Cindy Strnad on sat 29 sep 01
Dear Ruth,
When you say semi-opaque, I wonder if you mean
semi-matte? This is from some of your other
remarks that the glazes you have tried have
sometimes been too glossy. You also haven't
mentioned whether you are working at ^10, ^6, ^04,
or whatever, reduction or oxidation. I can only
help you at ^6 oxidation.
Xavier's Warm Jade Green is a beautiful blue-green
glaze which you can find at
http://www.frogpondpottery.com along with a number
of other beautiful and useful glazes. The base for
this glaze would be a good start for
experimentation with other colors. It is glossy,
though not "offensively" so.
Another glaze you might try is Opal Blue. Also
glossy but not mirror-bright. Though I wouldn't
recommend it for copper additions without testing,
it makes a good base for oxides less adept at
escaping than copper. This is a very simple recipe
and the blue color works best at slightly under
^6:
Kathy's Opal Blue (^5-6 oxidation, give it a nice
soak at the end for better color)
50 Frit 3134
25 EPK
25 Silica
02 Cobalt Oxide
04 Rutile
To get a dark blue-green, increase the EPK a bit
and decrease the frit--experiment to find
something you like.
This glaze will not make a dark yellow. It doesn't
have the chemistry for it. Dark yellow is a bit
more difficult. I've been experimenting with a
glaze once posted to Clayart which you might like.
Try it and see what you think. It's darker where
it's thin, but it may give you a starting point.
You might call this particular glaze a semi-matte.
It makes nice little runnels on a vertical
surface, just like an ash glaze.
Golden Fake Ash, ^6
===================
REDART.............. 29.00 28.71%
OM #4 BALL CLAY..... 21.00 20.79%
BONE ASH............ 5.00 4.95%
STRONTIUM CARBONATE. 9.00 8.91%
DOLOMITE............ 25.00 24.75%
GERSTLEY BORATE..... 10.00 9.90%
LITHIUM CARBONATE... 2.00 1.98%
========
101.00
CaO 0.45* 16.11%
Li2O 0.05* 1.04%
MgO 0.32* 8.16%
K2O 0.03* 1.81%
Na2O 0.02* 0.92%
SrO 0.12* 8.07%
TiO2 0.01 0.73%
Al2O3 0.21 13.37%
B2O3 0.08 3.72%
P2O5 0.03 3.01%
SiO2 1.05 40.13%
Fe2O3 0.03 2.94%
For another soft matte base to experiment with,
try my Crystal Matte as follows:
Cindy's Crystal Matte Base ^6
=====================
EPK KAOLIN.......... 30.00 30.00%
WHITING............. 21.00 21.00%
CUSTER FELDSPAR..... 20.00 20.00%
FRIT 3134........... 15.00 15.00%
SILICA.............. 14.00 14.00%
========
100.00
CaO 0.82* 17.22%
MgO 0.00* 0.03%
K2O 0.07* 2.49%
Na2O 0.11* 2.48%
TiO2 0.00 0.13%
Al2O3 0.45 17.09%
B2O3 0.15 4.00%
P2O5 0.00 0.08%
SiO2 2.50 56.17%
Fe2O3 0.01 0.30%
This glaze must have a slow cooling period. I
would suggest a hold of an hour or two or three at
around 1800 F to allow the crystals which give it
the matte finish to grow. If you don't have a
computer controller on your kiln, you'll just have
to guess and experiment with the cooling cycle
until you get it the way you want it. It makes a
nice blue. I haven't tried it with other oxides
yet, but I expect it to perform well.
Hoping this gives you and maybe some others a
starting point,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
becky schroeder on sat 29 sep 01
ruth
if you are firing in oxidation let me know off line. after 2 years of
testing i have found some beauts and will be happy to share them with you.
they may be what you are looking for.
becky schroeder
>From: Ruth Blake and David Blake
>Reply-To: Ceramic Arts Discussion List
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Semi-opaque glaze cone5/6
>Date: Sat, 29 Sep 2001 16:17:31 -0500
>
>I am trying to find a good base glaze that is semi-opaque to do some
>glaze color tests with.
>I have tried all the ones I could find in the books I have and have had
>no luck. They either come up too translucent, too glossy or don't seem
>to take the color well, or contain GB which I don't have.
