John Hesselberth on sun 30 apr 00
A few weeks ago, there was a post to Clayart from Clayart member Jennifer
Assinck who was trying to develop a set of limits that could be used for
"food surface" glazes. I missed that message, and I suspect several
other glaze oriented people did also, because I was set to NOMAIL while I
was at NCECA. At the time there was only 1 response that I could find in
the archives and it was of a fairly general nature.
A few days ago, Clayart member Sibylle reposted part of the message and I
could see that several members were beginning to pick up and use this set
of limits because they very much wanted to have such guidance. While
this effort was well-intended by Jennifer and she did quite a bit of
reading to develop the list, I must take a strong position that THIS LIST
OF LIMITS SHOULD NOT BE USED. I'm not shouting here; just emphasizing to
be sure the preceeding statement catches your eye. If you read nothing
else in this message, I want to be sure that the above statement is what
you take away. However (and this is what will make this message very
long) I want go on to explain some of what I know about this subject and
offer an alternative set of guidelines toward the end of the message. I
guess what I am really trying to do is bring together some bits and
pieces from a lot of Clayart posts in one comprehensive message in the
hopes you will save it and use it as a reference until better information
is developed.
I have included most of the original message because I feel it is very
well written and contains some issues that many of us struggle with and
are worth addressing. I have interspersed some comments or suggestions
periodically.
>I am hoping Clayart members can assist me in determining suitable upper
>limits
>for oxides in food surface glazes.
>
>I belong to a small guild of mostly hobby potters, and, until recently,
>most
>members used the studio facilities and the guild glazes that we have had
>analyze
>for safety.
>
>Some members have started to develop their own glazes at home, using
>recipes fro
>uncertain sources. The guild is not comfortable selling work glazed with
>unknow
>glazes on food surfaces. We do ask them to test their pots with vinegar
>for
>colour loss, and with rapid temperature changes for glaze fit. We are
>also
>considering asking them to confirm that the (food surface) glaze formula
>is
>balanced according to computer analysis.
Jennifer, the situation you describe here, I suspect, happens in
hundreds--maybe thousands--of places around the world. Developing your
own glazes is fun, the materials are readily availble and hope springs
eternal that we will be able to develop the most beautiful glaze the
world has ever seen. That is exactly why I started developing my own
glazes many years ago.
In a regulatory, or legal, sense there is little wrong with this. In
most countries only lead and cadmium are regulated when it comes to using
them in glazes and, if you stay away from using them, you can legally
sell any glaze as potters have done for thousands of years. If you use
lead or cadmium there are standards in most countryies that must be
adhered to. Many of us on Clayart and elsewhere, though, are working to
a self-imposed higher standard. We believe that, as a matter of good
craftsmanship and/or assuring the safety of our glazes to the maximum
extent possible, we should make our glazes very stable so that they leach
minimal amounts of the glaze materials into food or drink and are durable
in use.
The rest of my comments are addressed to those of us who hold ourselves
to that higher standard. If you are a person who thinks "don't worry
about it--no one has ever been harmed by lead and cadmium-free glazes and
a little extra copper or cobalt or manganese is just a nice dietary
supplement" then the rest of this message is not for you. I personally
think that is an irresponsible attitude when it comes to glazes for use
on food surfaces, but I understand some people continue to hold it. If
you do, then you may as well hit the delete key now. The remainder of
this message is for the rest of us like Jennifer and Sibylle and me and
hundreds of other Clayart members (more every day judging by my mail) who
are concerned about the stability of glazes used on food surfaces.
>
>I should also mention here, that functional pots must be fired to cone 6
>or
>higher, so there is little risk of toxins leaching through a porous clay
>body
>from an outside glaze.
>
>Although I realize that a balanced glaze can hold on to more oxide than
>an
>unbalanced glaze, I would like to establish upper limits for particular
>oxides i
>food surface glazes. Using a percent limit in the formula would be a
>clearer
>guide for our members than ppm in a leachate test, because that would
>serve as a
>beginning for choosing glazes. Even if a member were to get a mug tested
>for
>leaching (not likely for the hobbyist who is constantly experimenting),
>subsequent work may not be subject to the same firing conditions,
>resulting in a
>different degree of leaching.
Your concern here is real. While I am a strong proponent of testing, I
have come to realize that only a small percentage of people who are
formulating their own glazes are really going to test them. Assumming
you are concerned about what leaches from your glazes or those of your
students or peers (and I tried to dismiss those who aren't concerned in a
paragraph above) there are several approaches that could be taken.
