[Clayart] Glaze stability question

[Dragonbelly Ceramics via Clayart]

Hello, all,

As I learn more and more about glaze chemistry, it solves many problems and
also raises other questions.

I have a really beautiful and interesting cone 6 glaze that was shared with
me from a prior studio. It was a glaze that studio owner didn't know the
origin of and has been in production there for quite some time.

I'm now concerned that according to its chemistry, this glaze (Tom West
Green) is not a stable glaze and should not be used on food surfaces as it
has a R2O:RO ratio of 0.79:0.21 and a boron amount of 0.34.

However, I've had pieces glazed with it - mostly in combination with other
more stable glazes by chemistry (closer to 0.3:0.7) - that I've used for
years, including years of exposure to the dishwasher without any apparent
change to its surface quality. However, I know that this means little
without real testing.

This is the recipe:

Tom West Green, Cone 6 oxidation

Ferro Frit 3269: 27
Gerstley Borate: 15
Spodumene: 40
EPK: 18
Copper Carbonate: 5
Tin Oxide: 7

Is this a stable glaze? If not, is it worth the time and effort to try to
make it stable? (Or is this even possible?)

It's also the most costly glaze we have!

Best regards,
Lisa Cohen

--
http://www.dragonbellyceramics.com
* where imagination meets function*
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[joel joelfink.net via Clayart]

I'm not an expert by any stretch.

The RO/R2O looks fine to me. An 8 to 2 ratio is well enough within limits. I ran the formula given on Insight and it gave me a 3 to 7, which is best. The SiO2 to Al2O3 comes out at the top of Stull's map, but within the safe range so far as I understand the map. If someone else knows better, listen to them. I've never seen a glaze that landed on the top of the map, I'm guessing it is the Spodumene and the unusual frit.

I don't see anything to be worried about but the Boron, and if the glaze isn't getting on your shelves, it isn't a problem either. I wouldn't push it much hotter than 6 though.

Joel Fink
Ficklefink Pottery
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From: Clayart <clayart...@lists.clayartforum.com> on behalf of Dragonbelly Ceramics via Clayart <cla...@lists.clayartforum.com>
Sent: Friday, May 24, 2024 10:33 PM
To: Clayart international pottery discussion forum <cla...@lists.clayartforum.com>
Cc: Dragonbelly Ceramics <li...@dragonbellyceramics.com>
Subject: [Clayart] Glaze stability question

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[ronroy--- via Clayart]

Hi Lisa,

I'm tempted to say it must be unstable if you compare it's molecular
formula to our limits published in Mastering Cone 6 Glazes but here is
the problem.

First, it does not mean there are not stable glazes outside those
limits - we know there are.

Expansion is dangerously low, The alkalies (Sodium, potassium and
lithium) when added together are almost double our recommended,
alumina almost double and boron somewhat over. So an unusual glaze to
start with and compounded by a lot of copper.

Having said that - there is plenty of silica and that is the most
important oxide in terms of stability.

Because alumina is very hard to melt, all that alkali flux may do the job?

Best to leave some lemon slices on it at room temperature for 24 hours
to see any colour change or dulling of glaze surface. Wash any test
pieces first to remove any oil or other inhibitors.

I can raise the expansion using some Neph Sy and taking out some of
the spodumene - lowing the price and raising the expansion.

Always interesting finding unusual glazes and making them better.

RR

Ron Roy
ron...@ca.inter.net
Web page ronroy.net

[Dragonbelly Ceramics via Clayart]

Thank you for your reply! It's really a fabulous glaze. On an iron rich
clay, one dip, it's this rich orange-brown. On white clay, one dip, it's a
mossy green. Shifts to areas of metallic dark green where thick.

In the attached photo, the entire plate was dipped in Tom West Green. One
side got a second dip of Val's Turquoise gloss, and the other half was
another glaze from that former studio called Hare's Fur (it's a variegated
off white).

The strip in the middle and where the edge breaks orange is the Tom West
Green.

I think I will play with different levels of neph sye, if only to make the
glaze less expensive! Will also do a lemon test and report back!

Best,
Lisa

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[Dragonbelly Ceramics via Clayart]

Thanks, Joel. The wild thing is this glaze doesn't run, even when
overfired. Though i have seen it run in combination with another glaze, but
I suspect it was simple thickness.

Best,
Lisa

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