Anthony Allison on sat 3 apr 99
Hi All,
Hoping there is a glaze specialist out there who can help me with a problem.
This glaze does not "fit" my high fire stoneware body and causes cracking
after the pot is taken from the kiln. Sometimes the crack appears right
away, but sometimes it takes a couple of weeks. At any rate it is hoped
that some minor addition can be made without substantially altering the
quality of the glaze.
The current recipe is for cone 10
22% Flint
43% spodumene
17% EPK
18% Whiting
Thanks,
Tony
Paul Lewing on sun 4 apr 99
Tony,
You've got a huge amount of spodumene there,consequently a huge amount
of lithium, and so a very low coefficient of expansion. This is what's
causing your cracking problem. I just ran it through Insight, and I
think if you replaced half of the spodumene with custer spar, you would
raise the COE enough to correct the problem. This is because you'd be
replacing low-expansion lithium with high-expansion sodium and
potassium. You'd still have quite a lot of lithium for a cone 10 glaze,
so maybe the characteristics of this glaze that you like would not be
affected. If this is not enough to fix the problem, you could also try
taking out some of the EPK, as you also have quite a lot of alumina. I
hope this helps.
Paul Lewing, Seattle
Michael Banks on sun 4 apr 99
"Cracking" covers a multitude of sins and needs to be more specific, but
your glaze has a rather low thermal expansion which could be causing the
problem. I suggest replacing some of the spodumene with potash feldspar to
address this.
A suggested recipe is as follows:
19% Flint
33% spodumene
17% EPK
17% Whiting
14% Potash Feldspar
Michael Banks,
Nelson,
New Zealand
----------------------------Original message----------------------------
Tony Allison wrote:
Hoping there is a glaze specialist out there who can help me with a problem.
This glaze does not "fit" my high fire stoneware body and causes cracking
after the pot is taken from the kiln. Sometimes the crack appears right
away, but sometimes it takes a couple of weeks. At any rate it is hoped
that some minor addition can be made without substantially altering the
quality of the glaze.
The current recipe is for cone 10
22% Flint
43% spodumene
17% EPK
18% Whiting
Thanks,
Tony
Craig Martell on sun 4 apr 99
Hello Tony:
The problem is coming from the huge amount of Spodumene in the glaze. Spod
is basically a lithium spar and has a low coefficient or expansion. I
didn't calculate the expansion of this glaze but the cracking you are having
would more than likely indicate that the idea of too much spod is correct.
You need to reduce the amount of spodumene in the glaze until the clay-glaze
stress is relieved. You may want to lower the spod by 5% increments and
replace it with a higher expansion spar. Hopefully, this won't change the
glaze too much. Soda spar and Nepheline Syenite are higher expansion than
potash spar and will give good color.
If you're interested, this is a compression stress crack and totally the
opposite of tension stress, which will cause a glaze to craze, and won't
crack the pot. Normally, if a glaze is under too much compression it will
shiver but your case is extreme, and the pots are shattering.
regards, Craig Martell in Oregon
Fabienne Cassman on mon 5 apr 99
------------------
Hello,
Excessively low expansion cause shivering and excessively high expansion
cause crazing -- that what first came to mind. But, when I plugged his
formula in insight, I turned up a pretty low expansion number which would
suggest shivering could happen rather than crazing. I looked at the
lithium's COE which is indeed much lower than that of sodium and potassium.
Yet, the glaze crazes even before these higher expansion fluxes are
introduced.
It made no sense until I vaguely remembered someone mentioning that lithium
was a two-faced flux=3B it increases the COE if above x =25 (of the total
weight?) and decreases it below that number. Could someone post that
number again? Does anyone have the heart to explain why? I would assume
that in this case we are well above that number since the glaze crazed.
Thank you,
=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=
=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60
Faye http://clay.justnet.com
Yes, I have learned from my mistakes...
I can reproduce them exactly.
