Daraburn@AOL.COM on sun 2 jan 05
Thanks to all who jumped in on the porcelain question and also the seconds
basket phenomenon. Now I want to get in on the discussion about platter (and
casserole) bottoms. In my limited experience and knowledge it seems to me that a
large flat vessel with unglazed bottom would be stressed if glazed on the top
side and not on the bottom. Am I correct? I recently glazed the inside of a
mug and 1/3 of the way down on the outside but not the bottom 2/3, and my
glaze crazed really badly and never had before. Now I have some casseroles to
glaze and wonder if they would be stronger, more durable etc if glazed all over
and fired on prongs. I don't really want to do that, but am concerned about
the strength of the vessel.
Thanks,
Dawn in Tennessee
Kathi LeSueur on sun 2 jan 05
Daraburn@AOL.COM wrote:
>.............Now I want to get in on the discussion about platter (and
>casserole) bottoms. In my limited experience and knowledge it seems to me that a
>large flat vessel with unglazed bottom would be stressed if glazed on the top
>side and not on the bottom.......
>
>
>
I make lots of platters. Some are 20" long. I never glaze the bottoms. I
make lots of casseroles. I never foot or glaze the bottoms. I fire to
cone 10 reduction. I do not get these pieces returned. And I know that
people use them all of the time. I have at times had customers show me a
piece by another potter that cracked and were told, "yes, that happens
sometimes". So, I know that some clays can't take the stress. But, at
cone 10 stilts would probably deform the piece if I wanted to glaze it
all over.
If you are having a problem, I'd look to your clay before trying to
glaze the piece all over.
Kathi
Chuck Wagoner on sun 2 jan 05
We make mugs that are glazed on the inside and only down about a half
inch on the outside and have not had any trouble over the years. All our
casseroles and other cookware are "dry-footed". Sometimes we use wax
resist on the outside sides and leave a portion unglazed which I guess
would be a similar situation.
Our medium brown clay is fired to cone 6-7 and almost all our glazes
come from "Mastering Cone 6 Glazes".
Potter Wagoner (Not Porter, although I often throw pots while wearing a
sequin jacket and you can't buy our pots, you can only get them in boxes
of Breeze laundry detergent.)
40 miles west of Indy where the 20" of snow "poof" is all gone.
Cindy on mon 3 jan 05
Dear Dawn,
I'm guessing your glaze will craze on your other items in time. The
particular stresses on the casserole may have exacerbated the problem and
speeded it up. I understand that pretty much any glaze will eventually craze
given enough time, but you might want to adjust the fit of your glaze. If
you don't know how, send in your recipe and more scientific people than me
will be certain to offer you their help.
I always leave my large, flat bases unglazed and haven't had a crazing
problem. Still, you're right. It would probably be better to have them
glazed. They would be stronger, certainly. In the end, it's really your
decision.
I wouldn't try to fire your pieces on stilts, though. That's tricky in
anything but low-fire. Just cut a foot ring (maybe a double one), and glaze
everything but the foot ring. If you don't want the pot to appear to have a
foot ring, you can hollow out the center expanse and leave the edges intact.
Waxing can be tricky when you're trying to get wax on only the foot ring of
a large base. If the ring is deep enough, you could just wipe off the glaze
from the foot ring on a tile of wet, low-pile carpet. It's also a good idea
to make a couple of little cut-outs in the foot ring, to let air flow during
the firing.
Best wishes,
Cindy in SD
Ron Roy on wed 5 jan 05
It is not true that all glazes will eventually craze - I have a celadone
that has been on porcelain for over 30 years now and no crazing - and have
seen many pots in museums that are not crazed.
So - If the expansion of the glaze is low enough - compared to the
expansion of the clay body - you can certainly avoid crazing.
If the clay is not vitrified enough to exclude water - then the clay will
gradually rehydrate - and force the glaze to craze.
RR
I understand that pretty much any glaze will eventually craze
>given enough time, but you might want to adjust the fit of your glaze. If
>you don't know how, send in your recipe and more scientific people than me
>will be certain to offer you their help.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
wjskw@BELLSOUTH.NET on wed 5 jan 05
EEK!
Ron...does absorption of water into fired clay
(such as boiling for absorption tests, microwave use, etc.),
figure appreciably in this? If we spend time making the fit as
close as needed to avoid crazing, won't it eventually craze over
time due to moisture absorption, even in a vitrified body?
Understood that a vitrified body will do its best not to absorb
water, but "some" gets in, doesn't it? (Which is the whole point of
the boiling tests.)
Best,
a "becoming more confused"
Wayne Seidl
If the clay is not vitrified enough to exclude water - then the clay
will
gradually rehydrate - and force the glaze to craze.
RR
Ron Roy on fri 7 jan 05
Hi Wayne,
Keep in mind - in order for water to get into the clay the spaces between
the body constituants must be connected.
You can have a certain degree of absorption and the water cannot get in -
or out - like in leaking.
As I said - I have seen pots in museums that are are not crazed - hundreds
of years old - and my celadon has not crazed on my porcelain in 30 years.
That can only mean one thing - no water has gotten in to the clay. Also
remember - because there is moisture in the air - if it can get in it will
- eventually - if the clay is not vitrified enough.
Keep in mind - that crazing is not the worst glaze fault - while a grazing
glaze does make a pot weaker - there is some protection from heat shock
that comes with crazing.
Go too for with the anticraze and you can get into thermal dunting.
This is why it is so important to have clays vitrified enough to hold water
for functional ware - if the glaze does craze the pot will still not leak -
and it will not absorb water and therefore be useful in a microwave.
RR
>Ron...does absorption of water into fired clay
>(such as boiling for absorption tests, microwave use, etc.),
>figure appreciably in this? If we spend time making the fit as
>close as needed to avoid crazing, won't it eventually craze over
>time due to moisture absorption, even in a vitrified body?
>
>Understood that a vitrified body will do its best not to absorb
>water, but "some" gets in, doesn't it? (Which is the whole point of
>the boiling tests.)
>
>Best,
>a "becoming more confused"
>Wayne Seidl
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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