1) You could try to develop a comprehensive set of guidelines, as you
tried to do below, that would establish a safety "fence" within which
people would do no harm. As I will explain below, this is not feasible
given today's knowledge. Maybe in a few years, we will be able to do
this, but not yet.
2) You could limit the materials list to those for which the toxicity
concerns are minimal and be relatively certain no one is going to do
damage to other people's health. A list of materials like this has been
presented on Clayart several times over the years, but to make this memo
a comprehensive summary I have included my own version of it below.
Doing just this, however, does not assure that the glazes developed will
be durable in use. Even though they may look good, they may come appart
in the dishwasher or the surface may be etched by acidic foods. Of
course it they do come apart in these situations you may still be causing
your customers to consume glaze ingredients but they will be pretty
innocuous materials (like clay, sand, fertilizer, and rust).
3) You could use the concept of a "liner" glaze for food surface glazes.
For example, you could limit the materials list as in 2 above, combine it
with instructions to "stay within the limits for a balanced glaze" and
require some easy-to-do home tests. This, of course, assumes a couple
more things. First there has to be agreement on what the limits for a
balanced glaze are--various authors have published various sets of
limits. There is enough consensus on this, however, that I will also
give a set of limits below that I believe will assure a relatively
durable-in-use glaze. The bigger problem with this approach is that the
person developing the glaze must be able to do unity calculations either
manually or using one of the computer programs that are readily availble.
The easy-to-do home tests are 1) a 3 day vinegar soak test--if the glaze
changes noticeably in either color or surface sheen it is not durable for
functional ware, 2) the 30 cycle dishwasher test--again if color or
surface sheen change the glaze is not durable, 3) the knife marking
test--if you can leave visible marks on a glaze surface with a table
knife the surface is too abrasive or mat for good performance and 4) the
freezer/boiling water test. This last test involves several cycles of
putting a pot in your freezer for several hours and then removing it and
immediately filling it with boiling water. You then examine the glaze
under a low power microscope for signs of crazing. It is a test that
checks for clay/glaze "fit."
For people unable or unwilling to have their glazes professionally tested
for leaching, for whatever reason (lack of money, lack of access to a
testing lab, too much time/trouble or whatever), this is the option I
recommend. I should point out that I believe this is the method that
Craig Martell and Ron Roy, two of our most respected glaze gurus on
Clayart, use for their own work. Perhaps others do also. I believe that
being able to do unity calculations, or obtaining a computer program to
do them for you, is the "price of admission" for responsible glaze
formulation of glazes to be used on food surfaces. Also later in this
memo I have listed some of the glaze calculation programs available for
both PC and Mac machines and web site references to them. Overall, then,
this is the most conservative approach, but it is one that limits you
color palette considerably to whites and tans and browns. You can,
however, make a good range of glaze surfaces from semi-mats to glossies
and maybe even something approaching a mat.
4) The fourth approach is to not limit the materials list, but add to the
above easy-to-do home tests the requirement for formal leach testing.
While this approach clearly gives the best indication of glaze stability,
it is not without complicating factors. The largest complication is what
standards or "limits" do I use for the leaching test results. As we have
discussed fairly recently on Clayart there are various views on that.
The most conservative proposal is that offered by Monona Rossol. She
proposes that, lacking definitive information, we would be in the most
defensible position if we used the maximum contamination limits for
water. This is a very strict, difficult to achieve, standard; however it
is achieveable with some materials and some colors outside of the brown
family would be available. A problem with this approach is that there
are no water limits for several of the materials we use, e.g. cobalt,
lithium and manganese (although there is a secondary standard for
manganese). For boron there is a "health advisory" guideline of 0.6 mg/l
(please forgive my use of only U.S. standards here--I do not know the
standards country by country). For barium, cadmium, chromium, copper,
lead and nickel there are definitive maximum contamination levels for
water in the U.S. of 2.0, 0.005, 0.1, 1.3, 0.015 and 0.1 mg/l
respectively. There are, of course, lead and cadmium standards set by
the U.S. Food and Drug Administration and California which are from 7-300
times less strict than the above maximum contamination levels for water
depending on the type of vessel.
