Steven Barth on tue 21 may 96
Hello everyone,
Last week someone on the list spoke briefly about using only
unquestionably foodsafe glazes and mentioned he was also an
organic gardener for the last 30 years.
I lost the message and would like to speak to that person.
I would appreciate it if you would let me know who you are.
Please e-mail me directly.
Thanks.
Lynn Barth
Cherry Valley Pottery
Lynn Barth on sun 1 dec 96
I, too, asked the question, "What makes a glaze foodsafe?"
And to make a long story short ended up with the book "Keeping
Clay Work Safe and Legal" By M. Rossol. In it I found all the
ingredients for all the glaze recipes and the hazards associated with
them. I have not found this information in any of the other dozen or
so books on ceramics that I have.
Next, I went to the "ClayArt Glaze Database" at:
http://apple.sdsu.edu/ceramicsweb/glazedatabase.html
and got all the recipes at C6 ox. that did not contain anything
hazardous. That was about 11 recipes.
I have tested several of those and below are my findings:
I used clay bodies S213 (A cone 6 "porcelain"), S200 (Buff colored
stoneware), S192 Baldwin Stoneware, and S108 (Terracotta Stone-
ware).
IE Red C6 ox.
Tan to light brown (where thick), brown flecks, semi matt. Good
fit.
IE 41 C6 ox.
Supposed to be very white and glossy but for me it came out very
white but very dry and with big cracks. Not a good fit for me.
IE 49 C6 ox.
Translucent white, semi-matt. Good fit.
Soft Green w/ Pink Flecks C6 ox.
A light green gloss. Best overall green on terracotta. On porcelain,
S192 and S200, light green where thin, two coats looks brown,
dark green where pooled. Good fit. However, no pink flecks.
IE 35 C6 ox.
White. Translucent. Whiter where pooled, glossy. Good fit.
IE BLACK C6 ox.
Shiny, black brown, opaque. A real dark chocolate brown. Stays
put. Nice fit. For the oxide I used BIO.
IE50
Came out white translucent. Whiter where thicker. Glossy, good fit.
A stroke of RIO over this came out a nice reddish brown.
All the above were submitted by Brian Kemp, except for the
Soft Green W/ Pink Flecks submitted by Lori Wilkinson.
The following comes from "Glazes and Glazings Techniques" by
Greg Daly.
C6 ox. Base glaze plus Copper Carbonate
Nepheline Syenite 30
Whiting 26
Silica 30
Kaolin 14
+2 copper carbonate
A soft green. Opaque. Tannish green where pooled, semi-gloss.
For all the people who ask, "Is this ingredient foodsafe?", do
yourself a favor and get this book. It's ALL in there.
You can order it from NCECA, (National Council on Education for
the Ceramic Arts) at P.O. Box 1677, Bandon, OR 97411, 1-800-
99NCECA.
(And I have no monetary connection with this author).
Lynn Barth
Cherry Valley Pottery
LABARTH@worldnet.att.net
croft on thu 6 apr 00
------------------
I wonder if someone could help with teh following. I have a basic glaze that
works and wondered if adding the following colourants would still make for a
foodsafe glaze
Basic Glaze at cone5/6
20=25 EPK
20=25 Silica
20=25 Wollastonite
20=25 Ferro Frit 3134
20=25 Potash feldspar
then adding
10 - 15 =25 superpax for white
4=25 cobalt carbonate for blue
1.5=25 cobalt carbonate and 1=25 copper carbonate for med blue
1.3 =25 cobalt carb and .8=25 chrome carb for blue/green
.3=25 cobalt oxide and 5=25 rutile for light variegated blue
7=25 copper carbonate for vivid metallic green
Thanks - I appreciate any and all responses
Cindy Strnad on fri 7 apr 00
Ms/Mr Croft,
I can't speak for the other additives (they sound all right to me, but if
you want to know for sure, you can only have them tested), but 7% copper
carb is a bit high for a stable glaze. You have a very stable base glaze,
and that's a good start, but I'll bet that copper will leach.
