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results: ron roy black cone 6 glazes

updated sun 14 mar 99


John Hesselberth on sat 13 mar 99

Ron recently posted a recipe for a Cone 6 Black glaze which he felt would
be attractive and stable. After a couple people indicated they liked the
looks of this glaze, I decided to try it and have it tested for leaching.
The results, detailed below, are back and they are excellent.

Ron developed the recipe in 2 forms: First using Kona F-4 feldspar and
then, in an attempt to substitute potassium for some of the sodium with
G-200 and Custer feldspar. I tested the Kona and Custer Versions

Ron Roy Black #2

Kona F-4 Feldspar 21
Whiting 6
Talc 3
Frit 3134 26
EPK 17
Flint 27


Bentonite 2
Red Iron Oxide 9
Cobalt Carbonate 1

Ron Roy Black #3 (Custer)

Custer Feldspar 22
Whiting 4
Talc 5
Frit 3134 26
EPK 17
Flint 26


Bentonite 2
Red Iron Oxide 9
Cobalt Carbonate 1

I won't detail the unity formula numbers here; however you know that if
Ron developed it, it is WELL within limits.

Both were fired to Cone 6 at 3 o'clock. I considered both to have
identical appearance. They were a nice glossy black, but the black, to
my eyes, was just a little on the brown side of black.

Results are now back from Alfred. I tested for both iron and cobalt.
I'm not worried about iron from a safety standpoint, but I felt measuring
it quantitatively would give a more definitive result on durability in
use than the vinegar soak test. Also since I expected the cobalt numbers
to be very low, and perhaps too low to measure accurately, I thought it
might show difference between the two glazes more accurately. The Alfred
leaching results:

Co Fe
RR#2 0.076 mg/l 0.85 mg/l
RR#3 (Custer) 0.086 1.80

These are very, very low numbers. The only other glossy black glaze I
have tested had over 30 times the cobalt release, probably because it
also contained copper which was also very high. I would not hesitate to
use either of these as a food contact surface. The difference in results
between the two did not prove than K makes a more durable glaze than Na,
but then the difference was small and the total KNa was actually a little
higher, by my calculations, on #3 than it was on #2. It will take a lot
more data to sort that out.

I am going to continue to adjust this glaze to see if I can get a truer
black on my tan/light brown clay (Standard #306). I plan to increase the
cobalt a little and perhaps fiddle with the clay level. If I get it to
where I like it better, I'll test that and post the results. In the
meantime, let's all give Ron a great big THANK YOU for developing a very
acid resistant black glaze--done, by the way, purely from his
understanding of glazes and using glaze calculation as a tool.

One other note. I also confirmed, in a second test, that my technique of
overspraying my high copper green glaze with a stable, clear glaze works
to reduce copper leaching. I always feel better about a technique when I
can do it twice and get good results both times.

Happy glaze testing,

John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: web site:

"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