Veena Raghavan on sun 23 apr 00
To Ron Roy and all the Clayart glaze gurus, could you please help with
this.
Maybe there is a list somewhere, which indicates the safe leaching amounts
of the various oxides in fired glazes in connection with the food safety
concern, but I have not found it.
There is a great deal of concern and talk about food safety and the
leaching of oxides, and I for one would like to feel that the functional
items I am selling are foodsafe. I realize that the ultimate goal is to
have the ware fired in your own kiln tested. However, when the
not-so-experienced look at the glaze leaching information, how do they know
whether the amounts listed are above or below the foodsafety guidelines? Is
there any list and explanation of this and, if so, would someone be kind
enough to tell me where I could find it. I am referring to cobalt, copper,
lithium, and any other metals or minerals that might be considered
hazardous if they leach beyond a certain amount.
Thank you in advance for any help you can give me.
Veena
Veena Raghavan
75124.2520@compuserve.com
John Hesselberth on mon 24 apr 00
Hi Veena,
As we have discussed many times on this list in the last couple years,
there is no definitive answer to your question--there are only opinions.
The opinions range all over the map. At one end are those who say that
if it meets the FDA lead and cadmium standards it is food safe, i.e. you
don't have to worry about the other glaze constituents. In a regulatory
sense I guess they are correct. On the other end of the spectrum are
those who believe we would be well advised to meet the limits for maximum
contamination levels in drinking water. And there is probably every
opinion in between. There is no data to support any of the positions and
probably won't be in our lifetimes.
I'm personally on the end of the spectrum which says we should be
worrying about how stable our glazes are and trying to make them more
stable; however I believe water standards are more severe than can be
justified based on history and on the standards that have been set for
lead and cadmium. In the end you will have to make up your own mind what
you are going to do.
I'd suggest you search the archives for a more thorough discussion. It
has been exhaustively discussed.
Regards, John
Veena Raghavan wrote:
>----------------------------Original message----------------------------
>To Ron Roy and all the Clayart glaze gurus, could you please help with
>this.
>
>Maybe there is a list somewhere, which indicates the safe leaching amounts
>of the various oxides in fired glazes in connection with the food safety
>concern, but I have not found it.
>
>There is a great deal of concern and talk about food safety and the
>leaching of oxides, and I for one would like to feel that the functional
>items I am selling are foodsafe. I realize that the ultimate goal is to
>have the ware fired in your own kiln tested. However, when the
>not-so-experienced look at the glaze leaching information, how do they know
>whether the amounts listed are above or below the foodsafety guidelines? Is
>there any list and explanation of this and, if so, would someone be kind
>enough to tell me where I could find it. I am referring to cobalt, copper,
>lithium, and any other metals or minerals that might be considered
>hazardous if they leach beyond a certain amount.
>
>Thank you in advance for any help you can give me.
>
>Veena
>
>Veena Raghavan
>75124.2520@compuserve.com
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures
are inevitably projections of the minds of their creators." Bernard
Leach, A Potter's Book.
sibylle on wed 26 apr 00
Hi Veena,
some weeks ago, someone gave us the below list of oxides in glazes, I don't
remember who it was but am very thankful to have it.
Limits for foodsafe glazes:
No lead,barium, cadmium, mangan, ironcromate, nickel or vanadium .
0.1% chromoxid (Hamer: max. colour at1%; 0.1% for pink)
1.6% cobaltcarbonate (exeption: recipe for black glaze contains
5,5% cobaltcarbonate, used as a light decoration at the surface)
1.0% cobaltoxid (Hamer: max. colour at 1%)
5.5% coppercarbonate
3.0% copperoxid (Hamer: max. colour at 5%)
10.0% ironoxid (Hamer: max. colour at 12%)
2.0% Lithiumcarbonat
5.5% Rutil
12.6% Strontiumcarbonat
7.0% Zinnoxid
5.0% Titanoxid (Hamer: max. opak at 5%)
8.0% Zinkoxid
I translated it for me into german and now re-translated in english. Hope all
names are exact. If not, please contact me.
Sibylle
(germany)
Veena Raghavan schrieb:
> ----------------------------Original message----------------------------
> To Ron Roy and all the Clayart glaze gurus, could you please help with
> this.
>
> Maybe there is a list somewhere, which indicates the safe leaching amounts
> of the various oxides in fired glazes in connection with the food safety
> concern, but I have not found it.