David Hewitt on tue 6 apr 99
Tony,
I can confirm Paul's comments about the Lithium being far too high.
Recently I was involved in some tests where we did a progression blend
of adding Lithium Carbonate to a base glaze. The base glaze started with
.14 parts unity formula and incremented up to .41 parts. At .41 parts
Lithium we had severe shivering. My analysis of your glaze is that you
have .39 parts Lithium. I would aim to reduce this by a factor of three
to something like .13 with the changes that Paul suggests.
David
In message , Paul Lewing writes
>----------------------------Original message----------------------------
>Tony,
>You've got a huge amount of spodumene there,consequently a huge amount
>of lithium, and so a very low coefficient of expansion. This is what's
>causing your cracking problem. I just ran it through Insight, and I
>think if you replaced half of the spodumene with custer spar, you would
>raise the COE enough to correct the problem. This is because you'd be
>replacing low-expansion lithium with high-expansion sodium and
>potassium. You'd still have quite a lot of lithium for a cone 10 glaze,
>so maybe the characteristics of this glaze that you like would not be
>affected. If this is not enough to fix the problem, you could also try
>taking out some of the EPK, as you also have quite a lot of alumina. I
>hope this helps.
>Paul Lewing, Seattle
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Stanley Irvin on tue 6 apr 99
Dear Faye,
I don't know about lithium being a two-faced flux but if you have a
material like lithium that has low expansion it means that it also shrinks
less in the cooling cycle. The clay body actually shrinks more than the
glaze, puting the glaze under compression and therefore causing the glaze
to shiver (not crackle).
David Hewitt on thu 8 apr 99
Fabienne,
Your comment about lithium being two-faced is certainly interesting.
In response to am earlier post on this thread I made the following
contribution:-
"I can confirm Paul's comments about the Lithium being far too high.
Recently I was involved in some tests where we did a progression blend
of adding Lithium Carbonate to a base glaze. The base glaze started with
.14 parts unity formula and incremented up to .41 parts. At .41 parts
Lithium we had severe shivering. My analysis of your glaze is that you
have .39 parts Lithium. I would aim to reduce this by a factor of three
to something like .13 with the changes that Paul suggests."
I might have added the following in view of your comments below.
The tests referred to were carried out by Mike Bailey and myself on the
following glaze:-
Soda Feldspar 830
Dolomite 52
Zinc Oxide 33
Quartz 55
Lith. Carb. 30
Lithium Carbonate was added in ten steps of 10 parts with a total of 100
parts giving a final recipe total of 130 parts. This was tested on three
bodies, a buff stoneware, a white stoneware and a porcelain.
On all three bodies, crazing started and progressively increased as the
Lithium increased. However with the buff stoneware at 60 additional
parts and higher, the glaze shivered as well. similarly with the white
stoneware, but not with the porcelain.
If you calculate out the expansion coefficients for this recipe and for
each of the additions you do get an increase in the glaze coefficient.
This does not, however, explain the shivering that was observed. the
results were repeatable.
If anyone does have an explanation for this Mike Bailey and I would
certainly be very interested.
In message , Fabienne Cassman writes
>----------------------------Original message----------------------------
>------------------
>Hello,
>
>Excessively low expansion cause shivering and excessively high expansion
>cause crazing -- that what first came to mind. But, when I plugged his
>formula in insight, I turned up a pretty low expansion number which would
>suggest shivering could happen rather than crazing. I looked at the
>lithium's COE which is indeed much lower than that of sodium and potassium.
> Yet, the glaze crazes even before these higher expansion fluxes are
>introduced.
>
>It made no sense until I vaguely remembered someone mentioning that lithium
>was a two-faced flux=3B it increases the COE if above x =25 (of the total
>weight?) and decreases it below that number. Could someone post that
>number again? Does anyone have the heart to explain why? I would assume
>that in this case we are well above that number since the glaze crazed.
>
>Thank you,
>=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=
>=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60
>Faye http://clay.justnet.com
>
>Yes, I have learned from my mistakes...
