David Hendley on wed 22 sep 99
Thanks for this excellent information, John.
It certainly should not be boring to ANYONE making
functional pottery.
My suggestion is that this work is too important to
simply be presented as an e-mail, and should be
formally written and published.
This would serve two purposes.
First, it would ensure that the information is widely
available. It could then be quoted and used as a springboard
for possible further investigation.
Second, publication would earn payment which would
reimburse you for your testing expenses.
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/
----- Original Message -----
From: John Hesselberth
To:
Sent: Sunday, September 19, 1999 8:43 PM
Subject: Copper Leaching from Glazes (boring if you are not into the
technology)
| ----------------------------Original message----------------------------
| As many of you know I have been puzzled about why copper seems to be so
| difficult to keep in a glaze, at least at the levels for 4-5% where I
| have tested it. I have formed a working hypothesis about this and have
| set about testing it.
|
| Hypothesis: Colorant oxides are stable within a "good glass" base glaze
| up to a certain level depending on the exact structure of the glaze
| matrix and the characteristics of each particular oxide (i.e. if there
| are "holes" in the glaze matrix into which the colorant oxide will fit,
| it will be stable in the glaze until all the holes are filled--then, at
| higher levels, it will leach quite easily). Each colorant oxide will
| behave differently in a given base glaze depending on things like it
| molecular or ionic size, its electrical characteristics, and probably
| more.
|
| For an initial test of this hypothesis I took one of my standard base
| glazes and made sample cups with 5 different levels of copper: 0.5, 1.0,
| 2.0, 3.5 and 5.0% copper carbonate. I sent the samples to Alfred
| Analytical Lab and just got the results back.
|
| Copper Level, % Copper Leached, mg/L
|
| 0.5 0.07
| 1.0 0.17
| 2.0 0.14
| 3.5 4.73
| 5.0 14.37
|
| At 2 % or less copper, almost none leaches; these numbers are barely
| above the detectable limit of 0.05% and may not even be significantly
| different from each other. This particular glaze is very, very stable to
| copper leaching at low levels of copper in the base glaze. Somewhere
| above 2 % (straight line extrapolation would predict it is about 2.7%)
| significant amounts of copper begin to come out of this glaze. By the
| way, at 2% it gives a very attractive light-medium green color at cone 6
| oxidation.
|
| So, at least in a first test, I have added credibility to the hypothesis
| above. I won't be so bold as to claim the hypothesis is proven on the
| basis of a single test like this; however this does lead me to want to
| test more and gives me a hint that we may eventually be able to predict
| leaching propensity. Some of the questions that come to my mind are:
|
| 1. Will I get similar results for other colorant oxides? Do they all
| have a "limit of solubility" in a given base glaze? Are those limits
| dramatically different for the various colorant oxides?
|
| 2. We know that different base glazes behave substantially differently in
| their ability to hold colorant oxides. What are the important factors?
| Is being within traditional limits and making "good glass" one of them?
| Is there a simple way to predict which are stable?
|
| 3. Is there a ranking of the "limits of solubility" of colorant oxides
| in glazes, e.g. is iron always more "soluble" than cobalt which in turn
| is always more "soluble" than copper.
|
| 4. Would adding a second or third colorant oxide affect the above results
| on copper or do the oxides behave essentially independently of each
| other, each fitting into their own "holes" in the glaze matrix?
|
| For those who seem to want to read more into my leaching results than
| should be read, I must add the following caution: These results do not
| translate to your base glaze! Maybe someday a few years from now we will
| know how to translate them, but not yet. If I hear anyone suggest that
| it has now been proven that using copper at 2% or less gives a stable
| result, you will hear my scream no matter what part of the world you are
| in.
|
| What I am interested in from Clayart members are your thoughts, comments
| and suggestions on the above. I will continue to follow up on this work
| as I can afford it and have the time; however I would like my experiments
| to be as efficient as possible. I know there are some brilliant minds
| out there and I am wide open as to where to go next. Please let me hear
| from you if you have an interest in helping me figure out how to design
| the next experiment(s) or if you think you have a good mechanistic
| explanation of the above results or whatever.
|
|
|
| John Hesselberth
| Frog Pond Pottery
| P.O. Box 88
| Pocopson, PA 19366 USA
| EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
|
| "It is time for potters to claim their proper field. Pottery in its pure
| form relies neither on sculptural additions nor on pictorial decorations.
| but on the counterpoint of form, design, colour, texture and the quality
| of the material, all directed to a function." Michael Cardew in "Pioneer
| Pottery"
John Hesselberth on thu 23 sep 99
David Hendley wrote:
>My suggestion is that this work is too important to
>simply be presented as an e-mail, and should be
>formally written and published.
>This would serve two purposes.
>First, it would ensure that the information is widely
>available. It could then be quoted and used as a springboard
>for possible further investigation.
>Second, publication would earn payment which would
>reimburse you for your testing expenses.
Hi David,
Thanks for your comments. I do plan to try to publish when I have enough
to make a good article. Roland Hale and I have something cooking right
now in that area. I did apply to give a paper on this subject at NCECA
2000 but was rejected. Oh well, maybe next time.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
| |
|
