John Hesselberth on tue 4 mar 08
On Mar 4, 2008, at 1:10 PM, Lynn Goodman Porcelain Pottery wrote:
> I also teach beginning glaze classes using batch limits; they work
> well to give an introduction to what the different fluxes do, without
> throwing anything too complicated at people who don't have any
> experience with glazes at all. I have never been able to find low-
> fire batch limit formulas anywhere-- does anyone have any? I've
> compiled a list, but it leaves a great deal to be desired.
Hi Lynn,
There are a couple sets available in the literature. Zakin published
some in Ceramics: Mastering the Craft, page 126. I never have been
fond of these because they go way too low in allowable silica levels
and, in some cases, boron levels and result in "unmelted" mattes
which will not be very durable.
Green, in his 1980 book Pottery Glazing Basics lists "maximum amount
of common fluxes present in glazes". My notes say it is in Table 6,
but I have misplaced that book at the moment.
For cones 04-06, my own advice would be to pay attention to 3 things.
Silica from 2.5-4; alumina from 0.25-0.45 and boron from 0.7-1.25. Go
light on the alkaline earths and heavier on the alkalis.
Regards,
John
John Hesselberth on tue 4 mar 08
On Mar 4, 2008, at 1:10 PM, Lynn Goodman Porcelain Pottery wrote:
> I also teach beginning glaze classes using batch limits; they work
> well to give an introduction to what the different fluxes do, without
> throwing anything too complicated at people who don't have any
> experience with glazes at all. I have never been able to find low-
> fire batch limit formulas anywhere-- does anyone have any? I've
> compiled a list, but it leaves a great deal to be desired.
>
> Lynn
I just found Pottery Glazing Basics by Green. It gives an extensive
list of "flux limits" every 25 Deg C from 900 up to 1400. In Table 5
he also gives the same info for Silica and Alumina. Again I'm not
fond of the silica and alumina numbers--I think he goes way too low
to make durable glazes at earthenware temperatures. Once he gets up
to cone 6 and above they are better.
One point I should make regarding my view of how to make durable
glazes. Call it my current working hypothesis backed up by a fair
amount of leaching data and vinegar testing. I think the allowable
ranges for silica and alumina should be essentially the same
regardless of firing temperature. You have to have those in
appropriate amounts and ratios to each other to have a decent glass
when it gets back down to room temperature. Then what is important is
the mix of fluxes + boron to get the glaze fully melted at your
firing temperature. So for earthenware you need more boron and a
higher percent alkalis than you do at cone 6. At cone 10 you don't
need any boron and can lean heavily on alkaline earths.
That is in direct opposition to much of the literature and the
published glazes. They tend to start to reduce the amounts of silica
and alumina they use at lower temperatures, particularly when they
try to make matte glazes. To get to lower temperatures, they are
working off the theory of "more flux, less silica and alumina" where
I pursue "more effective flux, same silica and alumina". Time will
tell who is right I suppose, but I do know this. Almost every
unstable glaze (vinegar test or leaching data) I see has low silica--
generally 2.0 or below. The best glazes from a stability standpoint
are 2.5, preferably 3.0, or above. You can go too high also and those
limits may vary by temperature with more silica and alumina being OK
at high temperatures because that mix can still be thoroughly melted.
Regards,
John
Lynn Goodman Porcelain Pottery on tue 4 mar 08
I also teach beginning glaze classes using batch limits; they work
well to give an introduction to what the different fluxes do, without
throwing anything too complicated at people who don't have any
experience with glazes at all. I have never been able to find low-
fire batch limit formulas anywhere-- does anyone have any? I've
compiled a list, but it leaves a great deal to be desired.
Lynn
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
Lynn Goodman Porcelain Pottery on wed 5 mar 08
> One point I should make regarding my view of how to make durable
> glazes. Call it my current working hypothesis backed up by a fair
> amount of leaching data and vinegar testing. I think the allowable
> ranges for silica and alumina should be essentially the same
> regardless of firing temperature. You have to have those in
> appropriate amounts and ratios to each other to have a decent glass
> when it gets back down to room temperature. Then what is important is
> the mix of fluxes + boron to get the glaze fully melted at your
> firing temperature. So for earthenware you need more boron and a
> higher percent alkalis than you do at cone 6. At cone 10 you don't
> need any boron and can lean heavily on alkaline earths.
Thank you for the info!
Yes, I do agree with you. I use the batch limits as a starting point,
to allow students to see the difference in fluxes, as well as to
allow them to make a glaze in the first round of testing, whether or
not it's sturdy. The students are AMAZED that they are able to do
that-- then I follow it up with acid testing to see how stable and
durable that glaze is. Then they learn how to improve their glaze in
the next round of glaze tests. I find that starting with a glaze--
any glaze-- that they have formulated, gets them well and truly hooked.
Lynn
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
Edouard Bastarache Inc. on wed 5 mar 08
Hello,
check out Val Cushing's in GlazeChem,
or from Bob Wilt, the author of the
programme.
Gis la revido,
(A la revoyure)
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.pshcanada.com/Toxicology.htm
http://www.flickr.com/photos/30058682@N00/
http://www.flickr.com/photos/potier/20321056/
http://perso.orange.fr/smart2000/livres.htm
http://myblogsmesblogs.blogspot.com/
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