Carol Sandberg on mon 20 nov 00
Vince:
Are you saying that a black interior in a coffee mug is unsafe regardless of
the temperature it's been fired to? I have some wonderful salt glazed
porcelain coffee mugs with very dark interiors (navy blue-black). Should I
stop having coffee in them?
Please advise.....
Thanks,
Carol
----- Original Message -----
From: vince pitelka
To:
Sent: Monday, November 20, 2000 12:38 AM
Subject: Seeking Satin Doll
> Sometimes our workshop presenters will mix up a dynamite glaze, and leave
it
> here without leaving the recipe. Someone last summer left a rich black
> satin cone 10 reduction glaze called Satin Doll, and my students love it.
> It seems to be very reliable, and looks good with a lot of our other
glazes.
> It is obviously loaded with oxides, so we do not use it on food contact
> surfaces, especially since I do not have a recipe. But we just ran out,
so
> if any of you have a recipe for a glaze called satin doll I would
appreciate
> it if you would post it on the list.
> Thanks and best wishes -
> - Vince
>
> Vince Pitelka
> Home - vpitelka@dekalb.net
> 615/597-5376
> Work - wpitelka@tntech.edu
> 615/597-6801 ext. 111, fax 615/597-6803
> Appalachian Center for Crafts
> Tennessee Technological University
> 1560 Craft Center Drive, Smithville TN 37166
> http://www.craftcenter.tntech.edu/
>
>
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vince pitelka on tue 21 nov 00
> Are you saying that a black interior in a coffee mug is unsafe regardless
of
> the temperature it's been fired to? I have some wonderful salt glazed
> porcelain coffee mugs with very dark interiors (navy blue-black). Should
I
> stop having coffee in them?
Carol -
No, I don't think I suggested that at all. Of course it is impossible to
make generalizations about dark glazes. There are plenty of black gloss
glazes which are very stable. All I said was that I was being cautious
because we did not have a recipe for this glaze.
Regarding normal studio glazes, if the materials are locked in a stable
glass with a gloss surface, and if the glaze seems well-balanced in a glaze
calc program, then it is probably okay. Regarding your mugs, if you can get
the glaze recipe you can check it in a glaze-calc program to get some sense
of whether it is well-balanced, and you can see what percentages of metals
it contains. For example, the Val Cushing Satin Doll recipe I was looking
for has fairly small percentages of a variety of oxides, but a large
component of redart clay, giving a dense rich black color and a stable
glaze.
Thanks to those who supplied the recipe for Satin Doll. I appreciate it,
- Vince
Vince Pitelka
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
http://www.craftcenter.tntech.edu/
Martin Howard on tue 21 nov 00
As our glazes are so often coloured with metallic oxides, which can leach,
would it not be safer to do all the colouring with coloured slips, then just
cover with a clear safe glaze?
Then you can have black slip inside the coffee mug, just glazed clear, and
safe?
What do others think about this question?
We certainly seem to be in an era when what was safe last year is now
subject to questioning and higher standards of public health.
Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
England
martin@webbscottage.co.uk
John Hesselberth on tue 21 nov 00
Martin Howard wrote:
>As our glazes are so often coloured with metallic oxides, which can leach,
>would it not be safer to do all the colouring with coloured slips, then just
>cover with a clear safe glaze?
>
>Then you can have black slip inside the coffee mug, just glazed clear, and
>safe?
>
>What do others think about this question?
Hi Martin,
I guess it would work, but you would certainly limit your aesthetic
options. Glazes can be made to leach next to nothing if you know what
you are doing and glazes expand the range of aesthetic options available
(vs. colored slips covered by clear glazes) enormously. There are glazes
of nearly all colors (including black) that leach next to nothing. There
are also those I can draw the color right out of with a few minutes
exposure to weak acids like lemon juice or vinegar. It all boils down to
good craftsmanship which, I would think, would be required for either
route. I would guess that someone who will put a badly leaching glaze on
a functional piece of pottery would also find a way to screw up the
colored slip/clear glaze route.
Regards, John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
John Hesselberth on tue 21 nov 00
Carol Sandberg wrote:
>Are you saying that a black interior in a coffee mug is unsafe regardless of
>the temperature it's been fired to? I have some wonderful salt glazed
>porcelain coffee mugs with very dark interiors (navy blue-black). Should I
>stop having coffee in them?
Hi Carol,
You can make black glazes that are stable; you can also make ones that
are very unstable. See another response of mine in the "Toxicity and
Cone 10" thread for more info.
Regards, John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Jean Silverman on thu 23 nov 00
At 08:37 AM 11/21/00 +0000, Martin wrote:
>As our glazes are so often coloured with metallic oxides, which can leach,
>would it not be safer to do all the colouring with coloured slips, then just
>cover with a clear safe glaze?
>
>Then you can have black slip inside the coffee mug, just glazed clear, and
>safe?
I suspect (hope) Martin wrote this tongue-in-cheek to illustrate the taking
of things to absurd extremes. This issue of achieving perfect safety in
everything we do has gone way too far. Surely there is a medium route
between sitting by your kiln inhaling manganese fumes or selling cups with
a copper-green lead glaze, and leading a life of perfect safety, uniformity
and sterility in those hazzardous waste handling suits and never contacting
reality at all. T'aint possible! Life is full of hazzards. Clay (dust) is
bad for you, and the clay is probably the safest, least toxic material in
the studio. What about common sense? Is there anyone out there any more who
even knows what common sense is?
End of rant. Thank you all.
Jean Silverman
Plum Tree Pottery
41 Neal Mill Rd.
Newmarket NH 03857
603-659-2632
Ron Roy on sun 26 nov 00
I think it's good old common sense that got us where we are today.
Thousands of recipes given, sold and stolen and not any indication of which
are stable.
Clay suppliers selling clay that is not properly vitrified at the
recommended cone.
Studios where only the young and fit should be working.
On the contrary - common sense only works if you know what your are up
against. Being perfectly safe has little to do with discussions of this
type - learning has everything to do with it.
I think Martin asked a good question - it is one way of cutting down on the
leaching of toxins out of glazes. If you are a production potter you better
pay attention to this issue - if you want to be successful in the long run.
RR
>
>I suspect (hope) Martin wrote this tongue-in-cheek to illustrate the taking
>of things to absurd extremes. This issue of achieving perfect safety in
>everything we do has gone way too far. Surely there is a medium route
>between sitting by your kiln inhaling manganese fumes or selling cups with
>a copper-green lead glaze, and leading a life of perfect safety, uniformity
>and sterility in those hazzardous waste handling suits and never contacting
>reality at all. T'aint possible! Life is full of hazzards. Clay (dust) is
>bad for you, and the clay is probably the safest, least toxic material in
>the studio. What about common sense? Is there anyone out there any more who
>even knows what common sense is?
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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