>
> There is a great deal of concern and talk about food safety and the
> leaching of oxides, and I for one would like to feel that the functional
> items I am selling are foodsafe. I realize that the ultimate goal is to
> have the ware fired in your own kiln tested. However, when the
> not-so-experienced look at the glaze leaching information, how do they know
> whether the amounts listed are above or below the foodsafety guidelines? Is
> there any list and explanation of this and, if so, would someone be kind
> enough to tell me where I could find it. I am referring to cobalt, copper,
> lithium, and any other metals or minerals that might be considered
> hazardous if they leach beyond a certain amount.
>
> Thank you in advance for any help you can give me.
>
> Veena
>
> Veena Raghavan
> 75124.2520@compuserve.com
John Hesselberth on thu 27 apr 00
Hello Sibylle,
I don't know where this particular list came from; I haven't seen it
before. If you can find the original reference to it I would very much
like to see it. I would like to know how much research was done to
support it, and I will go back to the original author if I can find
her/him and ask.
However I must caution anyone tempted to use this list that my own
research would give me concern regarding parts of the list. For example,
a base glaze has to be extremely good (very stable) to hold 5% copper
carbonate without leaching large amounts of it. With many base glazes,
5.5% copper carbonate would result in leaching well in excess of 50 mg/l.
While, in the U.S., there is no law preventing using a glaze like that
with food, I certainly would not do it. I don't know what the laws might
be in Germany. On the other side of the question, I can show you a glaze
that will hold 2% cobalt carbonate with virtually no leaching; although I
am sure that some glazes with 1.6% cobalt carbonate would leach a lot.
Also, I also know manganese can be put in a glaze safely if it is done
correctly. The problem with manganese is in breathing the fumes during
firing--not necessarily in the fired product.
For other items on the list, I have little or no data, so I will refrain
from expressing an opinion at this time. I am very interested in this
list, however, and would repeat my plea to send me the original reference
for it if you can find it. I am trying to locate all of the original
references I can find regarding this general subject area of food safe
glazes. Thanks.
John Hesselberth
sibylle wrote:
>some weeks ago, someone gave us the below list of oxides in glazes, I don't
>remember who it was but am very thankful to have it.
>
>Limits for foodsafe glazes:
>
>No lead,barium, cadmium, mangan, ironcromate, nickel or vanadium .
>
>0.1% chromoxid (Hamer: max. colour at1%; 0.1% for pink)
>1.6% cobaltcarbonate (exeption: recipe for black glaze contains
> 5,5% cobaltcarbonate, used as a light decoration at the surface)
>1.0% cobaltoxid (Hamer: max. colour at 1%)
>5.5% coppercarbonate
>3.0% copperoxid (Hamer: max. colour at 5%)
>10.0% ironoxid (Hamer: max. colour at 12%)
>2.0% Lithiumcarbonat
>5.5% Rutil
>12.6% Strontiumcarbonat
>7.0% Zinnoxid
>5.0% Titanoxid (Hamer: max. opak at 5%)
>8.0% Zinkoxid
>
>I translated it for me into german and now re-translated in english. Hope all
>names are exact. If not, please contact me.
>
>Sibylle
>(germany)
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures
are inevitably projections of the minds of their creators." Bernard
Leach, A Potter's Book.
Wade Blocker on thu 27 apr 00
----------
> From: sibylle
> To: CLAYART@LSV.UKY.EDU
> Subject: Re: FOODSAFE LEACHING LIMITS FOR OXIDES
> Date: Wednesday, April 26, 2000 10:52 AM
>
> ----------------------------Original message----------------------------
> Hi Veena,
>
> some weeks ago, someone gave us the below list of oxides in glazes, I
don't
> remember who it was but am very thankful to have it.
>
> Limits for foodsafe glazes:
>
> No lead,barium, cadmium, mangan, ironcromate, nickel or vanadium .
>
> 0.1% chromoxid (Hamer: max. colour at1%; 0.1% for pink)
> 1.6% cobaltcarbonate (exeption: recipe for black glaze contains
> 5,5% cobaltcarbonate, used as a light decoration at the
surface)
> 1.0% cobaltoxid (Hamer: max. colour at 1%)
> 5.5% coppercarbonate
> 3.0% copperoxid (Hamer: max. colour at 5%)
> 10.0% ironoxid (Hamer: max. colour at 12%)
> 2.0% Lithiumcarbonat
> 5.5% Rutil
> 12.6% Strontiumcarbonat
> 7.0% Zinnoxid
> 5.0% Titanoxid (Hamer: max. opak at 5%)
> 8.0% Zinkoxid
>
> I translated it for me into german and now re-translated in english. Hope
all
> names are exact. If not, please contact me.