> I can reproduce them exactly.
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Fabienne Cassman on thu 8 apr 99
------------------
Hello,
I finally got things straightened out with much help. Thank you all once
again.
To summarize what happened:
I was under the impression that lithium oxide was causing crazing when used
in quantities beyond a fixed amount. However, it has a low coefficient of
expansion which would suggest that used in excess it would make the glaze
shiver. I was also envisioning shivering to be more like the gentle
crawling with some glaze popping off, but nothing drastic. On top of that,
shivering is sometimes described as crackling. Where I got some of those
notions, I do not recall.
Clarifying the difference between compression and tension puts it back into
prospective=3B it did for me at least:
Craig Martell explained it in a wonderful non-technical manner, =22If you
hold a piece of paper at opposite ends and pull real hard, it rips apart
and that is an analogy to crazing. If you push your
hands together, the paper crumples. If the paper was rigid, it would break
or crack and this is from compression. When shivering occurs, the glaze
matrix is literally pushed together so hard and violently that it cracks
and some of the interface is taken into the fracture, which causes chunks
of clay and fused glaze to pop off the pot.=22
I will take the time to do a test tile with a guaranteed shivering glaze so
when I see it described again or in real life, I won't go crazy again.
I also hear that there is something special about lithium carbonate=3B I
don't know what, yet. Hopefully, someone will volunteer some information
some time. :)
Cheers,
=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=
=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60=B0=BA=A4=F8,=B8=B8,=F8=A4=BA=B0=60
Faye http://clay.justnet.com
Yes, I have learned from my mistakes...
I can reproduce them exactly.
Suzanne Furman on mon 17 may 99
Does anyone know of a glaze that would look like shino and fires ^6
electric.....would be very interested in finding such a glaze.....thanks
Suzanne Kraman
Walnut Creek Pottery
Lori Pierce on tue 18 may 99
I have been wondering, but have not yet tried it, if one could do a carbon
trap bisque firing by smoking the hot bisqued pots in a bbq or some similar
way, then glaze and fire in the raku kiln ,Could one do the same to effect a
shino look in an electric kiln....or would the oxidizing firing eliminate
the desired carbon under the shino? Lori in New Port Richey, this is is
addressed to Suzanne in Walnut Creek and all you other knowing people.
-----Original Message-----
From: Suzanne Furman
To: CLAYART@LSV.UKY.EDU
Date: Monday, May 17, 1999 11:34 AM
Subject: Glaze help needed
>----------------------------Original message----------------------------
>Does anyone know of a glaze that would look like shino and fires ^6
>electric.....would be very interested in finding such a glaze.....thanks
>Suzanne Kraman
>Walnut Creek Pottery
>
Cindy Strnad on sun 18 mar 01
Hi, Sibylle.
While no one can tell you whether your glaze will leach chemicals into food
without laboratory testing, the preliminary tests you've done do sound good.
According to the limit formulas for Insight, it's a bit high in both silica
and alumina--maybe could stand to be fired hotter. But, if it's working for
you . . .
Check out http://www.frogpondpottery.com for information on how to have your
glaze tested for leachate. John Hessleberth has put together a very
informative web page there, which I'm sure will be of help to you.
Here is the Insight analysis, using the Material Data Table supplied with
Insight. I used Custer as the potash spar. Your analysis will be different,
as your materials will be different, but this should give you a general
idea.
CUSTER FELDSPAR..... 49.00 44.95%
SILICA.............. 25.00 22.94%
WHITING............. 20.00 18.35%
EPK KAOLIN.......... 15.00 13.76%
========
109.00
CaO 0.72* 11.61%
MgO 0.00* 0.02%
K2O 0.19* 5.19%
Na2O 0.08* 1.46%
TiO2 0.00 0.06%
Al2O3 0.49 14.44%
P2O5 0.00 0.04%
SiO2 3.90 67.01%
Fe2O3 0.00 0.18%
Cost/kg 0.41
Si:Al 7.88
SiB:Al 7.88
Expan 7.29
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
sibylle on sun 18 mar 01
Hello to all,
I need some information about the food safety of a glaze.
