Jean Lehman on tue 26 may 98
A potter friend of mine (Deborah Randolph) was looking through the Smith
and Hawken Catalog last week and was disturbed to find *Crackleware* for
sale. It is cast work, badly crazed. The comment says that a deliberate
mismatch between the clay and glaze creates the decorative crackling. It
says that because it is fired at a high stoneware heat, it is safe to use
for food.
Deborah called Smith and Hawkins, saying she was unhappy that a catalog of
such a high caliber wouldn't do better research on products they sell. She
told them she has been a potter for twenty five years and that she is sure
that a crackled glaze should not be used for food.
This morning (Memorial Day) she received a call from the company who makes
the Crackleware. They are located in southern France (a company called Jars
Ceramistes.) She talked with their technical expert, through a translator.
They wanted to know why she was concerned. She explained her position, and
said that we try hard in this country to make our products food safe before
selling them to the public.
The translator said that she just didn't understand... that this is
STONEWARE, and stoneware is NON POROUS!!! And furthermore, it has been
fired to 1280c. She responded that stoneware fired at that temp IS still
porous...
Then their argument changed to say that the cracking is only on the top
layer of glaze and doesn't go all the way down to the clay body. She told
him she didn't believe him!! And besides, she said, the crackle would still
be next to the food, and that would still make it good bacteria breeding
grounds.
His next argument was that *We have sold it for many years with no
problems...* Then he wanted her to send him a speciman of her own work,
which she said (and I completely agree) has nothing to do with this
situation.
Finally, he wants her to send him substantial proof of what she is saying.
She would like confirmation of her postition (or not) from the experts in
our group. She wants to know if she really is on solid ground here. She can
also dig out quotes and send them along to the company with a list of books
to check for themselves. Any information you could provide would help her a
lot. She feels strongly that we must give the public a product that is
safe, and was very pleased that Smith and Hawkens took her call seriously
instead of just ignoring it.
Thanks. I will give her all your comments, as she is not a computer user.
Jean
---------------------->
Jean Lehman, in Lancaster, PA
j_lehman@acad.FandM.EDU (that's an _underscore_ not a hyphen)
http://www.art-craftpa.com/lehman.html
Check out the 1998 Strictly Functional Pottery National at:
http://www.art-craftpa.com
Paulo Correia on wed 27 may 98
Jean,
Your friend may be right if the the "crackle is crazing" and wrong if the
"crazing is crackle".
Wile crazing represents one of the major glazing defects of a finished pot,
a fine network of hairline cracks produces a very decorative effect.
The Japanese potters use china ink to make the crackles more pronounced,
and another method is to use stain rubbed to reveal the crackles and then
refiring the pot. These methods and other variations are done with a glaze
that has an different shrinkage rate than the body.
Usually in stoneware , if we want to do *crackle ware* then we under fire
by 80-100C a normal glaze that matches the shrinkage rate of the body thus
producing crackles by incorrect firing that are then stained and then the
pot is refired at the correct temperature. This way we have a beautiful
Crackle Ware, with a Food Safe Glaze With NO CRACKS. This is the reason why
your friend Deborah Randolph can be right or wrong, only seeing the pots,
or knowing if they use the under firing technique...
About the stoneware temperature, we do have that low temperature stoneware,
and porcelain too.And even lower temperature believe me.
For more information on the subject there is a very good book;
Ceramic Techniques - Pravoslav Rada - 1989 by The Hamlyn Publishing Group -
Michelin House, 81 Fulham Road, London SW3 6RB - ISBN 0-600-56154-2
------------------------------------------------------------
Paulo Correia - Cerbmica Decorativa, Lda.
Portugal
P.Correia-Ceramica@ip.pt
BobWicks on wed 27 may 98
>> potter friend of mine (Deborah Randolph) was looking through the Smith
and Hawken Catalog last week and was disturbed to find *Crackleware* for
sale. It is cast work, badly crazed. The comment says that a deliberate
mismatch between the clay and glaze creates the decorative crackling. It
says that because it is fired at a high stoneware heat, it is safe to use
for food.
Deborah called Smith and Hawkins, saying she was unhappy that a catalog of
such a high caliber wouldn't do better research on products they sell. She
told them she has been a potter for twenty five years and that she is sure
that a crackled glaze should not be used for food.
This morning (Memorial Day) she received a call from the company who makes
the Crackleware. They are located in southern France (a company called Jars
Ceramistes.) She talked with their technical expert, through a translator.
They wanted to know why she was concerned. She explained her position, and
said that we try hard in this country to make our products food safe before
selling them to the public.
