Tom Buck on thu 23 feb 06
Greetings, John:
Thanks for your reaction to posts by Ron & me.
Glaze calculation via computer is a bit mysterious to all too many studio
potters, so I seek to encourage potters generally to learn the technique,
as part of their general education, not as a direct substitution for doing
glaze tests.
You, John, have conducted more such tests than most potters, often
as an investigative tool, to prove the reliability of your raw materials,
rather than to design a new glaze as such.
You developed a fresh approach to the "oilspot" glaze and
published your findings. ?I wonder how many tests you did before you had a
workable glaze? Probably many many more than the number undertaken by
David Hewitt and Michael Bailey of UK, who published their results a year
or three before you did. And because Hewitt, a chemical specialist and
potter, is versed in glaze calculation he was able to make one clear
conclusion after a few tests, namely that Magnesium Oxide content is a
critical component of an oilspot glaze, a conclusion not previsously
reported in the literature.
As Ron & I have said previously, glzcalc is a "guide" and does not
magically provide a solution to the problem of a new glaze design. But it
sure cuts down drastically the effort to find a new glaze design when
working with standard materials of well-established chemical composition
(which most potters use).
Empirical testing comes into its own when we seek to use materials
of uncertain or unknown composition, a route I seldom follow.
be well. peace Tom
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
John Britt on thu 23 feb 06
Tom,
Thanks for the post.
Yes, I agree with you that glaze calculation should be a part of a general
ceramics education but never a substitute for glaze testing.
Glaze calculation cannot exist without empirical (glaze) testing while
glaze testing can exist and flourish without glaze calculation, although
certainly glaze calculation can direct and inform glaze testing.
It is interesting that you mention oil spots as I came upon them with only
one test!! It was through observation of another firing where I saw some
glimmering hope of an oil spot and at the appropriate time tested it in an
oxidation firing.
Now this is by no way typical but it is true! Then I tested, not using any
Seger guidelines as I did not have any, but rather I used the Ian Currie
method. I ran one Currie blend and from that I found 3 other possible oil
spot recipes.
I do beg to differ with you on one point. I believe that the first mention
of MgO in Oil Spots is referenced in =93Stoneware Glazes=94 by Ian Currie on=
page 166, published in 1985 where he paraphrases an article by Brother
Anthony published in 1980. Brother Anthony talks about and gives possible
Seger Formula limits for Oil Spots. He talks about the need for CaO and
MgO and then he lists recipes in the O.S.1 chart where he varies the
source of MgO: talc, dolomite or magnesium carbonate.
Also, there is a mention in =93The Potter=92s Dictionary=94 by Frank and Jan=
et
Hamer (I have the 4th edition 1997, it could have been in an earlier
edition too, but I don=92t have that one.). It is under a =93Magnesium Oxide=
=94
where they talk of the late fusion of magnesium oxide as being valuable in
producing a mottled and oil-spot and hare=92s fur effect. Page 211.
( BTW. If anyone doesn't have the Hamer Dictionary you need to get it. It
is the most amazing reference manual. The more you read and the more
experience you have the more you find!)
I also seem to remember reading a very early dissertation mentioning MgO,
but cannot recall the date and document at this time.
Nevertheless, I think we are in agreement about glaze calculation....but I
prefer to give the edge to empirical (glaze) testing since so many factors
cannot be taken into account with the Seger Formula. When they come out
with a Glaze Calculation Program that takes into account the clay/glaze
interface, glaze thickness, firing cycle, cooling cycle, atmosphere,
solubility, and mesh size then I will be a believer.
I will take a test tile over a formula any day. You can=92t hug a formula.
Best to you and glad you are posting again.
John Britt
Lee Love on fri 24 feb 06
On Thu, 23 Feb 2006 00:08:27 -0500, Tom Buck wrote:
> Empirical testing comes into its own when we seek to use materials
>of uncertain or unknown composition, a route I seldom follow.
I agree with you about how glaze calculation is a great
investigative tool. I primarily use it after empirical testing, to
help me understand why things turned out the way they did.
For 99.9% of pottery history, folks did not use refined
materials that were made specifically for industrial use. When your
materials are primarily potter specific materials, that are not refined,
it is your only affordable method (you can't pay a lab to test each bag
of materials you purchase.)
Folks on the list have tried to paint traditional glaze
methods as being "unscientific." They are not "unscientific", but are
the most accurate and financially affordable method for unrefined materials.
I had my last standard Nami Jiro glaze (woodash/ball
clay/Amakusa stone 3\2\1 turn out differently that it has for several
years. I immediately thought that the problem might be the new wood
ash I was using. I used grape vine ash (one of Jean's english
students farms grapes It turned
out a mottled gold/yellow color. I always test my ash recipes using
synthetic ash in the glaze as a baseline. So I made the glaze
again, last firing with synthetic ash. I got the same gold color!
I then remembered that I bought a new bag of gariome /ball clay. My
next test will sub kaolin for most of the ball clay. The other
possible area to look at is the hakame slip which also uses the same
ball clay.
Thee gold is beautiful and I will keep using it. But it
isn't the nami jiro which is my main glaze, that I would also like to use.
I recommend everybody exploring ash glaze to have one version
using synthetic ash, so you can test and see if you have changes in
your other materials. It is good to have a baseline.
--
Lee Love
in Mashiko, Japan http://mashiko.org
http://seisokuro.blogspot.com/ My Photo Logs
Hank Murrow on fri 24 feb 06
On Feb 23, 2006, at 5:57 PM, John Britt wrote in part:
>
> Yes, I agree with you that glaze calculation should be a part of a
> general
> ceramics education but never a substitute for glaze testing.
>
> It is interesting that you mention oil spots as I came upon them with
> only
> one test!! It was through observation of another firing where I saw
> some
> glimmering hope of an oil spot and at the appropriate time tested it
> in an
> oxidation firing.
> When they come out with a Glaze Calculation Program that takes into
> account the clay/glaze
> interface, glaze thickness, firing cycle, cooling cycle, atmosphere,
> solubility, and mesh size then I will be a believer.
Amen to that, and somebody start working on it! I had been firing my
high alumina shinos for ten years before getting to cone 10 one time
and wondering in my fatigue if a soak in oxidation might just brighten
the fire color. I decided to let the kiln cool a little after finishing
the fire so that the glazes would not run, and then start it up with
enough gas/air to create a steady oxidizing fire. I held this condition
for four hours, and the result was spectacular color......... the same
sort of color that I got from the day before yesterday's firing.
Glorious! I hope some NCECA-bound folks will get to see the results at
the Aurora Gallery on the Sacagawea shuttle route.
Hunches, tests, grid tiles, and yes.........software & a laptop. Fondly
treasured in my toolkit.
Cheers, Hank
www.murrow.biz/hank
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