Phil Smith on wed 5 mar 03
Clayarters,
Are there any books that list proven, Stable, non-leaching
cone 10 reduction glazes?. Also, If a glaze does not have obvious toxic
materials does this make it safe by default?.
The instructor at our studio is under the impression that all high fire
glazes are safe regardless of the materials that are in them. From what I
have learned by reading ClayArt this is simply not the case. From what I
see the flux materials contain metals that one may not care to ingest in a
form other than a multi-vitamin or other more natural form.
Please refer me to reading materials and or recipes.
Thanks.
Phil...
John Hesselberth on wed 5 mar 03
On Wednesday, March 5, 2003, at 12:39 PM, Phil Smith wrote:
> Are there any books that list proven, Stable, non-leaching
> cone 10 reduction glazes?. Also, If a glaze does not have obvious toxic
> materials does this make it safe by default?.
> The instructor at our studio is under the impression that all high fire
> glazes are safe regardless of the materials that are in them. From
> what I
> have learned by reading ClayArt this is simply not the case.
>
Hi Phil,
You are right; your instructor is wrong. There are lots of cone 10
glazes that are not stable.
No, there is not a book that addresses this, but Ron Roy and I are both
convinced that if you follow the rules we developed for cone 6 you will
be pretty certain of having a good glaze. In short 1) have enough
silica, 2) have enough alumina, 3) thoroughly melt during firing (no
"unmelted" mattes, please), and 4) don't overload with colorants. A
specific example of one popular cone 10 glaze that is pretty unstable is
Oribe (too much copper--I've forgotten how much silica and alumina it
has), but there are plenty of others.
You can debate your question on safety until the end of time and not get
a definitive answer. However, there are issues other than safety with
unstable glazes. For example 5 mg/l of copper can make food taste
bitter. It is pretty easy to leach 5 mg/l from a glaze that is
overloaded with copper--Oribe will easily go there--so will a lot of
others. There is also the question of durability. This subject came up a
week or two ago and I pointed out I have some cone 10 mugs with the
color missing on the inside. I used them for coffee most every day for a
year of two and the interior glaze has leached/dissolved. To me it is a
matter of good craftsmanship to put stable glazes on functional pottery.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
June Perry on thu 6 mar 03
There are no books which list cone 10 glazes as being stable; but there may
be cone 10 stable glazes in books! :-)
Short of having glazes tested, you can run the recipes through software like
Tony Hansen's Insight and compare the results to the limit formulas as shown
in the software. I believe you can download a trial period of the software at
his site.
There are other good programs out there as well -- Matrix, for example. You
can also get a copy of Daniel Rhodes book "Clay and Glazes" and learn to do
the math manually or with a calculator to find out how many moles of silica,
alumina etc. are in the recipes you are using and compare those results with
published limit formulas.
And no, not all cone 10 recipes are safe. Firing to cone 10 will not make up
for a faulty glaze -- for instance, one that is too low in silica and/or
alumina to hold potential harmful oxides in solution, thereby, probably
releasing them into food under the right conditions.
I've run many so called "famous" published glazes through my software and was
pretty surprised to see that so many of them were under supplied with silica
and alumina. If that's the only problem with the glaze recipe, it's an easy
and fast correction to make, particularly with glaze software.
Regards,
June Perry
http://www.angelfire.com/art2/shambhalapottery/
Ron Roy on thu 6 mar 03
Hi Phil,
Not yet - if you read Mastering Cone 6 Glazes you will know how to find
that kind of information for any kind of glazes.
It is not true that cone 10 glazes are automaticaly durable - that is just
silly. I have never heard of any potter or institution doing anything to
verify how stable any cone 10 glazes are.
The job remains to be done and I would not be surprised to find that over
50% of the glazes in current use will not pass the testing.
RR
>Clayarters,
> Are there any books that list proven, Stable, non-leaching
>cone 10 reduction glazes?. Also, If a glaze does not have obvious toxic
>materials does this make it safe by default?.
>The instructor at our studio is under the impression that all high fire
>glazes are safe regardless of the materials that are in them. From what I
>have learned by reading ClayArt this is simply not the case. From what I
>see the flux materials contain metals that one may not care to ingest in a
>form other than a multi-vitamin or other more natural form.
>Please refer me to reading materials and or recipes.
>Thanks.
>
>Phil...
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
iandol on thu 6 mar 03
Dear Phil Smith,=20
You ask <cone 10 reduction glazes?.>>
Resolving the deathly silence I have had to similar questions I would =
suggest that no one really knows.