The recipe is:
potassium-feldspar 40%
flint 25%
whiting 20%
kaolin 15%
We add:
cobalt oxide 2,5%
RIO 3 %
chrome oxide 0,375%
manganese 0,3%
We fire in an electric kiln oxidation, at temperatures between cone 6
and 7, at 1250°C.
It is a wonderfull, not too glossy nor to matt deep black glaze.
We tested it in the dishwasher for about 30 times, had some acid in for
72 hours and did scratch it with a sharp knife as hard as we could do.
Everything seems to be good. No pinholing.
Because of the manganese and chrome, we only use it for decorating our
pots outside, not as a liner glaze. I would like to know, if there is
any need for a glaze calculation. (I don't have one, for here in germany
it is not possible to buy one for a macintosh computer.) And if you,
glaze gurus, think it is neccessary to get the additional information
about food safety, could someone please calculate it for me?
I'm sorry, I can't give you the seger formula. The glaze was developed
from a friend of mine . I'm sure she would be very happy if someone else
in the world will use it, so if you want, try, test.
Thank you for helping.
Sibylle
John Hesselberth on sun 18 mar 01
Hello Sibylle,
I'm glad to see you are still on Clayart. I have put your glaze into
HyperGlaze (which is a Macintosh program) and here is what it looks like.
Cone: 6,7
Testing: Untested
Surface texture: Matte
Firing: Ox.=20
=20
Recipe: Percent
Potash Feldspar 40.00
Kaolin 15.00
Whiting 20.00
Silica 25.00
100.00 %Total
Also add:
Cobalt Oxide 2.50
Red Iron Oxide 3.00
Manganese dioxide 0.30
Chromium Oxide 0.38
Possible Health Hazards:
Silica: free silica-wear a NIOSH approved dust mask when handling dry
material
Manganese Dioxide: TOXIC-avoid inhalation-wear a NIOSH approved dust mask
when handling dry material
Chromium Oxide: all chromium compounds should be viewed as potentially
carcinogenic
Unity Formula for Sibylle:
0.168 K2O 0.466 Al2O3 3.734 SiO2
0.073 Na2O 0.002 Fe2O3 0.003 TiO2
0.751 CaO 0.001 P2O5 8.0:1 Si:Al Ratio
0.007 MgO=20
Weight Percentage Analysis:
66.90 % SiO2
14.17 % Al2O3
4.72 % K2O
1.35 % Na2O
12.56 % CaO
0.08 % MgO
0.10 % Fe2O3
0.04 % P2O5
0.07 % TiO2
As you can see this glaze has plenty of silica and alumina and a reasonably
good balance of fluxes (some people would say it is high in calcium, but I
have seen stable glazes with more calcium than this). However I am very
concerned that it is matte because it is not fully melted rather than matte
because of recrystallization of some of the components in the glaze as it
cools. With no boron at all, I would expect this glaze might be a cone 10
glaze rather than a cone 6-7 glaze. And I would doubt that it is matte at
cone 10 with a Si/Al ratio of 8.0/1.
I would not personally use this glaze on food surfaces without having it
leach tested and seeing low leaching results. I am guessing that it will
leach quite a bit unless you fire it to a higher temperature.
Regards, John
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
> From: sibylle
> Reply-To: Ceramic Arts Discussion List
> Date: Sun, 18 Mar 2001 22:00:15 +0100
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: glaze help needed
>=20
> Hello to all,
>=20
> I need some information about the food safety of a glaze.
> The recipe is:
>=20
> potassium-feldspar 40%
> flint 25%
> whiting 20%
> kaolin 15%
>=20
> We add:
> cobalt oxide 2,5%
> RIO 3 %
> chrome oxide 0,375%
> manganese 0,3%
>=20
> We fire in an electric kiln oxidation, at temperatures between cone 6
> and 7, at 1250=B0C.