>Does anyone have a good base glaze recipe that they have had success
>with? I have a scale and experience mixing glazes but I don't know
>enough yet to devise my own. (I'm working on it)
>I'm trying to find a good mossy green, blue green and dark yellow that
>will work for some pots I have made. I figure by using the same base I
>can get glazes that marry well on the pot. These poor pots have been
>naked now for 4 months and I still am not even close to finding the
>right glaze.
>Ruth
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
_________________________________________________________________
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Ruth Blake and David Blake on sat 29 sep 01
I am trying to find a good base glaze that is semi-opaque to do some
glaze color tests with.
I have tried all the ones I could find in the books I have and have had
no luck. They either come up too translucent, too glossy or don't seem
to take the color well, or contain GB which I don't have.
Does anyone have a good base glaze recipe that they have had success
with? I have a scale and experience mixing glazes but I don't know
enough yet to devise my own. (I'm working on it)
I'm trying to find a good mossy green, blue green and dark yellow that
will work for some pots I have made. I figure by using the same base I
can get glazes that marry well on the pot. These poor pots have been
naked now for 4 months and I still am not even close to finding the
right glaze.
Ruth
Joan & Tom Woodward on sun 30 sep 01
Joanne,
This glaze looks nice to me too, but can you describe the finish more? Is
it a satin? And have you tested for food safety? Thanks, Joan (also in the
West!)
----- Original Message -----
From: Joanne Van Bezooyen
To:
Sent: Sunday, September 30, 2001 8:21 AM
Subject: Re: Semi-opaque glaze cone5/6
> Hi Ruth,
> The following glaze might work for you. Start with it and then make
changes
> It may be too opaque for you. It is blue/green breaking to a golden light
> brown.
> Custer Feldspar.......30
> EPK (kaolin)........30
> Whiting........20
> FF3134......20
> Copper Carbonate.....3.6
> Red Iron Oxide.....2.4
> Rutile.....3.6
> Cobalt Carbonate.....1
> (I know it adds up past 100 because of the colorants, but it works great)
John Hesselberth on sun 30 sep 01
Hi Joanne,
I guess its my day to spot problems. I don't mean to rain on any parades,
but I am concerned when I see glazes as unstable as this one will be without
any mention that it should only be used for decorative work. Its silica
level is 2.06 and with 3.6% copper carbonate I would almost guarantee that
it will fail the vinegar test and leach a lot of copper. It just won't hold
up in use; I would hate to see anyone putting this on functional work.
Since Ruth didn't say what kind of work she did, I feel its necessary to
raise a red flag here.
Xavier's Warm Jade Green, which someone mentioned earlier, is a good stable
satin glaze which could be used as a base. It does require slow cooling,
though, to give a satin finish. I think I can predict that all properly
formulated satin or matte glazes require slow cooling to get that surface.
That surface results from recrystallization of some of the glaze components
and that takes time to happen. If you are getting matte surfaces by just
turning off a typical underinsulated electric kiln and letting it cool
naturally there is a very high probability the glaze is not durable. In
that situation, you are more than likely generating the matte surface from
incomplete melting of the glaze or from a very low Si/Al ratio.
If you can't find the recipe for Xavier's Warm Jade Green, go to my web site
(URL below) and click on Glaze Stability.
Regards,
John
on 9/30/01 10:21 AM, Joanne Van Bezooyen at joannevb@EARTHLINK.NET wrote:
> Hi Ruth,
> The following glaze might work for you. Start with it and then make changes
> It may be too opaque for you. It is blue/green breaking to a golden light
> brown.
> Custer Feldspar.......30
> EPK (kaolin)........30
> Whiting........20
> FF3134......20
> Copper Carbonate.....3.6
> Red Iron Oxide.....2.4
> Rutile.....3.6
> Cobalt Carbonate.....1
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.
Diane Winters on sun 30 sep 01
Hi Ruth,
I'm guessing you're looking for a matt or satin-matt glaze, as you refer to
a problem with some of the tested glazes being too glossy. I'm also
guessing, though you don't say, that you're firing electric. Nor did you
say if food safety is an issue.
Getting additional, different glaze formulas may or may not be a solution.
I do work with semi-opaqe satin-matt glazes at ^5-6, but mine will go
translucent, even glossy, plus exhibit an almost infinite variety of
additional characteristics, depending on how fast I fire to ^6 and how fast
I cool, as well as thickness of application. I fire and cool slow - would
be happy to share my firing schedule with you if you want.