My own approach is to use formal leach testing to help me make glazes
that are way toward the stable end of the spectrum, but I don't
personally feel bound by the water standards where they exist. In many
cases, however, my glazes do meet the maximum contaminations levels for
water; my use of copper is the main exception where I have decided to aim
for an interim goal of 10 mg/l. I say interim goal, because I will
continue to try to learn to how to make my copper-colored glazes even
more stable in my ongoing research.
So in the end, if you use this fourth approach, you do have to make your
own decision on what goals you will accept for yourself. But it does
provide the most quantitative information on how your glazes will perform
on exposure to foods (95% of which are acidic). I would also add that if
any material you test for leaches more than, say, 10 mg/l, you have a
glaze which could be made more stable without limiting your color palette
in any meaningful way. If your glazes leach more than 30 mg/l of any
material you have a very unstable glaze which is probably not suitable
for functional use (food surface of not). Today there are a number of
copper-containing glazes in use which will leach in the range of 10-50
mg/l.; I don't think those glazes should be used on food surfaces.
>
>While I anticipate that Clayart members may respond that one should
>always test
>one's glazes for leachates, please keep in mind that I am discussing
>hobbyists
>who are not likely to do this. This is my attempt to put some control on
>the
>situation, and give us some means of making a decision when we are
>presented wit
>a pot with an unknown glaze on the day before the sale. We will also be
>spendin
>time educating our more adventurous members, and encouraging all members
>to
>consider using white or clear liners more often.
>
>>From my reading of the Clayart archives, I have come up with the
>following list
>of oxides and recommended upper limits. I would appreciate feedback from
>Clayar
>members on these plans in general, and on the following glaze limits.
>
>The Potter's Dictionary (p. 235), by Hamer, gives upper limits for
>producing ful
>strength colour without becoming matt, but these may be more than one
>would want
>in a food-surface glaze.
>
>I chose some of the following limits based on the amounts we have in our
>balance
>guild glazes. For those materials we don't use, I referred to Hamer.
>
>UPPER LIMITS FOR FOOD-SURFACE GLAZES
>
>No lead, barium, cadmium, manganese, iron chromate, nickel or vanadium.
>
>0.1% Chromium oxide (Hamer: max colour at 1%; 0.1% for pinks)
>1.6% Cobalt carbonate (Exception: Guild recipe for black glaze
>containing 5.5%
>cobalt carb, used as light decoration on food surface)
>1.0% Cobalt oxide (Hamer: max colour at 1%)
>5.5% Copper carbonate
>3.0% Copper oxide (Hamer: max colour at 5%)
>10.0% Iron oxide (Hamer: max colour at 12)
>2.0% Lithium carbonate
>5.5% Rutile
>12.6% Strontium carbonate
>7.0% Tin oxide
>5.0% Titanium dioxide (Hamer: max opacity at 5%)
>8.0% Zinc oxide
The list above that you propose is a problem in that there is presently
no way to tell how much colorant or opacifier (or in the case of lithium
carbonate, strontium carbonate and zinc oxide how much flux) a given
glaze will hold. As you note, most of this information is from Hamer and
Hamer and was derived to define the point of maximum color, i.e. the
point at which further additions of color will not have a visible effect.
That, unfortunately, can be quite different from the amount that leaches
from a glaze on exposure to acids. Even staying within the limits of a
balanced glaze will not assure leach free performance of colorants used
at the above levels; although it will increase the odds of the glaze
being stable. I have seen copper containing glazes that are within
limits of a balanced glaze and still leach 20-25 mg/l of copper. I do
not have any data on chromium nor have I seen any in the literature;
however your suggestion to forbid use of iron chromate, but allow 0.1%
chromium oxide is, in itself, inconsistent. More that likely the final
composition in the glaze will be chromium oxide regardless of which of
the two starting materials is used.
The other place where there are virtually no data is in the combination
of materials. There is simply no understanding of possible interactions
between, say, chromium, cobalt, copper and iron and what effect that may
have on leaching.
And, of course, if these levels of colorants were put in an unbalanced
glaze the odds are much higher that the leaching will be very high
(although I have seen some unbalanced glazes which give very good
leaching performance--the limits for balanced glazes are not perfect nor
fully understood).
So bottom line: I cannot recommend use of this set of guidelines. What
I do recommend is following approach #3 or #4 above.
