John Hesselberth on wed 2 dec 98
I promised to begin publishing some results of my glaze leaching studies
when I had enough information that made sense. What follows are my
results on a very nice glossy blue Cone 6 Glaze.
The recipe is the same as published in Chappell as glaze SG-28 except the
amounts of the colorants have been increased. I may have also seen this
glaze on Clayart a few months ago called Chappell's Blue. I have named
it (or renamed it as the case may be) Broken Blue because the blue tends
to break up into little globs or patterned areas of blue. In between the
blue patches/areas the glaze is nearly transparent and the clay color
shows through. It also accentuates texture nicely, because the high
spots are transparent and the blue migrates to the low spots. I use it
on a light brown clay (Standard Ceramic 306) with excellent results. For
a glossy blue (of which I am personally not terribly fond) it is one of
the nicer ones I have seen.
Even though it contains a LOT of the dreaded, but seductive, Gerstley
Borate, I have found it to be well behaved. It seems to tolerate firing
to Cones 5-6.5 with little or no difference in appearance; although I
have only tested leaching at Cone 6. Also for those who like simple
glaze recipes, it doesn't get much simpler than this. The only question
in my mind is whether or not the Gerstley Borate will come back to bite
those who use it.
The recipe:
50.0 Gerstley Borate
15.0 EPK
35.0 325 mesh Flint
Add:
1.5 Cobalt Carbonate
3.0 Copper Carbonate
1.0 Dark Rutile
This results in a unity formula of:
0.004 K2O
0.176 Na2O
0.815 CaO
0.004 MgO
0.231 Al2O3
0.874 B2O3
3.584 SiO2
Note that the above numbers are outside of most limit formulas on calcium
and boron and are on the low side for alumina and the high side for
silica. If boron were treated as a flux instead of a stabilizer, this
would change the numbers considerably. Also, with 0.815 Ca you might
expect this glaze to be a semigloss or even a semi mat; however it is
glossy--no other word for it. Apparently the high boron level is really
changing things vs. traditional thinking limit formula thinking. I have
not done the limit formula calculation with boron as a flux, but I bet
Ron Roy will since he is an advocate of including it there. Let us know
the results Ron.
Now for the good news on leaching. My test was done for me by the Alfred
Analytical Laboratory and sample preparation was by the method I have
previously recommended to the Clayart forum (see
http://www.frogpondpottery.com/glaze test.html). With the high level of
boron, I checked for it as well as for copper and cobalt. Firing was to
Cone 6 at 4 o'clock in an electric kiln. The results:
Boron: 0.196 mg/L (or ppm)
Copper: 0.41 mg/L
Cobalt: O.063 mg/L
These are some of the lowest numbers I have seen for any glaze tested so
far. With these numbers it kind of goes without saying, but this glaze
also passes the 24 hour vinegar soak and the 3-month-in-the-dishwasher
tests.
I am very interested in what this glaze will do in other people's hands
(with your source of Gerstley Borate, your glaze preparation, your kiln,
etc.). If you try it, please let me know the results. I would hope at
least a few Clayarters would also have it tested for leaching and let me
know the results. So far, my posts on this subject have produced mostly
major yawns from Clayart members. Only about 6 people seem to be
seriously concerned about or interested in the subject. Are there more
of you out there? If so, test this glaze (including spending $30 to have
it tested for copper and cobalt leaching) and let me know the results.
Happy glaze testing. John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"The whole aim of practical politics is to keep the populace alarmed, and
hence clamorous to be led to safety, by menacing it with an endless
series of hobgoblins, all of them imaginary." H.L. Mencken, 1925
John Hesselberth on thu 3 dec 98
In my post yesterday on this subject I somehow got an extra space in the
URL for directions for testing glazes for leaching. The correct address
is
http://www.frogpondpottery.com/glazetest.html
There should be no spaces in the above. If any have sneaked back in,
take them out.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"The whole aim of practical politics is to keep the populace alarmed, and
hence clamorous to be led to safety, by menacing it with an endless
series of hobgoblins, all of them imaginary." H.L. Mencken, 1925
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