>
> Sibylle
> (germany)
>
>
>
>
> Veena Raghavan schrieb:
>
> > ----------------------------Original
message----------------------------
> > To Ron Roy and all the Clayart glaze gurus, could you please help with
> > this.
> >
> > Maybe there is a list somewhere, which indicates the safe leaching
amounts
> > of the various oxides in fired glazes in connection with the food
safety
> > concern, but I have not found it.
> >
> > There is a great deal of concern and talk about food safety and the
> > leaching of oxides, and I for one would like to feel that the
functional
> > items I am selling are foodsafe. I realize that the ultimate goal is to
> > have the ware fired in your own kiln tested. However, when the
> > not-so-experienced look at the glaze leaching information, how do they
know
> > whether the amounts listed are above or below the foodsafety
guidelines? Is
> > there any list and explanation of this and, if so, would someone be
kind
> > enough to tell me where I could find it. I am referring to cobalt,
copper,
> > lithium, and any other metals or minerals that might be considered
> > hazardous if they leach beyond a certain amount.
> >
> > Thank you in advance for any help you can give me.
> >
> > Veena
> >
> > Veena Raghavan
> > 75124.2520@compuserve.com
Sibylle,
Thank you for your posting. Zinnoxid is Tin oxide, Zinkoxide is zinc oxide,
the other translations are obvious. Mia in ABQ
WHew536674@cs.com on thu 27 apr 00
Sybille,
Thanks for posting the limits for foodsafe glazes. I have another question:
If a slip, under a glaze, contains more than the limit, does the same apply?
I feel like that is a dumb question, but since clayarters say no question is
a dumb one, I'll bare my ignorance and ask.
Joyce A.
Whew536674@cs.com
sibylle on fri 28 apr 00
Hello John,
I'm very sorry, I can't give you the original reference and I know nothing about
the research.
This list was given by clayart some weeks ago, as I remember from a woman. She
asked if someone knows, wether these limits are o.k. She herself found it
somewhere in the net. I kept it for some weeks, waiting for an answer of someone
who knows more about this limits than me. As there raised no objections I
accepted them for me, although I thought 5,5% copper would be a little bit
difficult.
Perhaps the women who gave this list could write again and give you further
information you need.
For I gave this list also to other potters here in town, please give me any
information you can get (or even express your opinion, surely it is based on muc
more knowledge than mine).
Concerning your info about mangan, what it the correctly way to use mangan in a
glaze? And can I colour my clay by kneading (or wedging, if you like) with manga
without danger, working with a mask or can it poison me through the skin?
There are laws about food-safety for glazes in germany. It is forbidden to use
lead, cadmium and more than 25 % barium. Concerning cobalt I think there are no
fixed limits but you are supposed to mark it.
Sibylle
from (south)germany, where spring has changed to summer suddenly
sibylle on fri 28 apr 00
Hello Joyce,
concerning this list for foodsafe glazes meanwhile (because of John
Hesselberths writing) I have my doubts and await some discussion about this
limits and think so should you.
And for your question, I think food safety it such an important theme that it
is better asking one question to much than one less.
I'm not sure but if your glaze is a really stable one, there should be no risk
with slip containing more than this limits. But in fact I have not enough
experience and knowledge to answer this exactly. Perhaps someone of the glaze
gurus could answer this, please.
Sibylle
(germany)
----------------------------Original message----------------------------
Sybille,
Thanks for posting the limits for foodsafe glazes. I have another question:
If a slip, under a glaze, contains more than the limit, does the same apply?
I feel like that is a dumb question, but since clayarters say no question is
a dumb one, I'll bare my ignorance and ask.
Joyce A.
Whew536674@cs.com
John Hesselberth on sat 29 apr 00
>Thanks for posting the limits for foodsafe glazes. I have another question:
>If a slip, under a glaze, contains more than the limit, does the same apply?
>I feel like that is a dumb question, but since clayarters say no question is
>a dumb one, I'll bare my ignorance and ask.
>Joyce A.
>Whew536674@cs.com
Hi Joyce,
I'll try to answer your question, but it will take a while so please read
patiently. My primary response, for now, would have to be that these
limits should not be used in the first place. I have searched the
literature extensively for work that might have been done in this area
and have found very little--certainly nothing that would support a list
of limits of this type. I must have been off line when it was originally
published or I would have questioned it then.