>=20
> It is a wonderfull, not too glossy nor to matt deep black glaze.
> We tested it in the dishwasher for about 30 times, had some acid in for
> 72 hours and did scratch it with a sharp knife as hard as we could do.
> Everything seems to be good. No pinholing.
>=20
> Because of the manganese and chrome, we only use it for decorating our
> pots outside, not as a liner glaze. I would like to know, if there is
> any need for a glaze calculation. (I don't have one, for here in germany
> it is not possible to buy one for a macintosh computer.) And if you,
> glaze gurus, think it is neccessary to get the additional information
> about food safety, could someone please calculate it for me?
>=20
> I'm sorry, I can't give you the seger formula. The glaze was developed
> from a friend of mine . I'm sure she would be very happy if someone else
> in the world will use it, so if you want, try, test.
>=20
> Thank you for helping.
>=20
> Sibylle
>=20
> _________________________________________________________________________=
_____
> Send postings to clayart@lsv.ceramics.org
>=20
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>=20
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
james.e.champion@US.ARMY.MIL on thu 2 jun 05
It's an adventuresom live being dsylicic!
I've been mixing test glazes from a few sources,
one of which is cone 6 glazes by Bailey.
Using the T13 base glaze
Soda feldspar (F-4) 43.0%
China clay (EPK) 5.0%
Bentonite 2.0%
Whiting 22.0%
Zinc Oxide 6.0%
Flint or Quartz 21.5%
(note that my base glaze is off by 0.5% on the whiting.
The book calls for 22.5% of whiting.)
I added what I thought to be Manganese dioxide
to the base glaze of 500 milgrams
I mix three of these base glazes,
which in reality I mixed 4% Magnesium carbonate in one,
6.0% Magnesium carbonate in another,
and 8.0% Magnesium carbonate in the last one.
I know that Magnesium carbontae provides an active flux at high temperatures,
produces matt surfaces, increases glaze viscosity,
and improves adhesion of glaze to clay.
I also know that Manganese dioxided is a fine particle ceramic colorant,
which usually produces browns in glazes
but can give purples in combination with cobalt or alkaline fluxes.
How do I fix this glaze?
When correcrted Will the Magnesiun carbonate provide an alkaline fluxe truning what
I hoped to be a tan to brown glaze to purple?
If so can the base glaze e correcrted and then
another colorent be added instead of the Manganese dioxided?
Daniel Semler on sat 4 jun 05
Hi James,
I have some cone 10 data that may help but I don't know.
I have a low MgO gloss base I've been blending colours into. Its got 0.47%
(0.04 molar equivs) MgO and 11.4% (0.74 molar equivs) CaO. I got nice honey/tan
browns between 5 and 10% MnO2. From what I've read you might be pretty safe from
getting purple at cone 6. In the end you'll need to try it.
Given toxicity concerns with high manganese concentrations you might want to
try another colourant, but your concentration seems low - see question below.
Your post sounds like you have already mixed tests. What were the results ?
Also, what was your intention when adding the mag carb. ? If you're worried
about it turning the glaze purple what was the reason you added it ?
One final note. Simply adding mag carb. is reducing the silica and alumina
content to a point where durability may become a concern for functional use.
Actually, a second final thing. By "500 milgrams", do you mean 0.5 gram ? If
so, with the additions of 4-8% mag carb. this below 0.5%. I got no colour with
this concentration of MnO2.
Thanx
D
karen gringhuis on sat 4 jun 05
Jim -
One doesn't have to be dyslexic to make your mistake!
Better what you did than the other way around.
Try just throwing in the MANGANESE now and going for
it. The MAGnesium carb. will make the glaze a bit
more matt than without it but shouldn't affect it
drastically.
Keep good notes about your "revised" version - you may
have a winner.