I would propose that if a glaze is Matt, or Opaque without the =
deliberate addition of an opacifier such as Tin Oxide or Zircon Silicate =
then it is suspect.
I am forming a firm opinion that it is always necessary to start with a =
base glaze that is always transparent and craze free if the composition =
is to have any hope of passing the Acid Test
Furthermore, to answer your second question <obvious toxic materials does this make it safe by default?.>> I would =
suggest that if Whiting or any other Carbonate which will degenerate to =
a soluble oxide is used to give opacity, translucency, mattness or =
additional fluxing power then in all probability it will fail the Acid =
Test.=20
I cannot understand why people guess answers when they do not know =
factually what "Is" through experimental investigation. But I suspect it =
is a function of laziness.
Best regards, and thanks for asking the hard questions.
Ivor Lewis.
Michael Wendt on thu 6 mar 03
RR,
A few years back, John H asked us to send him the recipes of glazes we had
had tested and I sent him the data I got back from Alfred Analytical on the
cone 10 glaze I use. All of the oxide tests fell below the release levels
allowed in drinking water. There must be some others out there who have done
the same and I am surprised the results were not at least mentioned in the
book.
I do agree that it is not realistic to claim a heavily colored glaze is safe
unless the oxides presumed to be released are either made non soluble by the
glaze (which must be verified by lab testing) or are inherently safe to
ingest (calcium, sodium, magnesium and potassium).
Regards,
Michael Wendt wendtpot@lewiston.com
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
http://www.wendtpottery.com
RR wrote:
Hi Phil,
Not yet - if you read Mastering Cone 6 Glazes you will know how to find
that kind of information for any kind of glazes.
It is not true that cone 10 glazes are automaticaly durable - that is just
silly. I have never heard of any potter or institution doing anything to
verify how stable any cone 10 glazes are.
The job remains to be done and I would not be surprised to find that over
50% of the glazes in current use will not pass the testing.
RR
John Hesselberth on fri 7 mar 03
On Thursday, March 6, 2003, at 01:44 AM, iandol wrote:
> I would propose that if a glaze is Matt, or Opaque without the
> deliberate addition of an opacifier such as Tin Oxide or Zircon
> Silicate then it is suspect.
>
> I am forming a firm opinion that it is always necessary to start with a
> base glaze that is always transparent and craze free if the composition
> is to have any hope of passing the Acid Test
Hi Ivor,
It is reasonable to be suspicious of mattes--certainly they are more
likely to be unstable than glossy glazes. However the high calcium
mattes we developed for Mastering Cone 6 Glazes are extremely stable to
acids if properly melted during firing and not overloaded with
colorants. But they can take quite a lot of colorant, though, without
problem--you can get medium greens, dark blues and browns with pretty
negligible leaching. The data are detailed in the book.
I think your supposition is more accurate with so-called alumina
mattes. I have not found a way to make one of them stable to my
satisfaction. But overloading with one of the alkaline earths to force
the glaze into the matte region seems to be OK (within reason) if you
keep plenty of silica (>2.5) and alumina (.25-.45 at cone 6) in the
glaze.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on sat 8 mar 03
Hi Michael,
You have my admiration for having your glazes tested - I was not aware.
Perhaps you would be willing to share that research with me - or better
still the list in general.
Can you tell me which oxides you had them tested for - and did you add
copper as a control?
I am also very interested in anyone else who has had any cone 10 glazes
tested. I know it is sometimes a problem because so much hangs on sharing
the recipes - there are ways around that and I would be agreeable to
respecting that stipulation.
I see no reason to think less of any potter who does not want to share
their recipes - in many cases it has taken years of hard work to come up
with them - and besides - who wants to see them in the booth next door at a
show.
I work for many production potters, glaze makers and clay suppliers - I
don't keep, collect or publish their glazes or clay bodies - not what a
consultant wants to get a reputation for doing. I am always ready to bind
myself legally to non disclosure. That is never an issue here and no one
can dispute that.
Best regards,
RR
>A few years back, John H asked us to send him the recipes of glazes we had
>had tested and I sent him the data I got back from Alfred Analytical on the
>cone 10 glaze I use. All of the oxide tests fell below the release levels
>allowed in drinking water. There must be some others out there who have done
>the same and I am surprised the results were not at least mentioned in the
>book.
>I do agree that it is not realistic to claim a heavily colored glaze is safe
>unless the oxides presumed to be released are either made non soluble by the
>glaze (which must be verified by lab testing) or are inherently safe to
>ingest (calcium, sodium, magnesium and potassium).
