Diane Winters on sun 30 sep 01
Hi John,
Thanks for your response and information on this glaze.
I don't use Toshiko Green, per se, but I guess it's quite probable my forest
green is just as unstable. If you'd be willing to take the time, would you
consider analyzing it? I'd appreciate it greatly. Barring that - I'm sure
you must be more than busy enough with your own work - if I do a little
empirical test, submerge a test tile sideways into an inch or so of lemon
juice for a couple hours, will I see enough myself? Actually, I'll try it
anyway - just don't have a test tile at home now.
My ^6 oxidation forest glaze is:
Custer spar 35.5
Whiting 22.2
EPK 8
Calcined Kaolin 6.9
Cornwall Stone 17.7
Zinc 5.3
Gerstley Borate 4.4
+Copper carb. 5
+Titanium dioxide .4 (that's point 4)
Thanks,
Diane Winters
diane@winterstileworks.com
John Hesselberth wrote:
>
>Hi Diane, Ruth and others,
>
>Please be aware that this glaze, Toshiko Green, is an extremely unstable
>glaze. It is one of the 3 I used in my NCECA presentation last March to
>show how bad attractive glazes can be. I can suck the color right out of
>this glaze with lemon juice in less than 2 hours. It simply does not have
>enough silica (1.6) to be stable. I personally wouldn't even put it on
>decorative tiles unless you're sure they won't ever see lemon juice, acid
>rain, vinegar or the like. The glaze as is, nor the base, are not ones I
>would recommend to anyone making anything that might remotely be called
>functional. That would include things like tiles embedded in end tables or
>installed on a kitchen backdrop.
>
>Regards,
>
>John
John Hesselberth on mon 1 oct 01
on 10/1/01 1:31 AM, Diane Winters at diane@WINTERSTILEWORKS.COM wrote:
> Hi John,
>
> Thanks for your response and information on this glaze.
> I don't use Toshiko Green, per se, but I guess it's quite probable my forest
> green is just as unstable. If you'd be willing to take the time, would you
> consider analyzing it? I'd appreciate it greatly. Barring that - I'm sure
> you must be more than busy enough with your own work - if I do a little
> empirical test, submerge a test tile sideways into an inch or so of lemon
> juice for a couple hours, will I see enough myself? Actually, I'll try it
> anyway - just don't have a test tile at home now.
>
> My ^6 oxidation forest glaze is:
>
> Custer spar 35.5
> Whiting 22.2
> EPK 8
> Calcined Kaolin 6.9
> Cornwall Stone 17.7
> Zinc 5.3
> Gerstley Borate 4.4
> +Copper carb. 5
> +Titanium dioxide .4 (that's point 4)
Yes Diane, I'm sorry to say this one is almost as bad as Toshika Green. It
has a silica level of 1.9 when you need to be at least at 2.5 and preferably
3.0 or above to have a stable glaze. This is particularly true when you use
copper as a colorant. It is the most difficult colorant to keep in a glaze.
The unity formula is at the bottom of this note.
There is a simple screening test you can use if you don't understand the
chemistry. It hasn't been described on Clayart for quite a while so I'll
take a minute and do that. Take a clean test tile and submerge half of it
in a glass or pan of regular white or cider vinegar for three days. Rinse
the tile, dry it and then let it air dry for a few more minutes. If there
is a change in gloss or sheen between the submerged and unsubmerged portions
you have a glaze which is of questionable stability. It might be OK, but
further testing is probably warranted. If there is a change in color
between the two section you have a very unstable glaze that just isn't
suitable for use on functional work. This test is not foolproof, but it is
a pretty good screening test. I would predict that your forest glaze will
lose significant color in 3 days and maybe in two hours.
I normally use the vinegar test. I use the lemon juice test only for "show"
when I want to demonstrate by photo or live how bad glazes can be. Somehow
audiences relate better to a slice of lemon sitting on a saucer.
Incidentally there is a photo of what lemon juice will do to Toshika Green
on page 10 of the March 2001 issue of Ceramics Monthly. Toshika Green is on
the left.
Now, back to the book which, of course, will have lots more information on
this subject as well as a host of very stable glazes to use as starting
points in your own glaze development.
Regards, John
----------------
Glaze name: Forest
Cone: 6
Testing: Untested
Firing: Ox. or Red.
Recipe: Percent
Custer Feldspar 35.50
Whiting 22.20
EPK 8.00
Calcined Kaolin 6.90
Cornwall Stone 17.70
Zinc Oxide 5.30
Gerstley Borate 4.40
Totals: 100.00 %
Also add:
Copper Carbonate 5.00
Titanium Dioxide 0.40
Unity Formula for Forest:
0.118 K2O 0.380 Al2O3 1.938 SiO2
0.080 Na2O 0.046 B2O3 0.003 TiO2
0.620 CaO 0.003 P2O5 5.1:1 Si:Al Ratio
0.015 MgO
0.167 ZnO
Web site: http://www.frogpondpottery.com Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
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