james & becca sydnor on sat 22 mar 97
Hi,
I've got a question about crazing that I would love some input on. I
have been changing my clay formula to make it whiter and have now begun
to have problems with my Temple crazing. Here are the recipes:
Clay
100 lbs. Goldart Fireclay
50 PV Clay
50 No. 6 Tile clay
20 Silica
15 Custer Feldspar
(I just added a little more silca from 10 lbs to 20 lbs in hopes
hopes of some relief.)
Glazes:
35 g. Custer spar
19.7 Dolomite
2.6 Whiting
22.7 Kaolin
20 Flint
2 Tin Oxide
I am thinking of changing the glaze by adding 2% boron, Removing 2%
Dolomite and adding 4% Kaolin.
If you have any other suggestions on changing the clay body or the glaze
to help me it would be greatly appreciated. Direct e-mails would be
great I don't think my server posts everything or something.
Thanks
Jim Sydnor
Tony Hansen on sun 23 mar 97
> Have now begun
> to have problems with my Temple crazing. Here are the recipes:
> .......
> I am thinking of changing the glaze by adding 2% boron, Removing 2%
> Dolomite and adding 4% Kaolin.
In my experience your change will not be enough. If the glaze is crazing
it is not out a little, it is probably out a lot. I'd do the changes on
the oxide level rather than with the materials.
--
Tony Hansen, IMC
Emily & Kurt Kiewel on fri 17 sep 99
I'm having a very confusing problem. I'm making a set of dinnerware and
last night noticed that there was some cracks on a dinner plate. Also, the
color is a very transparent celadon and when I put some water on the plate
the cracks darkend and bled. Looked terrible! I can't even imagine what
happens with beets!
At first I thought the problem crazing and decided to up the silica and
decrease the feldspar a little in the recipe. But then I noticed a few
other things. A large platter glazed in the same celadon was not crazed
at all-out of the same firing. None of the bowls, cups, or salad plates
were crazed, and one of the dinner plates had not crazed. Is it from not
glazing the bottom? The plates are approx. 11 3/4 diam. and inside the
foot, about 7 1/2 inches is not glazed. Then why is the one plate and even
larger platter not crazed? Also, the salad plates glazed the same way (8
3/4 diam. and 5 1/2 inched unglazed) are not crazed. I'm confused!
My other thought was that maybe the cracks were caused by shivering. The
plates that have the cracks are slightly curved. Does this make sense?
The one dinner plate with no cracks is pretty flat, but the platter has a
very slight indentation in the center. If the problem is shivering then
adjusting the silica/feldspar should also fix the problem, right?
I'm pretty careful about not unloading too soon. I don't crack my kiln
until 200 C and then wait until it is almost the same temp as outside. The
one other thought I had was the the center of my plate setters has a hole
in it and that it was cooling too fast there, but still, I have that one
dinner plate with no cracks.
Here's the recipe-cone 10
EPK 10
Whiting 20
Flint 30
Soda Spar 40 (I use Minn spar)
Bentonite 2
Red Iron Oxide 1.5
Any help will be greatly appreciated as I have 3 orders for dinnerware in
this glaze (it's really beautiful)!
Thanks-thanks-thanks-Emily
Ron Roy on sat 18 sep 99
Hi Emily,
Send this back to me but with as detailed a description of the clay as
possible. Are you firing oxidation - Tell me how long a firing you do -
particularly from red heat to cone 10 - and describe the cracks - the
pattern and tell me if there is any difference after freezing in your
fridge for 24 hours - put one that did not craze in as well. RR
>----------------------------Original message----------------------------
>I'm having a very confusing problem. I'm making a set of dinnerware and
>last night noticed that there was some cracks on a dinner plate. Also, the
>color is a very transparent celadon and when I put some water on the plate
>the cracks darkend and bled. Looked terrible! I can't even imagine what
>happens with beets!
