The Goodsons on thu 8 feb 07
Ron,
Hello! I am following your last replies on this subject with great=20
interest.
You wrote to Bonnie:
"Heating a glaze will not craze it - however - if heated then cooled quic=
kly
then perhaps but I can see this happening in any "normal situation - if t=
he
expansion of the glaze is right for the clay.
If the glaze is too big for the clay (the opposite of crazing) then it co=
uld be."
You wrote to Eleanor:
"All the high calcium glazes in our book have a low expansion - they woul=
d
be the last to craze."=20
I am finding your remarks very helpful, as always. I too have a mug tha=
t has a "crazed look" in the bottom. This glaze has a thermal expansion o=
f 412, which is on the low side for my clay. Common sense tells me that c=
razing would not be possible. When I use this mug, I heat the water in it=
, in the microwave, so if the heating and cooling were causing the "crazi=
ng? - shivering?" it should be on the sides as well- and it is not. Also=
, this glaze was overfired a little and the glaze ran and pooled in this =
area. The pooled area is the only place where I see this. I would like =
to pose another question. When the glaze runs, does the chemical makeup s=
tay the same? or could it possibly change?, causing the pooled runny area=
s to contain more fluxing materials and less alum and silica and therefor=
e having a higher expansion in just these areas? Or is that completely i=
mpossible? You know I am just learning, so please bare with my question=
s. I would appreciate your thoughts on these questions. Thanks so much =
for all the help.
Constantly contemplating glazing,
Linda Goodson
208 East Rhodes Street
Lincolnton, NC 28092
goodfun@charter.net
Ron Roy on sun 11 feb 07
Hi Linda,
Good to hear from you!
As you know - glaze fit is always different on different clays - so my
first answer is a question - have you tested that clay with our fit testing
glazes in chapter 5? That should tell you if a glaze with a calculated
expansion of 412 is low enough to not craze on that body.
Many times the advice - to prevent crazing - is to not apply it too thick.
The reason for that is - glazes are influenced by the clay they are fired
on. They pick up some of the material from the clay and - the clay, being
short of fluxes (compared to the glaze) will help lower the expansion of
the glaze.
If a glaze is running and pooling or is just on thick - it will not have
the same kind of contact with the clay body - this is probably why the
pooling glaze is crazing.
I suggest you lower the expansion of that glaze - or - find a clay body
with a higher COE.
A glaze that pools indicates to me that it needs more alumina to slow the
pooling - and some more silica to lower the expansion in this case - so
adding clay - say 2% at a time in a short line blend will tell you if this
a practical way to solve the problem.
RR
>I am finding your remarks very helpful, as always. I too have a mug that
>has a "crazed look" in the bottom. This glaze has a thermal expansion of
>412, which is on the low side for my clay. Common sense tells me that
>crazing would not be possible. When I use this mug, I heat the water in
>it, in the microwave, so if the heating and cooling were causing the
>"crazing? - shivering?" it should be on the sides as well- and it is not.
>Also, this glaze was overfired a little and the glaze ran and pooled in
>this area. The pooled area is the only place where I see this. I would
>like to pose another question. When the glaze runs, does the chemical
>makeup stay the same? or could it possibly change?, causing the pooled
>runny areas to contain more fluxing materials and less alum and silica and
>therefore having a higher expansion in just these areas? Or is that
>completely impossible? You know I am just learning, so please bare with
>my questions. I would appreciate your thoughts on these questions.
>Thanks so much for all the help.
>
>Constantly contemplating glazing,
>Linda Goodson
>208 East Rhodes Street
>Lincolnton, NC 28092
>goodfun@charter.net
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
The Goodsons on sun 11 feb 07
Dear Ron,
I hope all is well with you. Thanks so much for the response.
Yes, I have mixed the glaze tests from Chapter 5 and a thermal expansion
of 412 should be on the low side for the clay I was using. That is why
I was so confused. I didn't realize that the thickness of the glaze
played such an important role. You wrote:
"If a glaze is running and pooling or is just on thick - it will not have
the same kind of contact with the clay body - this is probably why the
pooling glaze is crazing."
And yes, it is much thicker in the area in which the glaze has pooled.
I will reformulate as you suggest adding more silica and alumina. I
find all of this most interesting and really appreciate all the great
advice.
Best regards,
Linda Goodson
Lincolnton, NC
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