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metals leaching from glazes--status report (long)

updated wed 14 oct 98


John Hesselberth on tue 13 oct 98

Note: This message is long and will not be of interest to many. However I =
a specific need and request at the end of the message. Before you delete =
please page down to the last several paragraphs and read those. If you can
help, I would appreciate it.

Many thanks to those of you who responded to my request for literature
references on metals leaching from glazes. Interestingly, I only got a =
specific literature citations=3B however I got several excellent suggestions=
where to look. My most successful attempt to date has been to ask the =
Ceramic Society to do a literature search for me. Their capability to do a
computer search goes back to 1976 and they gave me excellent service. Who
should know the ceramic literature better than they? The output provided a =
start for me to dig farther back. Part of the problem is that much of this =
was done a long time ago. Hermann Seger developed the unity formula, also
called the Seger Empirical Formula, in the later part of the 1800s (he died =
1894). A couple of the references that address glaze durability, and that I=
most interested in reading, are from the 1920s. A list of the references I
consider most interesting, at this point in time, is at the end of this =
Be aware, though, that I have not yet read all of them--some may not be

What I have extracted and assimilated right now is mostly in the category of
=B3bits of information=B2. I don=B9t have enough yet to try to synthesize =
it into
knowledge. In most any research program I have ever been involved with, one
goes through a =B3divergent=B2 stage where you collect any and all possibly =
information you can gather and seek ideas and hypotheses from every possible
source. You then go into a convergent stage where you begin to sort out, =
and narrow the focus of your work. I am clearly in the divergent stage on =
one so, please, send me any other pieces of information that you think might
help me.

Some preliminary bits of information that I find potentially interesting and

How much metal, of the various metals we use, can leach from a glaze and =
have it be =B3food safe=B2? Answer: No one knows. Of course, in most =
lead and cadmium are regulated and we must follow the laws with respect to
those. However, those are the only two metals addressed by regulation and =
seems to be a complete void of information on other metals. Monona Rossol
challenges us to use allowable drinking water levels as a conservative, safe
side level, but she also notes that the allowable leaching levels for lead =
cadmium are many times what is allowed in water. Indeed, allowable leaching
concentrations for lead from ceramic glazes are 6-33 times higher than the
concentration allowed in drinking water. Could a similar factor be applied =
copper, cobalt and other metals? No one knows. Another bit of information:
Copper concentrations of 10 ppm can give fruit juice an unpalatable, bitter
taste. Levels of 2-20 ppm are regularly seen from copper-colored lead-free
green glazes. Do we want our pitchers to give orange juice a bitter taste? =
think not. This question of how much is safe will not be answered =
any time soon. Each potter will have to determine for her/himself what =
they are willing to accept. Some criteria, other than human safety, can be =
to set upper limits (dishwasher durability, no affect on taste, 24 hour =
soak without noticeable loss of color, etc.) Clearly less leaching is =
however that will have to be balanced against aesthetic considerations and =
people will buy. Fewer burgers and fries would be better for our health =
but most people don=B9t care=3B life is full of choices. Since I am not =
to work in this area I plan to leave it to others while I focus on how we =
minimize the amounts of metals that leach with minimal impact on the =
of the glazes we develop and use.

So how is the leaching of metals related to glaze durability as defined by =
formulas? Answer: I don=B9t know. It may or may not be known by anyone. =
is the most studied metal and, of course, that is of little interest to most=
us since we don=B9t use lead. If there is a large body of data on metals =
than lead, I haven=B9t found it yet. It may be in some of the references =
below that I haven=B9t been able to put my hands on yet. Some miscellaneous=
The books that list limit formulas in one form or another give no hint where
those limit formulas came from or what criteria were used to set them. One =
the opinion they just appeared out of the mist=3B however I am sure there is=
definitive work behind them (probably focusing on lead) if I can find it. =
was probably done in the 1920s. Or it may be locked up in corporate =
records that haven=B9t ever been made public. I do get the opinion that =
some of
it may have been compiled over the years by simply looking at =B3good =
glasses=B2 vs
=B3bad glasses=B2. Some, like the specific formulas (e.g. Cone 10 Temmoku =
formula) Richard Burkett has built into HyperGlaze, have come from his =
of the known glaze formulas for that type of glaze. More on all of this =
after I
get and read some of those articles from the 1920s.

What next? As I=B9ve already mentioned, I=B9ll be digging deeper into the
literature. Several articles are on their way to me from the American =
Society Information Center.

I do have one very specific request, however, where many ClayArt members can
help. I would like to begin accumulating a data base of glaze recipes where=
standard acid leaching test has been done on the resulting glaze. Do you =
any data? Are you willing to share it with me and help? I have had several
glazes tested myself and more are on their way to the Alfred Analytical
Laboratory. I will include all of them. What I would need from you is the
recipe (I will do the unity calculations myself so they are all done one way
using one set of raw material analyses--I use HyperGlaze), description of =
resulting glaze, the firing conditions and the lab results. Also the =
results of
any tests you have done like the 24 hour vinegar soak or 1 month in the back=
your dishwasher. If you label the recipe as confidential I will keep it =
way and only use it to extract relevant information that affects leaching. =
=B3public domain=B2 recipes, though, I would eventually want to make results
available to this group and, perhaps, the potter world at =
where more than one test has been done on a given glaze (as much as we have
discussed Floating Blue surely a few people have tested it). I know that an
uncontrolled data set like this can=B9t be used to draw firm conclusions, =
but it
should be useful in getting some idea of where we are and in guiding future

If you have, and are willing to share, such data (I hope you will inundate =
please send it to me at:


John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366-0088

By the way, if any of you are concerned over my qualifications to pursue
research in this area I would be happy to send you my C.V. from my last =
(chemical engineer by formal training=3B 30 years in industrial R=26D).

As long as I sense interest from ClayArt members I will continue to update =
on the progress of this work periodically. I have become very interested in=
myself and will pursue for my own interest in any case. Thanks in advance =
any data you are willing to share with me and help you can give me.


Bibliography (Preliminary)

1. J. R. Taylor and A. C. Bull, Ceramics Glaze Technology (Oxford, Pergamon
Press, 1986)

2. Turner, W. E. S. and Dimbleby, V., J.Soc. of Glass Tech., 10, 304-358 =

3. A. Paul, Chemistry of Glasses (London, Chapman and Hall, 1982)

4. Peddle, C., J. Soc. Glass Tech., 5, 195-268 (1921)

5. Turner, W. E. S. and Winks, F., J.Soc. of Glass Tech., 9, 389-405 (1925)

6. Koenig, J. H. and Earhart, W. H., Literature Abstracts of Ceramic Glazes
(Philadelphia, College Offset Press, 1951)

7. Eppler, Richard A., =B3Formulation and Processing of Ceramic Glazes for =
Lead Release,=B2 Ind. Ceramique, 706, 362-72, 1977

8. Camera, B., and Schaeffer, H. A., =B3The Study of Chrome, Cobalt and =
Iron As
Colouring Elements in Silicate Glasses, Glaze Frits and Ceramic Stains -- =
2: Stain/Frit Reactions and the Leaching Behavior of Various Glazes,=B2 =
Forum International, 59=5BN-2=5D, 88-93, 1982.

John Hesselberth
Frog Pond Pottery
Pocopson, PA 19366 USA
EMail: web site:

=22The whole aim of practical politics is to keep the populace alarmed, and =
clamorous to be led to safety, by menacing it with an endless series of
hobgoblins, all of them imaginary.=22 H.L. Mencken, 1925