Material List for Approach #3
For approach #3 the materials list would be (I believe this list is
documented on Tony Hansen's www.digitalfire.com site)
kaolin
ball clay
flint or silica
gerstley borate
sodium or potassium feldspars
whiting
wollastonite
nepheline syenite
talc
dolomite
bentonite
Frits 3124, 3134, 3110, 3195 (or others than do not contain lead or
barium)
zirconium opacifiers (Zircopax, Superpax, etc.)
Titanium dioxide opacifier
Rutile
Iron Oxide
There are probably a couple other materials I have missed here, but this
list will give good access to calcium, sodium, potassium, magnesium,
boron, alumina and silica for the base glaze as well as zirconium and
titanium based opacifiers and iron as a colorant. Some people would add
strontium carbonate and zinc oxide to give more choice of fluxes.
The limits I would use for approach #3 would be:
for Cone 6 for Cone 10
(KNa)2O 0.1-0.3 0.1-0.3
CaO 0.2-0.6 0.3-0.7
MgO 0.0-0.3 0.0-0.4
Al2O3 0.25-0.4 0.3-0.7
B2O3 0.15-0.35 0.0-0.3
Silica 2.5-4.0 3.0-5.0
As I noted above, others will have slightly different views on the above
limits; however I doubt anyone would argue that this is a "bad" set.
This set is not a direct copy of any published set, but rather is an
amalgam of published sets and what I have learned from the leaching
studies I have done or seen. If anyone does think it is a "bad" set
please let me know.
Glaze Calculation Programs
I am aware of three commercial glaze calculation programs that are widely
available. In alphabetical order (so as to try not to indicate a ranking
or priority), by author's last names, they are:
HyperGlaze by Richard Burkette (availably only for the Macintosh). More
information can be found at: http://members.aol.com/hyperglaze/index.html
Matrix by Lawrence Ewing (available for both the Macintosh and PCs).
More information can be found at: http://www.Matrix2000.co.nz
Insite by Tony Hansen (currently available only for the PC unless you
choose to run a PC emulation program on your Macintosh--a Macintosh
native program is promised in the future). More information can be found
at: http://www.digitalfire.com
Additional information on less well known calculation packages can be
found on the Ceramics Web at:
http://art.sdsu.edu/ceramicsweb/glazesoftware.html
Glaze Testing Instructions
I am only aware of one lab that makes this service readily available at
modest cost. That laboratory is Alfred Analytical Laboratory in Alfred
Station, New York. Alfred Analytical is a privately owned business and is
not a part of Alfred University. The basic cost of having one glaze
tested is $10US for the leaching and an additional $10US for each
material tested for (testing a glaze for cobalt and copper would cost
$30). While I work closely with Roland Hale at Alfred Analytical I have
no financial interest in recommending them. They are simply the only
place I know to get this kind of work done professionally and
economically. If anyone is aware of other such labs, I would be glad to
mention them whenever I discuss this subject.
For complete instructions on how to have your glazes tested, visit my web
site at:
http://www.frogpondpottery.com/glazetest.html
So, you still ask, is there some variant of Jennifer's list above that
would make available some colors other than whites, tans and browns while
having assurance that the glazes would be reasonably stable. I'm going
to be practical here and recognize that there are some people, who are
concerned about the stability of their glazes, but who are simply not
willing to limit themselves to the color palette of Approach #3 or are
not willing or able to take Approach #4. To them I will outline an
approach #5 WHICH I DO NOT RECOMMEND but, on the other hand, would rather
have them use than to do nothing at all or use approaches #1 or 2.
NOT RECOMMENDED Approach #5 (but here it is anyway)
Use the materials list for approach #3, stay within the limits for
approach #3, do the simple-to-do home tests for approach #3 and add up to
1% cobalt carbonate and 2% copper carbonate (or the molar equivalents of
cobalt and copper oxides) to the materials list. This will add light
greens and medium blues to the available color palette.
So, Jennifer, Sibylle and others, I am finally reaching the end of what I
think needs to be said here. I am certainly near the end of any
definitive knowledge I have on the subject.
If I sense any interest in having the above written in an easier-to-use
format that can be used in the classroom or to pass out to other potters
I will rewrite it into a several page document that can be downloaded and
read on Adobe Acrobat. Let me know if you want that--it will take a bit
of work but I am willing to do it if there is interest. That would also
give me the opportunity to include any thoughts that others contribute in
any followup discussion. Otherwise, please consider saving this document
and any followup additions from the other glaze-knowledgeable people on
the list.
Peace, John
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.
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