>
>At first I thought the problem crazing and decided to up the silica and
>decrease the feldspar a little in the recipe. But then I noticed a few
>other things. A large platter glazed in the same celadon was not crazed
>at all-out of the same firing. None of the bowls, cups, or salad plates
>were crazed, and one of the dinner plates had not crazed. Is it from not
>glazing the bottom? The plates are approx. 11 3/4 diam. and inside the
>foot, about 7 1/2 inches is not glazed. Then why is the one plate and even
>larger platter not crazed? Also, the salad plates glazed the same way (8
>3/4 diam. and 5 1/2 inched unglazed) are not crazed. I'm confused!
>
>My other thought was that maybe the cracks were caused by shivering. The
>plates that have the cracks are slightly curved. Does this make sense?
>The one dinner plate with no cracks is pretty flat, but the platter has a
>very slight indentation in the center. If the problem is shivering then
>adjusting the silica/feldspar should also fix the problem, right?
>
>I'm pretty careful about not unloading too soon. I don't crack my kiln
>until 200 C and then wait until it is almost the same temp as outside. The
>one other thought I had was the the center of my plate setters has a hole
>in it and that it was cooling too fast there, but still, I have that one
>dinner plate with no cracks.
>
>Here's the recipe-cone 10
>
>EPK 10
>Whiting 20
>Flint 30
>Soda Spar 40 (I use Minn spar)
>Bentonite 2
>Red Iron Oxide 1.5
>
>Any help will be greatly appreciated as I have 3 orders for dinnerware in
>this glaze (it's really beautiful)!
>Thanks-thanks-thanks-Emily
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Ellie Blair on tue 16 dec 03
Here is another crazing question for those of you out there who do low =
fire white clay. I haven't worked much with low fire for the past =
several years since my focus is crystalline, but I got a consignment for =
Christmas for some small white ware with seasonal painting on the front. =
I used Amaco underglazes and glazed with Amaco's low fire Super Clear. =
I used Terra Blanc clay that fires to 04. The clay was extremely wet =
when I got it and was hard to throw because it wanted to collaspe. I =
let the ware dry completely and did a slow bisque to 08. I glazed with =
Amaco and let it dry overnight and did a fast glaze fire to 08. I had =
problems with reds burning out if I went to 04. This load started to =
craze about 4 days after I took it out of the kiln. It was so bad in =
some pieces that they weren't water tight. What could have caused this. =
My process wasn't different from ones I had followed in the path except =
for the extra caution because of the wet clay. Could this have done it.
Any help would be appreciated.
Ellie Blair
Blair Pottery
Lawrence, Kansas 66047
Ron Roy on sun 21 dec 03
Hi Ellie,
Any earthenware clay (read under vitrified) will leak - with even one craze
line or pinhole - just a matter of time.
To compound the problem - moisture will enter under vitrified clay and -
eventually - the clay will rehydrate a bit and the glazes - even if well
fitted in the beginning - will craze. It is called moisture crazing.
Industry gets a round this to some degree by using high talc bodies which
help solve the problem two ways. They have a high expansion rate (high
contraction rate) so they combat the crazing that is such a difficult
problem at earthenware temperatures - and - reduces the propensity of such
bodies to rehydrate over time.
The question then is about the Terra Blanc - is it a high talc body? and is
it fired high enough to get the maximum benefit from the talc or dolomite?
RR
>Here is another crazing question for those of you out there who do low
>fire white clay. I haven't worked much with low fire for the past several
>years since my focus is crystalline, but I got a consignment for Christmas
>for some small white ware with seasonal painting on the front. I used
>Amaco underglazes and glazed with Amaco's low fire Super Clear. I used
>Terra Blanc clay that fires to 04. The clay was extremely wet when I got
>it and was hard to throw because it wanted to collaspe. I let the ware
>dry completely and did a slow bisque to 08. I glazed with Amaco and let
>it dry overnight and did a fast glaze fire to 08. I had problems with
>reds burning out if I went to 04. This load started to craze about 4 days
>after I took it out of the kiln. It was so bad in some pieces that they
>weren't water tight. What could have caused this. My process wasn't
>different from ones I had followed in the path except for the extra
>caution because of the wet clay. Could this have done it.
>Any help would be appreciated.
>
>Ellie Blair
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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