Good luck.
---------------------
It's an adventuresom live being dsylicic!
I've been mixing test glazes from a few sources,
one of which is cone 6 glazes by Bailey.
Using the T13 base glaze
Soda feldspar (F-4) 43.0%
China clay (EPK) 5.0%
Bentonite 2.0%
Whiting 22.0%
Zinc Oxide 6.0%
Flint or Quartz 21.5%
(note that my base glaze is off by 0.5% on the
whiting.
The book calls for 22.5% of whiting.)
I added what I thought to be Manganese dioxide
to the base glaze of 500 milgrams
I mix three of these base glazes,
which in reality I mixed 4% Magnesium carbonate in
one,
6.0% Magnesium carbonate in another,
and 8.0% Magnesium carbonate in the last one.
I know that Magnesium carbontae provides an active
flux at high temperatures,
produces matt surfaces, increases glaze viscosity,
and improves adhesion of glaze to clay.
I also know that Manganese dioxided is a fine particle
ceramic colorant,
which usually produces browns in glazes
but can give purples in combination with cobalt or
alkaline fluxes.
How do I fix this glaze?
When correcrted Will the Magnesiun carbonate provide
an alkaline fluxe truning what
I hoped to be a tan to brown glaze to purple?
If so can the base glaze e correcrted and then
another colorent be added instead of the Manganese
dioxided?
Karen Gringhuis
KG Pottery
Box 607 Alfred NY 14802
__________________________________
Yahoo! Mail
Stay connected, organized, and protected. Take the tour:
http://tour.mail.yahoo.com/mailtour.html
Daniel Semler on sun 5 jun 05
Hi James,
I see now, following Karen Gringhuis' post, that I completely misread your
post. My apologies.
I suspect the higher mag carb (6 and 8) may move a bit. The 8 is going to be
short on silica and alumina for cone 6 and the 4 is low. I'm not that concerned
about the colour - it may be ok. If this is a small test batch fire it up and
see. Then try the way you intended it and see how that is.
Thanx
D
mel jacobson on wed 10 jan 07
this is forward.
HELP WITH BUTTERMILK GLAZE
I googled buttermilk glaze and found an old plea for the recipe which
someone had lost. I found this recipe in the !976 December CM and
have used it for around 20 years. It was one of my main, reliable glazes. I
have been having lots of trouble with it for the last 2 years. I keep
testing it. This is the recipe:
BUTTERMILK ^9-10
Gerstley Borate-----10.5
Dolomite------------ 6.8
Whiting--------------9.0
Custer Feldspar----29.3
EPK--------------- -6.8
Talc----------------13.5
Flint----------------24.1
Zircopax-------------8.0
The glaze gets bubbles on it. they aren't popped bubbles. If I fire it
really hot, say ^11, it will melt right. This is hard for me to do because
my other glazes can't get that hot. I would actully LIKE to bring this
recipe down to a real cone 9 but I really need it to work at cone 10 like
it always used too. This is supposedly a cone 9 glaze. I historically fired
it to hard cone 10. I thought maybe the ingredients changes were making a
difference--the gerstley borate change to gillespie, but I recently came
across
300 # of the old gerstley borate and tested it again and it still turned
out weird. Any ideas out there? Anyone having similar problems with this
glaze? Diane Millner
In response to a question about Reitz green from Chris Trabka On Dec 11,
2006, at 6:36 PM
I have used this glaze alot. I find it works better when there is no iron
in the clay body. I also add .5 lithium and epsom salts.
REITZ GREEN-- a hunter green with blue where thick
Neph. Sey--------70
Whiting-----------6
Gerstley Borate--2(Gillespie)
Petalite-----------15
Geo Kaolin-------8
cobalt carb----1
rutile-----------1
lithium --------.5
from: mel/minnetonka.mn.usa
website: http://www.visi.com/~melpots/
Clayart page link: http://www.visi.com/~melpots/clayart.html
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