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
Chris Schafale on sat 8 apr 00
As I'm sure others will tell you, you have to get the glaze tested in
order to know for sure. However, I would be willing to bet $10 that
the variation with 7% copper would not be appropriate for food use.
Copper, we're told, is hard to keep in glazes, and if you have
enough that the surface looks metallic, you almost certainly have
more than the glaze can hold.
Chris
> ----------------------------Original message----------------------------
> ------------------
> I wonder if someone could help with teh following. I have a basic glaze that
> works and wondered if adding the following colourants would still make for a
> foodsafe glaze
> Basic Glaze at cone5/6
> 20% EPK
> 20% Silica
> 20% Wollastonite
> 20% Ferro Frit 3134
> 20% Potash feldspar
> then adding
> 10 - 15 % superpax for white
> 4% cobalt carbonate for blue
> 1.5% cobalt carbonate and 1% copper carbonate for med blue
> 1.3 % cobalt carb and .8% chrome carb for blue/green
> .3% cobalt oxide and 5% rutile for light variegated blue
> 7% copper carbonate for vivid metallic green
>
> Thanks - I appreciate any and all responses
>
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
John Hesselberth on sat 8 apr 00
Hi Mr/Ms/Mrs Croft (Do you have a first name? We're a pretty friendly
bunch)
The short answer to your question is that there is no way to tell but to
test your glazes. But seconding what Cindy Strnad said, the one that is
most suspect is 7% copper. I haven't seen a glaze yet that will hold
that much copper. The only one I have been told about that uses 7%
doesn't even pass the 3 day vinegar test. If vinegar will change the
color of your glaze in a 2-3 day soak, then the glaze is totally
unsuitable for functional work (I too am a functional potter, not a
utilitarian one to comment briefly on another thread). For sure enough
copper would leach to make food taste bitter and since you say it is
metallic, I know there is a lot of loose copper waiting to contaminate
someone's food.
The base glaze itself is higher in calcium than I have seen in other
stable glazes, but that may or may not be important. We just don't have
enough information to be able to predict stability from composition. The
other components are in range of normal limit formulas which is a good
place to start if you are trying to make a stable glaze.
With respect to the colors other than metallic green, I'd test them in
order of highest concentration of colorants, i.e test the 4% cobalt
carbonate first. If it is stable the cobalt in the other
cobalt-containing ones probably is also. Then test the cobalt/copper one
for copper; then the cobalt/chrome one for chrome. If all of those are
stable, there isn't any need to test the one with cobalt and rutile. I
might also test the one containing 15% zirconium. Normally zirconium is
not a problem, but that is a very high level. I'd put that lower on my
priority list though.
You can find testing instructions on my web site at
http://www.frogpondpottery.com/glazetest.html
There is also some additional information on glaze stability and "food
safe" on my site if you are interested.
And as I always do, I will ask you to share the results of any testing
you do with Clayart. I am trying to build a list of tested glazes--you
will find a few already posted on my site.
Regards, John
croft wrote:
>----------------------------Original message----------------------------
>------------------
>I wonder if someone could help with teh following. I have a basic glaze that
>works and wondered if adding the following colourants would still make for a
>foodsafe glaze
>Basic Glaze at cone5/6
>20% EPK
>20% Silica
>20% Wollastonite
>20% Ferro Frit 3134
>20% Potash feldspar
>then adding
>10 - 15 % superpax for white
>4% cobalt carbonate for blue
>1.5% cobalt carbonate and 1% copper carbonate for med blue
>1.3 % cobalt carb and .8% chrome carb for blue/green
>.3% cobalt oxide and 5% rutile for light variegated blue
>7% copper carbonate for vivid metallic green
>
>Thanks - I appreciate any and all responses
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
The only things in life that are certain are death and taxes; however
only taxes come once a year. Anonymous
Ron Roy on tue 11 apr 00
This a common glaze in use now - it has a lot of CaO - more than the limits
for such glazes recommend. I would be interested in seeing the resuts of a
leach test with 2% copper added - there is plenty of silica and alumina for
a cone 6 glaze so it may be stable - only testing will tell.
