June Perry on thu 9 oct 03
Dear Ron,
The latest version and I think an earlier one also, had cone 6 limits for
copper bearing glazes. Here is what the newest Insight shows for the limits of
alumina and silica for Cone 6 copper bearing glazes:
All2O3 .29-.64
SiO2 2.4 -4.7
He does mentions using more B2O3 or frits for increased melt if you are going
to use the upper limits.
As you can see it is pretty high, and I was curious as to what limits you use
for copper bearing glazes.
I see that he has yours and Johns limits in there as well . Yours and John's
cone 6 limits are shown as:
Al2O3 - 0.25- 0.5 5
SiO2 - 2.5 - 4
His cone 3 - 7 non copper bearing limits are much lower than your cone 6
limits, topping out at .35 for alumina and 3.5 for silica - not too big a
difference for the silica, but quite a bit of different in the alumina between the
two sets of limits.
I guess what we need to do it create, using the limits as a general guide
line, batch, fire and then test.
Regards,
June
http://www.angelfire.com/art2/shambhalapottery/
John Hesselberth on thu 9 oct 03
Hi June,
I am going to jump in here for a minute. While we did develop a set of
"limits" that are analagous to other literature limits our focus was on
making stable glazes. For that there are 4 things we found important.
1. Have silica of at least 2.5, preferably above 3.0. Above 4.0 watch
out for poor melting. 2. Have alumina of 0.25-0.45. 3. Thoroughly melt
the glaze and 4. Don't overload with colorants. Chapter 4 of our book
expands on these points considerably. It has been our experience that
these "rules" are much more reliable than "limits" for assuring
stability. For example, most limits set 0.7 or thereabouts as the
upper limit for calcium. I can make very stable glazes at 0.85 or
higher. Most limits for boron top out at 0.4 or so for cone 6.
Waterfall has a boron level of 0.77 and is very stable. Limits are good
places for beginners to start in order to maximize their chances of
early success. They also "limit" you from making some very attractive
AND stable glazes, and I strongly recommend experienced glaze
developers like you switch over to the 4 rules and forget limits.
Recognize that limits were all derived by examining a series of glazes
that looked like they gave "good glass" and looking at the ranges of
the various oxides they contained. There was NO science involved in
deriving limits--or if there was it never made it to the published
literature. I have done a very extensive literature search to find a
scientific foundation for them and came up with essentially zip. That
literature search is documented in an article I have on my web site.
Our work on stability, while a long way from perfect, has a much better
scientific foundation.
While "limits" are a traditional method, I find them of little use in
my work at this point and no longer think that way. While I don't have
the data to prove it yet, I believe the same 4 rules I quoted above are
valid at any temperature, although the upper limit on alumina probably
moves up with increasing firing temperature (because you can better
follow rule 3).
And as a last point, the copper level in a glaze only affects the
balance of fluxes you need to get good melting and the balance of
fluxes (+ boron) you need to get good melting is a bit too complicated
to express simply the way traditional limits are specified. We may
someday be able to give a simple formula to specify the level of fluxes
needed, but that is a ways off--probably a long way off.
Regards,
John
On Thursday, October 9, 2003, at 03:29 PM, June Perry wrote:
> Dear Ron,
>
> The latest version and I think an earlier one also, had cone 6 limits
> for
> copper bearing glazes. Here is what the newest Insight shows for the
> limits of
> alumina and silica for Cone 6 copper bearing glazes:
>
> All2O3 .29-.64
> SiO2 2.4 -4.7
>
> He does mentions using more B2O3 or frits for increased melt if you
> are going
> to use the upper limits.
>
> As you can see it is pretty high, and I was curious as to what limits
> you use
> for copper bearing glazes.
