RR
>I am still confused about the basics. Does the thermal expansion and the
>firing shrinkage the same thing, or a part of thermal expansion?
>
>Regards,
>
>
>Hodaka Hasebe
>hasebeh@rochester.rr.com (Hodaka Hasebe)
>Pittsford, NY
>716-264-9698
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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