By itself there is no problem - there are no toxins present - the other
question is - is it durable - perhaps this needs the acid test to determine
that - anyone left this glaze in vinegar for a week?
No problem adding super pax - it will only make the glaze more durable.
4% cobalt is over kill - and getting close to the amount that cannot be
held in solution as a glaze cools. - I suspect this glaze will leach cobalt
at that consentration - question is - will it be a problem?
The next three I would imagine would be OK
7% copper in any glaze is going to leach big time and it's going to change
colour when next to a lot of different foods - don't put it on surfaces
that can come in contact with food or drink.
My general rule is use as little of a colouring oxide as suits your purpose
in glazes that will come in contact with food - that goes double for
copper.
There - I guess you could describe me as being in a rather confident mood
tonight - don't know what got into me.
RR
>----------------------------Original message----------------------------
>------------------
>I wonder if someone could help with teh following. I have a basic glaze that
>works and wondered if adding the following colourants would still make for a
>foodsafe glaze
>Basic Glaze at cone5/6
>20% EPK
>20% Silica
>20% Wollastonite
>20% Ferro Frit 3134
>20% Potash feldspar
>then adding
>10 - 15 % superpax for white
>4% cobalt carbonate for blue
>1.5% cobalt carbonate and 1% copper carbonate for med blue
>1.3 % cobalt carb and .8% chrome carb for blue/green
>.3% cobalt oxide and 5% rutile for light variegated blue
>7% copper carbonate for vivid metallic green
>
>Thanks - I appreciate any and all responses
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Cindy Strnad on wed 12 apr 00
Hi, Ron.
You're talking about Tony Hansen's 5x5 glaze, aren't you? I've kept vinegar
in it for a week--longer, actually, with 4% copper, and had no change in
color from below the water line to above. I sure wouldn't think of using 7%
copper for anything but pretty baubles, though.
Four percent of cobalt would make it nearly black, I think. I don't really
like it with cobalt anyway, but it makes a nice semi-opaque white for me
with 7% zircopax and a pretty, translucent green with 4% copper. I also like
it with; red clay in place of half the EPK, 4% rutile, and (I think) 10% RIO
for a tawny tan.
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
Ron Roy on mon 17 apr 00
The only way to be sure is to hace it tested - remember - if you can see
changes it means the glaze is really bad - like awful.
4% copper is going to be very hard to keep in any glaze - have you left
orange juice in for an hour and tasted it? - that would be a better test.
I seem to remember Jon Hesselberth saying he had that glaze tested - but
not with that much copper.
There is plenty of alumina and silica in that glaze so it should be OK if
you don't overload it with colouring oxides. It is oversupplied with CaO
and when this happens it can lead to less than durable glazes - has to do
with how different molecules get used up during the melt. Sometimes there
are some left over and they compromise the stability of the glaze.
RR
>----------------------------Original message----------------------------
>Hi, Ron.
>
>You're talking about Tony Hansen's 5x5 glaze, aren't you? I've kept vinegar
>in it for a week--longer, actually, with 4% copper, and had no change in
>color from below the water line to above. I sure wouldn't think of using 7%
>copper for anything but pretty baubles, though.
>
>Four percent of cobalt would make it nearly black, I think. I don't really
>like it with cobalt anyway, but it makes a nice semi-opaque white for me
>with 7% zircopax and a pretty, translucent green with 4% copper. I also like
>it with; red clay in place of half the EPK, 4% rutile, and (I think) 10% RIO
>for a tawny tan.
>
>Cindy Strnad
>earthenv@gwtc.net
>Earthen Vessels Pottery
>RR 1, Box 51
>Custer, SD 57730
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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