>Regards,
>Michael Wendt wendtpot@lewiston.com
>Wendt Pottery
>2729 Clearwater Avenue
>Lewiston, Idaho 83501
>1-208-746-3724
>http://www.wendtpottery.com
>RR wrote:
>
>Hi Phil,
>
>Not yet - if you read Mastering Cone 6 Glazes you will know how to find
>that kind of information for any kind of glazes.
>
>It is not true that cone 10 glazes are automaticaly durable - that is just
>silly. I have never heard of any potter or institution doing anything to
>verify how stable any cone 10 glazes are.
>
>The job remains to be done and I would not be surprised to find that over
>50% of the glazes in current use will not pass the testing.
>
>RR
>
>______________________________________________________________________________
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>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sun 9 mar 03
As John has pointed out - the factors that determine a glazes stability are
not obvious by observation.
Crazing is not a factor - it is the quality of the glass - the chemistry
and the melt which are the determining factors. If an unstable glaze is
crazed - because the acids or alkalines have more surface to work on - then
leaching will be increased. If the glaze is stable to start with the extra
surface is not a significant factor.
Many of the glazes in our book are matte and semi matte but are still durable.
Some are way over the limits - especially with CaO and are stable.
Melting a glaze well and inducing mattness through slow cooling is a
productive way to get both durability and mattness. I cannot say always
because the chemistry still has to be right - matte glazes that are matte
because they don't have enough silica - well - good luck.
There is plenty of room for investigation here - and you are right Ivor -
guessing and observation is not worth any thing is this particular game.
I have to also add that glaze fit needs to be included as part of this
subject - no use having a durable glaze if it is falling off the pot or
cracking it.
RR
>I would propose that if a glaze is Matt, or Opaque without the deliberate
>addition of an opacifier such as Tin Oxide or Zircon Silicate then it is
>suspect.
>
>I am forming a firm opinion that it is always necessary to start with a
>base glaze that is always transparent and craze free if the composition is
>to have any hope of passing the Acid Test
>
>Furthermore, to answer your second question <>obvious toxic materials does this make it safe by default?.>> I would
>suggest that if Whiting or any other Carbonate which will degenerate to a
>soluble oxide is used to give opacity, translucency, mattness or
>additional fluxing power then in all probability it will fail the Acid
>Test.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Michael Wendt on sun 9 mar 03
Ron,
I am happy to share the recipe for the glaze I had tested. It is a line
blend of the standard chun base from my pottery course at the University of
Idaho in the early 70s, Frank Cronk, instructor(without the tin oxide) plus
a low expansion glaze I got from "Ceramic Science for the Potter".
As with all glazes, it works for me because it has been empirically fit to
the clay body it is placed upon over hundreds of trial adjustments until it
no longer resembles the original glazes that it was derived from.
Kingman Feldspar 28.3 (Custer works, just not tested)
Flint 23.0
Helmer Kaolin 14.0
Calcium Carbonate 7.7
Neph Sye (A270) 7.7
Talc (Pioneer) 6.4
Barium Carbonate 6.2
Gertsley Borate 5.7
(I have 500 lbs of the old Gertsley I got years ago. I have subbed frit 3134
for GB with indistinguishable results but have not had that formula tested).
The original chun has zinc oxide in it so I add it or leave it out depending
on the requirements of the colorant.
Insight lists: Si:Al = 8.15
SiB:Al = 8.35
CTE = 7.1 x 10^-6/ degree C
Suggested limits for drinking water: Ba 2.0, Fe 0.3, Mn not listed , Co not
listed, Zn 5.0
results all fell well below suggested PELs in parts per million as follows:
Ba < 0.2, Fe < 0.06, Mn trace, Co trace, Zn <0.05
I didn't send a copper sample. Next time I test, I will do frit 134 and ask
for copper to be tested.
My claim is that these results are valid only for my clay body mixed my way
and fired to witness cone 10 and cooled in the same fashion I cool my
wares.
cone 9-12 vitreous white stoneware Body :
39 Helmer slaked to a thin slurry and screened at least 60
mesh.
28 Custer 200 mesh Feldspar
9 A 270 Nepheline Syenite
15 125 mesh silica
9 200 mesh silica
Mixed and dried to proper moisture, this body throws as well as I need and
is translucent in thin sections, the way porcelain is. To see pots made with
it go to http://www.wendtpottery.com and click on the Helmer button. I added
some pictures of my larger pots so you could judge the throwing character of
the clay. Hard to believe a body with only 39% clay can work, but it does
and the glaze remains craze free for many years.
The body always influences the results a person gets with a glaze.
Regards,
Michael Wendt wendtpot@lewiston.com
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
wendtpottery.com
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