If you get translucence even in a slow firing , then you can try adding
increments (anywhere from 2-10%) of any of several opacifiers - most of my
glazes use zinc oxide (as both flux and opacifier), I also sometimes use
Superpax, and there's always tin oxide and titanium oxide. Different
opacifiers will affect each glaze a little differently; there's no right
answer as to which to use, so see for yourself.
For colorants I rely mostly on oxides/carbonates. Since each melts at a
different temp and contributes to the flux, I can't simply use a single
"base glaze". For each color I've had to adjust my basic glaze to
compensate (for example increasing the EPK, in a few instances the whiting,
etc.). And I've had to pull out the zinc entirely to get the red/burgundy
range.
The original glaze that was the seed for my tinkering is a glaze I learned
as Toshiko Green (which is obviously no longer green if you eliminate the
copper carb.) below:
Potash Feldspar 35.7
Whiting 22.4
EPK 13.4
Cornwall Stone 17.8
Zinc Oxide 10.7
+ Bentonite 1.7
+ Copper carb. 1.7 (for green)
This glaze breaks considerably from cool green to dark brown, depending on
thickness and underlying texture. Since this didn't suit my purposes, for
my green glazes I cut the zinc approximately in half to minimize the break.
NOTE: I make decorative wall tile - can't vouch for this as being a food
safe glaze.
Happy glaze testing,
Diane Winters
diane@winterstileworks.com
in Oakland/Berkeley by the Bay (now feeling for and missing lower Manhattan
where I lived 21 years before moving here; in my former corporate/legal life
worked at One Liberty Plaza adjacent to the south tower. Thankfully all
friends and former co-workers accounted for)
Ruth Blake wrote:
>
>I am trying to find a good base glaze that is semi-opaque to do some
glaze color tests with.
I have tried all the ones I could find in the books I have and have had
no luck. They either come up too translucent, too glossy or don't seem
to take the color well, or contain GB which I don't have.
Does anyone have a good base glaze recipe that they have had success
with? I have a scale and experience mixing glazes but I don't know
enough yet to devise my own. (I'm working on it)
I'm trying to find a good mossy green, blue green and dark yellow that
will work for some pots I have made. I figure by using the same base I
can get glazes that marry well on the pot.
>Ruth
>
Ababi on sun 30 sep 01
Hello Ruth
The Base I send you here, I do not remember where it is coming from, some
one from the list.
First the original:
Waxy White^6 ox.
============
QUARTZ.............. 23.50 23.50% ...................use flint
ENGLISH KAOLIN...... 14.90 14.90%......Any kaolin
NEPHELINE SYENITE... 14.50 14.50%
WOLLASTONITE........ 37.20 37.20%
GERSTLEY BORATE..... 9.90 9.90%
========
100.00
CaO 0.87* 21.12%
MgO 0.02* 0.38%
K2O 0.03* 1.20%
Na2O 0.08* 2.19%
TiO2 0.00 0.02%
Al2O3 0.21 9.28%
B2O3 0.10 3.03%
SiO2 2.40 62.61%
Fe2O3 0.00 0.17%
Si:Al 11.45
SiB:Al 11.92
Expan 7.27
It is very nice, looks like it is "matte" but pretty glossy you can see the
undercoloring.
A lot of Cao, probably cause's it. Now I will change it for you
Waxywhite 2
===========
NEPHELINE SYENITE... 10.3 10.32%
WOLLASTONITE........ 35.5 35.50%
FRIT 3134........... 12.7 12.70%
TALC................ 1.1 1.12%
EPK KAOLIN.......... 17.2 17.17%
FLINT............... 23.2 23.19%
========
100.0
CaO 0.87* 21.25%
MgO 0.02* 0.41%
K2O 0.01* 0.58%
Na2O 0.09* 2.50%
TiO2 0.00 0.07%
Al2O3 0.21 9.22%
B2O3 0.10 3.03%
P2O5 0.00 0.05%
SiO2 2.40 62.76%
Fe2O3 0.00 0.14%
Si:Al 11.56
SiB:Al 12.04
Expan 7.21
You can add oxides I would do so gently.