The translator said that she just didn't understand... that this is
STONEWARE, and stoneware is NON POROUS!!! And furthermore, it has been
fired to 1280c. She responded that stoneware fired at that temp IS still
porous...
Then their argument changed to say that the cracking is only on the top
layer of glaze and doesn't go all the way down to the clay body. She told
him she didn't believe him!! And besides, she said, the crackle would still
be next to the food, and that would still make it good bacteria breeding
grounds.
His next argument was that *We have sold it for many years with no
problems...* Then he wanted her to send him a speciman of her own work,
which she said (and I completely agree) has nothing to do with this
situation.
Finally, he wants her to send him substantial proof of what she is saying.
She would like confirmation of her postition (or not) from the experts in
our group. She wants to know if she really is on solid ground here. She can
also dig out quotes and send them along to the company with a list of books
to check for themselves. Any information you could provide would help her a
lot. She feels strongly that we must give the public a product that is
safe, and was very pleased that Smith and Hawkens took her call seriously
instead of just ignoring it.
Thanks. I will give her all your comments, as she is not a computer user.
Jean
---------------------->
Jean Lehman, in Lancaster, PA
j_lehman@acad.FandM.EDU (that's an _underscore_ not a hyphen)
http://www.art-craftpa.com/lehman.html
Check out the 1998 Strictly Functional Pottery National at:
http://www.art-craftpa.com<<
Jean:
You stick to your guns on the safety issue regarding crackle stoneware. If
your factory representative wants to hear the full story on the safety I
suggest that you send a CC of your message to Monona Rossol as she is a
nationally known industrial ceramic toxicologist. She will set him straight
with no uncertain terms.
Bob
Paulo Correia on wed 27 may 98
Jean,
About these glazes I can only send you the general characteristics of them,
and you may judge by yourself if they are food safe or unsafe. The glazes I
know are from Wolf E. Matthes - Keramische Glazuren, copyright 1985
Verlagsgesellshaft Rudolf Muller Gmbh.
Luster glazes are alkali-boron with low content of lead fusing at low
temperature, with addition of metal oxides like CuO, CoO, Fe2O3, TiO2,
MoO3, MnO2, NiO, Bi2O3, V2O5 and UO2, or small amounts of noble metallic
compounds, like AuCl2, AgCO3, etc. During the firing of the glaze at the
cooling down, the atmosphere is highly reducing between 800C and 500C, thus
forming a extremely thin surface of metallic oxide or pure metal at the
glaze surface, that will interfere with the reflection and diffraction of
the light in the glaze much the same way of an oil spot on water.
------------------------------------------
Yellow luster glaze - C. Fiddis
SK 04a/03a
0.42 Na2O 0.21 Al2O3 1.9 SiO2
0.21 K2O 0.70 B2O3
0.37 CaO
+ 1% AgNO3
1% Bi2O3
42.9 Frit 90167 Degussa
30.6 Frit 90158 Degussa
12.2 whiting
11.8 kaolin
2.5 Quartz
+1.0 Silver nitrate
1.0 Bismuth oxide
Try changing the oxides to achieve other shades.
---------------------------------------------------------------------
Blue luster glaze - Wolf E. Matthes
SK 03a/2a
0.417 Na2O 0.057 Al2O3 2.313 SiO2
0.249 K2O 0.170 B2O3
0.249 Ca
0.085 PbO
+1% CoCO3
1% Bi2O3
70.0 Frit 1233 M&M
20.0 Frit 90187 Degussa
10.0 Kaolin
+1.0 Cobalt carbonate
1.0 Bismuth oxide
--------------------------------------------------------------
So I think that only knowing the glaze formula, or trough a glaze test we
could be sure about the safety of the glaze.
Sometimes even knowing that a commercial glaze has a low lead content isn't
enough, because of the variables involved in firing and the interference of
other oxides used in the on glaze decoration, they may promote the release
of lead from the glaze.
I expect that my contribution be helpful to you, or at least the recipes
may be useful for someone out there.
Paulo Correia - Ceramica Decorativa, Lda.
Portugal
P.Correia-Ceramica@ip.pt
BBC on thu 28 may 98
Are these food safe glazes, they have been used for years at the local
college, and I was told they were??
I hope so...
White: Np Sy 4724, Dolomite 1000, EPK 1420, tin oxide 700, Gert Borate
1900, Bentonite 300.....
Blue: Np sy 2365, Gert Borate 1350, Silica 1015, epk 270, red iron oxide
100, cobalt oxide 50, rutile milled 200, Bentonite 100......