To very briefly summarize what I have learned from the literature or
found from my own research.
1. There is currently no way to predict ahead of time whether a glaze
will be stable or not; although it is more likely to be stable if you
keep silica above 3.0 (for a cone 6 glaze) and have a mixture of 3 or
more fluxes. If you don't understand glaze chemistry well enough to know
what the preceding sentence means, I will certainly be willing to try to
help you. A number of other Clayarters will also. Testing, which can be
done inexpensively at Alfred Analytical, is presently the only way to be
certain whether or not a glaze is stable.
2. Copper is the most difficult colorant to keep in a glaze. Some
glazes will hold 2-3% pretty well. A rare couple of glazes will hold
4-5%. But some otherwise attractive glazes will not even hold 2%.
Again, you must test if you want to know. Instructions for testing can
be found on my web site at
http://www.frogpondpottery.com/glazetest.html
There are also a number of glaze recipes, that I or others have tested,
posted on my site. To find them go from my home page (URL below) to the
section on glaze stability.
3. There has been a list posted recently of relatively harmless glaze
materials, e.g. feldspars, most clays, silica, whiting, etc. I don't
have it in front of me at the moment, but I would concur that, if you use
only those materials, you don't have to worry about harming anyone. You
will not necessarily have a stable glaze that will be durable in use--it
may still disintegrate in the dishwasher for example--but you won't be
poisoning anyone.
4. The best approach for food surfaces, in my view, is to make stable
glazes containing modest amounts of opacifiers and colorants like iron
oxide, zircopax, tin oxide, rutile, copper carbonate/oxide, cobalt
carbonate/oxide, even manganese oxide and the HAVE THEM TESTED to be sure
they don't leach significant amounts of those materials. At the very
least do a 3 day vinegar soak and look for changes in either surface
sheen or color. I personally stay away from nickel, chrome, vanadium and
cadmium as colorants; although even those can be used safely if you know
what you are doing and TEST. I also stay away from barium and lead as a
fluxes but, again, they can be used safely if you know what you are doing
and TEST. I have seen so much reluctance on the part of many of us
potters to test, though, that I think most of us should steer clear of
these questionable materials on food surfaces.
To try to get at your specific question though. A top coating of a
stable glaze can help "seal" an unstable slip or underglaze. But this
cannot be predicted. You have to test to be sure. I have made an
unstable copper-containing glaze much more stable by coating it with a
thin coat of a clear stable glaze.
And my last plea: if you do test please share the results with me and
Clayart. I am keeping track of all the glazes that have been tested that
I can find. Where the people who have paid for the testing have given
permission, the results and the recipes are posted on my site.
The list is slowly growing: Thanks to Rick Shanks and Anne Worner for
contributing the latest additions. Rick's glaze (a cone 5 peach glaze)
is the first glaze containing Cerdec cadmium stains that I have seen
tested (and it passed with flying colors). Anne tested a deep blue cone
10 glaze that she developed herself--it is also very stable.
Sorry for the long and somewhat wandering nature of this post, but one
question begs another in this very complex and frustrating area. I know
what we all want are simple answers. Unfortunately they don't yet exist.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures
are inevitably projections of the minds of their creators." Bernard
Leach, A Potter's Book.
John Hesselberth on sat 29 apr 00
sibylle wrote:
>Concerning your info about mangan, what it the correctly way to use mangan
>in a
>glaze? And can I colour my clay by kneading (or wedging, if you like) with
>manga
>without danger, working with a mask or can it poison me through the skin?
Hi Sibylle,
The primary danger from use of manganese is in breathing the fumes that
are released during firing. It is exposure to these fumes that has
resulted in a few potters having nervous system problems that very much
resemble Parkinson's disease. You should always vent your kiln very
well, but it is even more important if manganese is present in your
glazes. I would imagine it would be particularly dangerous for raku
potters who are much more exposed to the flames and smoke than in other
types of firings.
We actually consume quite a bit a manganese in our everyday diet so it is
not among the most toxic of materials to ingest. Nonetheless, we should
be careful in handling it. Certainly a good dust mask (preferably HEPA
type) should be worn when mixing glazes containing manganese dioxide. I
am not aware of any problems being reported as a result of wedging clay
containing granular manganese. Certainly of lot of potters use that type
of clay. If anyone else has information of that type hopefully they will
post it.