Visit my glaze pages perhaps you look for something else
Ababi Sharon
ababisharon@hotmail.com
http://members4.clubphoto.com/ababi306910/
----- Original Message -----
From: "Ruth Blake and David Blake"
To:
Sent: Saturday, September 29, 2001 11:17
Subject: Semi-opaque glaze cone5/6
> I am trying to find a good base glaze that is semi-opaque to do some
> glaze color tests with.
> I have tried all the ones I could find in the books I have and have had
> no luck. They either come up too translucent, too glossy or don't seem
> to take the color well, or contain GB which I don't have.
> Does anyone have a good base glaze recipe that they have had success
> with? I have a scale and experience mixing glazes but I don't know
> enough yet to devise my own. (I'm working on it)
> I'm trying to find a good mossy green, blue green and dark yellow that
> will work for some pots I have made. I figure by using the same base I
> can get glazes that marry well on the pot. These poor pots have been
> naked now for 4 months and I still am not even close to finding the
> right glaze.
> Ruth
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
Joanne Van Bezooyen on sun 30 sep 01
Hi Ruth,
The following glaze might work for you. Start with it and then make changes
It may be too opaque for you. It is blue/green breaking to a golden light
brown.
Custer Feldspar.......30
EPK (kaolin)........30
Whiting........20
FF3134......20
Copper Carbonate.....3.6
Red Iron Oxide.....2.4
Rutile.....3.6
Cobalt Carbonate.....1
(I know it adds up past 100 because of the colorants, but it works great)
Joanne L. Van Bezooyen
Art Gecko Designs
11220 E. Via Madre
Tucson, AZ 85749
Studio:(520) 760-1584 voice or fax
Home: (520) 749--1685
http://artgeckotile.com
----- Original Message -----
From: "Ruth Blake and David Blake"
To:
Sent: Saturday, September 29, 2001 2:17 PM
Subject: [CLAYART] Semi-opaque glaze cone5/6
> I am trying to find a good base glaze that is semi-opaque to do some
> glaze color tests with.
> I have tried all the ones I could find in the books I have and have had
> no luck. They either come up too translucent, too glossy or don't seem
> to take the color well, or contain GB which I don't have.
> Does anyone have a good base glaze recipe that they have had success
> with? I have a scale and experience mixing glazes but I don't know
> enough yet to devise my own. (I'm working on it)
> I'm trying to find a good mossy green, blue green and dark yellow that
> will work for some pots I have made. I figure by using the same base I
> can get glazes that marry well on the pot. These poor pots have been
> naked now for 4 months and I still am not even close to finding the
> right glaze.
> Ruth
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
John Hesselberth on sun 30 sep 01
on 9/30/01 6:53 AM, Diane Winters at diane@WINTERSTILEWORKS.COM wrote:
>
> The original glaze that was the seed for my tinkering is a glaze I learned
> as Toshiko Green (which is obviously no longer green if you eliminate the
> copper carb.) below:
>
> Potash Feldspar 35.7
> Whiting 22.4
> EPK 13.4
> Cornwall Stone 17.8
> Zinc Oxide 10.7
> + Bentonite 1.7
> + Copper carb. 1.7 (for green)
> This glaze breaks considerably from cool green to dark brown, depending on
> thickness and underlying texture. Since this didn't suit my purposes, for
> my green glazes I cut the zinc approximately in half to minimize the break.
> NOTE: I make decorative wall tile - can't vouch for this as being a food
> safe glaze.
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
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.
Paul Lewing on mon 1 oct 01
on 10/2/01 12:43 AM, John Hesselberth at john@FROGPONDPOTTERY.COM wrote:
> I have to conclude that strontium is a slightly stronger
> flux than is calcium. Does anyone else have data on that?
John, I have no data to back this conclusion up, but in my experience it is.
But then, in my experience, calcium isn't nearly as strong a flux at
mid-range temperatures as most people say it is.
Paul Lewing, Seattle
Wanda Holmes at Alistia on mon 1 oct 01
John, you mention "a very low SI:Al ratio". What ration should I look for
in a stable glaze?