Red: epk 200, silica 1200, feldspar 800, talc 360, gert borate 1280, red
iron oxide 600......
Sincerely,
Gloria
-----Original Message-----
From: Jean Lehman
To: Multiple recipients of list CLAYART
Date: Tuesday, May 26, 1998 9:00 AM
Subject: Technical advice needed
----------------------------Original message----------------------------
A potter friend of mine (Deborah Randolph) was looking through the Smith
and Hawken Catalog last week and was disturbed to find *Crackleware* for
sale. It is cast work, badly crazed. The comment says that a deliberate
mismatch between the clay and glaze creates the decorative crackling. It
says that because it is fired at a high stoneware heat, it is safe to use
for food.
Deborah called Smith and Hawkins, saying she was unhappy that a catalog of
such a high caliber wouldn't do better research on products they sell. She
told them she has been a potter for twenty five years and that she is sure
that a crackled glaze should not be used for food.
This morning (Memorial Day) she received a call from the company who makes
the Crackleware. They are located in southern France (a company called Jars
Ceramistes.) She talked with their technical expert, through a translator.
They wanted to know why she was concerned. She explained her position, and
said that we try hard in this country to make our products food safe before
selling them to the public.
The translator said that she just didn't understand... that this is
STONEWARE, and stoneware is NON POROUS!!! And furthermore, it has been
fired to 1280c. She responded that stoneware fired at that temp IS still
porous...
Then their argument changed to say that the cracking is only on the top
layer of glaze and doesn't go all the way down to the clay body. She told
him she didn't believe him!! And besides, she said, the crackle would still
be next to the food, and that would still make it good bacteria breeding
grounds.
His next argument was that *We have sold it for many years with no
problems...* Then he wanted her to send him a speciman of her own work,
which she said (and I completely agree) has nothing to do with this
situation.
Finally, he wants her to send him substantial proof of what she is saying.
She would like confirmation of her postition (or not) from the experts in
our group. She wants to know if she really is on solid ground here. She can
also dig out quotes and send them along to the company with a list of books
to check for themselves. Any information you could provide would help her a
lot. She feels strongly that we must give the public a product that is
safe, and was very pleased that Smith and Hawkens took her call seriously
instead of just ignoring it.
Thanks. I will give her all your comments, as she is not a computer user.
Jean
---------------------->
Jean Lehman, in Lancaster, PA
j_lehman@acad.FandM.EDU (that's an _underscore_ not a hyphen)
http://www.art-craftpa.com/lehman.html
Check out the 1998 Strictly Functional Pottery National at:
http://www.art-craftpa.com
Ron Roy on tue 2 jun 98
Hello again Gloria,
I have looked at the molecular formula of your glazes and my comments
follow. It is best to provide the total of the recipe to ensure the numbers
are all there and accurate. It is also necessary to provide the cone fired
to as glazes are evaluated according to final firing temperature. I am
making the assumption these are fired to cone 6 in oxidation???
Glaze #1 - total 10044: By itself it is OK but it is short of silica for
cone 6. If toxic colourants are added they will probably be released under
some circumstances into food - how much can only be determined by testing.
Glaze #2 (blue) total = 5450. A durable glaze - cobalt approx. 1% - I would
say that would be near the upper limit for cobalt oxide - maybe a bit more
for the carbonate.
Glaze #3 (red) total 4440. Not a durable glaze - short of both silica and
alumina. It may be possible that some boron may release from this glaze -
but I don't think it would be a significant amount - the only way to find
out would be to have it tested. I think it is safe to say this is a food
safe glaze but it will be affected (attacked) by both acid and alkali in
other words - dull with use over time.
Again, I have assumed a gloss firing to cone 6 (measured with a large
pyrometric cone) - if that is not so I would have to revaluate them for the
temperature they are being fired to. If these are fired without a witness
cone - say with a sitter - then we don't know what temp. they are actually
being fired at.
>----------------------------Original message----------------------------
>Are these food safe glazes, they have been used for years at the local
>college, and I was told they were??
>I hope so...
> White: Np Sy 4724, Dolomite 1000, EPK 1420, tin oxide 700, Gert Borate
>1900, Bentonite 300.....
>Blue: Np sy 2365, Gert Borate 1350, Silica 1015, epk 270, red iron oxide
>100, cobalt oxide 50, rutile milled 200, Bentonite 100......
>Red: epk 200, silica 1200, feldspar 800, talc 360, gert borate 1280, red
>iron oxide 600......
>
>Sincerely,
>Gloria
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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