For some more information on this subject also see my response to Joyce.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures
are inevitably projections of the minds of their creators." Bernard
Leach, A Potter's Book.
Ron Roy on sun 30 apr 00
I have a different opinion about this particular subject.
Remember - even a small amount of oxide in an unstable glaze can
leach excessive amounts of everything into foods under certain
conditions.
This kind of list should only be used if it has been determined (by
testing) that the glaze is reasonably stable to start with.
I I think 5.5 Copper Carb. (as John Hesselberth has noted) would be
too much in most if not all the glazes we use.
There are certain oxides I think are benign - perhaps rutile,
manganese in glaze (but dangerous for the glazer), Strontium does
not appear to be toxic but keep in mind there is a small amount of
Barium present in most grades, iron oxide is everywhere so it can't
be a serious threat, Titanium helps to make glazes more durable as
does Tin and Zirconium oxide.
I could go on and generalize more but my basic point is - it depends
on the basic durability of each individual glaze and the potential
toxicity of the oxides we use - and individual susceptibility to
various oxides.
My advice is - if you don't want to have your glazes tested - use an
uncoloured glaze as a liner which is made from non toxic oxides.
RR
>----------------------------Original message----------------------------
>Hi Veena,
>
>some weeks ago, someone gave us the below list of oxides in glazes, I don't
>remember who it was but am very thankful to have it.
>
>Limits for foodsafe glazes:
>
>No lead,barium, cadmium, mangan, ironcromate, nickel or vanadium .
>
>0.1% chromoxid (Hamer: max. colour at1%; 0.1% for pink)
>1.6% cobaltcarbonate (exeption: recipe for black glaze contains
> 5,5% cobaltcarbonate, used as a light decoration at the surface)
>1.0% cobaltoxid (Hamer: max. colour at 1%)
>5.5% coppercarbonate
>3.0% copperoxid (Hamer: max. colour at 5%)
>10.0% ironoxid (Hamer: max. colour at 12%)
>2.0% Lithiumcarbonat
>5.5% Rutil
>12.6% Strontiumcarbonat
>7.0% Zinnoxid
>5.0% Titanoxid (Hamer: max. opak at 5%)
>8.0% Zinkoxid
>
>I translated it for me into german and now re-translated in english. Hope all
>names are exact. If not, please contact me.
>
>Sibylle
>(germany)
>
>
>
>
>Veena Raghavan schrieb:
>
> > ----------------------------Original message----------------------------
> > To Ron Roy and all the Clayart glaze gurus, could you please help with
> > this.
> >
> > Maybe there is a list somewhere, which indicates the safe leaching amounts
> > of the various oxides in fired glazes in connection with the food safety
> > concern, but I have not found it.
> >
> > There is a great deal of concern and talk about food safety and the
> > leaching of oxides, and I for one would like to feel that the functional
> > items I am selling are foodsafe. I realize that the ultimate goal is to
> > have the ware fired in your own kiln tested. However, when the
> > not-so-experienced look at the glaze leaching information, how do they know
> > whether the amounts listed are above or below the foodsafety guidelines? Is
> > there any list and explanation of this and, if so, would someone be kind
> > enough to tell me where I could find it. I am referring to cobalt, copper,
> > lithium, and any other metals or minerals that might be considered
> > hazardous if they leach beyond a certain amount.
> >
> > Thank you in advance for any help you can give me.
> >
> > Veena
> >
> > Veena Raghavan
> > 75124.2520@compuserve.com
Ron Roy
93 Pegasus Trail,
Scarborough, Ontario
Canada. M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Ron Roy on sun 30 apr 00
Hi Sibylle,
The danger from Manganese is in breathing the dust and kiln fumes as
I understand it.
I would not work with clay or glazes that have powdered manganese in
them. If I was forced to I would adopt some very stringent methods to
avoid any dust accumulation in the studio and proper kiln room
ventilation.
RR
>Concerning your info about mangan, what it the correctly way to use
>mangan in a
>glaze? And can I colour my clay by kneading (or wedging, if you
>like) with manga
>without danger, working with a mask or can it poison me through the skin?
>
>There are laws about food-safety for glazes in germany. It is forbidden to use
>lead, cadmium and more than 25 % barium. Concerning cobalt I think
>there are no
>fixed limits but you are supposed to mark it.
>
>Sibylle
>from (south)germany, where spring has changed to summer suddenly
Ron Roy
93 Pegasus Trail,
Scarborough, Ontario
Canada. M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
| |
|