I am using a strontium matte base from Pete Pinell that has a ratio of
5.1:1. Most of the limit formulas I'm aware of show the Al to be high and
the Si to be in range. They do not, however, address the ratio. I have
discovered that the surface is not as hard as I'd like (I can scratch it
with a dental tool) and I'm working to correct that. I have not yet
performed leaching tests, but with this dicussion I'm starting to wonder if
it is stable. The limit formula is:
Na2O 0.36 Al2O3 0.58 SiO2 2.95
K2O 0.11 TiO2 0.01
MgO 0.01 Fe2O3 0.01
CaO 0.02
Li2O 0.05
SrO 0.46
Alumina:Silica ratio is 1.00:5.10
Neutral:Acid ratio is 1.00:5.12
Alk:Neut:Acid ratio is 1.00:0.58:2.96
Wanda
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of John Hesselberth
Sent: Sunday, September 30, 2001 9:29 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Semi-opaque glaze cone5/6
Hi Joanne,
I guess its my day to spot problems. I don't mean to rain on any parades,
but I am concerned when I see glazes as unstable as this one will be without
any mention that it should only be used for decorative work. Its silica
level is 2.06 and with 3.6% copper carbonate I would almost guarantee that
it will fail the vinegar test and leach a lot of copper. It just won't hold
up in use; I would hate to see anyone putting this on functional work.
Since Ruth didn't say what kind of work she did, I feel its necessary to
raise a red flag here.
Xavier's Warm Jade Green, which someone mentioned earlier, is a good stable
satin glaze which could be used as a base. It does require slow cooling,
though, to give a satin finish. I think I can predict that all properly
formulated satin or matte glazes require slow cooling to get that surface.
That surface results from recrystallization of some of the glaze components
and that takes time to happen. If you are getting matte surfaces by just
turning off a typical underinsulated electric kiln and letting it cool
naturally there is a very high probability the glaze is not durable. In
that situation, you are more than likely generating the matte surface from
incomplete melting of the glaze or from a very low Si/Al ratio.
If you can't find the recipe for Xavier's Warm Jade Green, go to my web site
(URL below) and click on Glaze Stability.
Regards,
John
on 9/30/01 10:21 AM, Joanne Van Bezooyen at joannevb@EARTHLINK.NET wrote:
> Hi Ruth,
> The following glaze might work for you. Start with it and then make
changes
> It may be too opaque for you. It is blue/green breaking to a golden light
> brown.
> Custer Feldspar.......30
> EPK (kaolin)........30
> Whiting........20
> FF3134......20
> Copper Carbonate.....3.6
> Red Iron Oxide.....2.4
> Rutile.....3.6
> Cobalt Carbonate.....1
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.
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
John Hesselberth on mon 1 oct 01
Hi Wanda,
I didn't mean to imply that low Si/Al ratio is bad in and of itself
(although below 4 might be--I just don't have enough data yet). Down to
about 5 it can be OK if both silica and alumina are high enough. I think
silica should be at least 2.5 and preferably above 3. Alumina should be at
least 0.25 and preferably 0.35 or a little higher. The problem is that when
you go to low Si/Al ratios too many people do it by cutting silica. That's
a good way to guarantee an unstable glaze. If you do it by keeping silica
up, then you have to worry about whether the glaze will melt or not, but
that is a different kind of worry.
By the way, Pete's glaze is a good stable one with respect to leaching. You
can find the results of my tests on the Weathered Bronze Green variant on my
website. It is a very unusual glaze and Pete, himself, cautioned about
using it at cone 6--it is originally a cone 9/10 glaze. But in my tests it
is stable IF you fire it to a full large cone 6 tip touching (that's usually
not small cone 6 in a kiln sitter as I have held forth on ad nauseum in
earlier posts). I have to conclude that strontium is a slightly stronger
flux than is calcium. Does anyone else have data on that? I'm not at all
sure an equivalent glaze with calcium in place of the strontium would melt
at cone 6 without the help of some boron. But I'll also bet Pete's glaze is
unstable as all get out at cone 4 and maybe 5. I think it just makes it
over the line at cone 6. It's a nice glaze, but I have also heard it can be
scratched easily. Maybe you're a little low on firing temperature. Leach
testing would tell you that.
Regards,
John
on 10/1/01 11:57 AM, Wanda Holmes at Alistia at wanda.holmes@ALISTIA.COM
wrote:
> John, you mention "a very low SI:Al ratio". What ration should I look for
> in a stable glaze?