> I see that he has yours and Johns limits in there as well . Yours and
> John's
> cone 6 limits are shown as:
>
> Al2O3 - 0.25- 0.5 5
> SiO2 - 2.5 - 4
>
> His cone 3 - 7 non copper bearing limits are much lower than your cone
> 6
> limits, topping out at .35 for alumina and 3.5 for silica - not too
> big a
> difference for the silica, but quite a bit of different in the alumina
> between the
> two sets of limits.
>
> I guess what we need to do it create, using the limits as a general
> guide
> line, batch, fire and then test.
>
>
http://www.frogpondpottery.com
http://www.masteringglazes.com
June Perry on fri 10 oct 03
Thanks John for your feedback. I agree that some of the best glazes we have
seen over the years have been those that broke the rules!
I have a CaO eutectic matt with .89 Cao and it's a gorgeous glaze. It's
#7473V3 on page 1 of the C6 test tiles on my web page. I basically used Parmalee's
information for developing the glaze except I added a hair Mgo in addition to
the Kna.
I enjoy glaze testing more than anything I think and like to play with
pushing the envelope as well. I haven't taken the boron up that high and I think it
will be fun to play with that a bit.
I recently purchased your book and am very impressed with both the quality of
the information, glazes and the book itself. I haven't tested any of the
glazes yet, but after checking your web page yesterday and seeing the new photos
you posted, I'm definitely going to test them when I can find the time.
Right now I'm playing with a promising rutile blue at cone 6. The first test
ran but the color is an exquisite light, electric, chun blue which I hope I
can maintain when I up the silica and alumina. I'm keeping my fingers crossed!
Warmest regards,
June
http://www.angelfire.com/art2/shambhalapottery/
Ron Roy on sun 12 oct 03
I don't know where Tony got his limits and if they are supposed to be for
stable glazes - in the end you will need to test in order to find out. I
suggest you ask him what he bases his limits on.
We are at the beginning of finding out about stable glazes - there is much
to learn and whatever limits are finally arrived at - there will still be
stable glazes outside them.
Some mixtures are going to melt better than others because of the
proportions - there is no way to forecast melt for instance - we can only
learn by melting and testing.
There are many stable cone 6 copper bearing glazes in our book of course -
and John used copper in all the glazes as a bench mark.
You can draw your own conclusions about this from comparing the fit test
glazes in chapter - they were all tested for copper release - #5 (high
expansion) is not recommended as a liner glaze for instance. It has 2.57
SiO2 and a lot of KNaO - it is clearly not a stable glaze with 5% copper
but fits the limits you include below.
Also John included 6% rutile along with the 5% copper - and notes that
rutile always seemed to improve stability.
RR
>The latest version and I think an earlier one also, had cone 6 limits for
>copper bearing glazes. Here is what the newest Insight shows for the limits of
>alumina and silica for Cone 6 copper bearing glazes:
>
>All2O3 .29-.64
>SiO2 2.4 -4.7
>
>He does mentions using more B2O3 or frits for increased melt if you are going
>to use the upper limits.
>
>As you can see it is pretty high, and I was curious as to what limits you use
>for copper bearing glazes.
>I see that he has yours and Johns limits in there as well . Yours and John's
>cone 6 limits are shown as:
>
>Al2O3 - 0.25- 0.5 5
> SiO2 - 2.5 - 4
>
>His cone 3 - 7 non copper bearing limits are much lower than your cone 6
>limits, topping out at .35 for alumina and 3.5 for silica - not too big a
>difference for the silica, but quite a bit of different in the alumina
>between the
>two sets of limits.
>
>I guess what we need to do it create, using the limits as a general guide
>line, batch, fire and then test.
>
>
>Regards,
>June
>http://www.angelfire.com/art2/shambhalapottery/
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ababi on sun 12 oct 03
I have a question about stable glazes:
Is it connected in any way to horse hair?
Ababi Sharon
Glaze addict
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
Sent: Sunday, October 12, 2003 6:57 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: glaze for week oct 3 For Ron Roy
I don't know where Tony got his limits and if they are supposed to be
for
stable glazes.
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