>
> I am using a strontium matte base from Pete Pinell that has a ratio of
> 5.1:1. Most of the limit formulas I'm aware of show the Al to be high and
> the Si to be in range. They do not, however, address the ratio. I have
> discovered that the surface is not as hard as I'd like (I can scratch it
> with a dental tool) and I'm working to correct that. I have not yet
> performed leaching tests, but with this dicussion I'm starting to wonder if
> it is stable. The limit formula is:
>
> Na2O 0.36 Al2O3 0.58 SiO2 2.95
> K2O 0.11 TiO2 0.01
> MgO 0.01 Fe2O3 0.01
> CaO 0.02
> Li2O 0.05
> SrO 0.46
>
> Alumina:Silica ratio is 1.00:5.10
> Neutral:Acid ratio is 1.00:5.12
> Alk:Neut:Acid ratio is 1.00:0.58:2.96
>
> Wanda
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.
Wanda Holmes on tue 2 oct 01
Thanks, John. Very helpful. Let me clarify on the scratching issue. The
Weathered Bronze Green variant does not scratch in my tests. The other
color variants I've tried do. The difference seems to be the TiO2 in the
Weathered Bronze Green. I have tests underway to see how much TiO2 I can
add to the other variants without ruining the color - and to see if the
scratching problem is solved.
I am firing to cone 6, large self-supporting cone, tip touching the kiln
shelf.
I also have tests underway to fire the original glazes to cone 8 to
determine the impact of higher firing on the scratching issue. I want to
use a cobalt and manganese variant on some floor tile, so scratching is a
big concern.
Wanda
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of John Hesselberth
Sent: Monday, October 01, 2001 7:44 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Semi-opaque glaze cone5/6
Hi Wanda,
I didn't mean to imply that low Si/Al ratio is bad in and of itself
(although below 4 might be--I just don't have enough data yet). Down to
about 5 it can be OK if both silica and alumina are high enough. I think
silica should be at least 2.5 and preferably above 3. Alumina should be at
least 0.25 and preferably 0.35 or a little higher. The problem is that when
you go to low Si/Al ratios too many people do it by cutting silica. That's
a good way to guarantee an unstable glaze. If you do it by keeping silica
up, then you have to worry about whether the glaze will melt or not, but
that is a different kind of worry.
By the way, Pete's glaze is a good stable one with respect to leaching. You
can find the results of my tests on the Weathered Bronze Green variant on my
website. It is a very unusual glaze and Pete, himself, cautioned about
using it at cone 6--it is originally a cone 9/10 glaze. But in my tests it
is stable IF you fire it to a full large cone 6 tip touching (that's usually
not small cone 6 in a kiln sitter as I have held forth on ad nauseum in
earlier posts). I have to conclude that strontium is a slightly stronger
flux than is calcium. Does anyone else have data on that? I'm not at all
sure an equivalent glaze with calcium in place of the strontium would melt
at cone 6 without the help of some boron. But I'll also bet Pete's glaze is
unstable as all get out at cone 4 and maybe 5. I think it just makes it
over the line at cone 6. It's a nice glaze, but I have also heard it can be
scratched easily. Maybe you're a little low on firing temperature. Leach
testing would tell you that.
Regards,
John
on 10/1/01 11:57 AM, Wanda Holmes at Alistia at wanda.holmes@ALISTIA.COM
wrote:
> John, you mention "a very low SI:Al ratio". What ration should I look for
> in a stable glaze?
>
> I am using a strontium matte base from Pete Pinell that has a ratio of
> 5.1:1. Most of the limit formulas I'm aware of show the Al to be high and
> the Si to be in range. They do not, however, address the ratio. I have
> discovered that the surface is not as hard as I'd like (I can scratch it
> with a dental tool) and I'm working to correct that. I have not yet
> performed leaching tests, but with this dicussion I'm starting to wonder
if
> it is stable. The limit formula is:
>
> Na2O 0.36 Al2O3 0.58 SiO2 2.95
> K2O 0.11 TiO2 0.01
> MgO 0.01 Fe2O3 0.01
> CaO 0.02
> Li2O 0.05
> SrO 0.46
>
> Alumina:Silica ratio is 1.00:5.10
> Neutral:Acid ratio is 1.00:5.12
> Alk:Neut:Acid ratio is 1.00:0.58:2.96
>
> Wanda
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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