Arnolds Home Improvements on tue 27 aug 02
Hello All !!!!!!!!
I have a clear cone 6 glaze that works and looks great except on bowls,
where it crazes like crazy. All other shapes no problem.
I bisque to cone 08 and fire to cone 6, use standards 119 and 181 stoneware
clay. I have heard that you can add silica to a glaze to stop crazing, but
if you add silica how much and when you add silica do you have to remove
some of something else? If so what?
Here is the recipe if someone could take a look and see if they can spot a
problem I sure would appreciate it !!!
Clear cone 6
Frit 3134 21
Nepheline Syenite 19
Wollastonite 20
EPK 16
Silica 24
Thanks in advance for any help anyone can offer!!!!!!!!!!!
Gene Arnold
mudduck@advi.net
iandol on wed 28 aug 02
Dear Gene Arnold,
My gut reaction is to suggest substituting a Potash felspar for the Neph =
Sy, 1 to 1, as a first step. This takes Sodium, which is notorious for =
causing crazing out of the equation. Because you are using a fair amount =
of frit this will provide the melting environment so I do not see there =
being a great change in maturing temperature.
If you want to go the Silica addition way, do a line blend with 2% =
increments up to about ten percent. This may move the cone value =
upwards.
Best regards,
Ivor Lewis
Ababi on wed 28 aug 02
Hello Arnold the glaze I believe is a " daughter glaze" of Tony Hansen's 5/20
5 20 TONY HANSEN
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Cone deg.C. -
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
EPK Kaolin 20.00
Custer feldspar 20.00
Wollastonite 20.00
FRIT 3134 20.00
Flint 20.00
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Seger Weight%
KNO 0.220 4.94%
CaO 0.753 12.99%
MgO 0.027 0.33%
Al2O3 0.376 11.76%
P2O5 0.001 0.05%
B2O3 0.224 4.79%
SiO2 3.522 65.07%
TiO2 0.003 0.08%
K2O 0.075 2.16%
Na2O 0.146 2.77%
Al:Si 9.38
Expan. 6.98
ST 352.35
I use the Insight's expansion in matrix. Probably a bit different but not a big deal
Now yours clear
I belive the problem is with the Nepheline S.
CLEAR ^6
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Cone deg.C. -
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
EPK Kaolin 16.00
Nepheline Syenite 19.00
Wollastonite 20.00
FRIT 3134 21.00
Flint 24.00
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Seger Weight%
KNO 0.241 5.12%
CaO 0.733 13.20%
MgO 0.027 0.34%
Al2O3 0.342 11.18%
P2O5 0.001 0.04%
B2O3 0.224 5.01%
SiO2 3.371 65.05%
TiO2 0.002 0.06%
K2O 0.032 0.96%
Na2O 0.209 4.16%
Al:Si 9.87
Expan. 7.21
ST 348.39
You have a little less alumina
More KNO
Less CaO
and a little less silica.
From my many RAKU tests I learnt that it was very easy to get crackles inside a bawl
while much harder in the outside. My English is not reach enough to explain you why it
is like stretching on the outside and kind of folding, breaking in the inside.
The first thing I would try is to change the recipe into the original 20/5
Or look for a new base
Ababi Sharon
Glaze addict
Kibbutz Shoval Israel
localpotter@walla.co.il
http://members4.clubphoto.com/ababi306910/
http://www.milkywayceramics.com/cgallery/asharon.htm
---------- Original Message ----------
>Hello All !!!!!!!!
>I have a clear cone 6 glaze that works and looks great except on bowls,
>where it crazes like crazy. All other shapes no problem.
>I bisque to cone 08 and fire to cone 6, use standards 119 and 181 stoneware
>clay. I have heard that you can add silica to a glaze to stop crazing, but
>if you add silica how much and when you add silica do you have to remove
>some of something else? If so what?
>Here is the recipe if someone could take a look and see if they can spot a
>problem I sure would appreciate it !!!
>Clear cone 6
>Frit 3134 21
>Nepheline Syenite 19
>Wollastonite 20
>EPK 16
>Silica 24
>Thanks in advance for any help anyone can offer!!!!!!!!!!!
>Gene Arnold
>mudduck@advi.net
>______________________________________________________________________________
>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 wed 28 aug 02
Hi Gene,
It is a little surprising that this glaze is crazing--maybe your clays
are a little out of the normal range in expansion.
I would do 2 things. First I would increase silica to 35 (without
lowering the other numbers) and then do a line blend with your original.
With silica at 35 you will surely lower expansion enough to get out of
crazing and the glaze should still melt.
Crazing can also be caused by uneven or too rapid cooling. If, for
example, you were to blow on the surface of the glaze after it had
solidified but while it was still very hot you can precipitate crazing
even if the glaze fits the clay very well. This is more likely to
happen with thin coats of glaze . It happens because the glaze shrinks
(cools) sooner than the clay and isn't strong enough to resist
cracking. Practitioners of raku do this on purpose to get a more
crackled surface. So the second thing I would do is put on a thicker
coat of glaze on your bowls and make sure they are buried in the middle
of the kiln where they will cool more uniformly. For sure don't put
them on the top shelf and crack the lid early.
Good luck.
Regards,
John
On Tuesday, August 27, 2002, at 07:46 PM, Arnolds Home Improvements
wrote:
> Hello All !!!!!!!!
>
> I have a clear cone 6 glaze that works and looks great except on bowls,
> where it crazes like crazy. All other shapes no problem.
>
> I bisque to cone 08 and fire to cone 6, use standards 119 and 181
> stoneware
> clay. I have heard that you can add silica to a glaze to stop crazing,
> but
> if you add silica how much and when you add silica do you have to remove
> some of something else? If so what?
>
> Here is the recipe if someone could take a look and see if they can
> spot a
> problem I sure would appreciate it !!!
>
> Clear cone 6
> Frit 3134 21
> Nepheline Syenite 19
> Wollastonite 20
> EPK 16
> Silica 24
>
>
>
Margaret Barlow on thu 29 aug 02
Hello Gene,
Here is how I see this problem.
First I see you are using 2 different clays. If you are using a different
clay for the bowls that could be the reason but if it is crazing on both
clay bodies and only on the bowls I do not know why except it might be
application. And if this glaze is crazing on bowls it is probably close to
crazing, and you may even get some delayed crazing, on other forms. I put
this glaze on Insight and this is how it looks.
FRIT 3134........... 21.00
NEPHELINE SYENITE... 19.00
WOLLASTONITE........ 20.00
EPK KAOLIN.......... 16.00
SILICA.............. 24.00
========
100.00
CaO 0.73* 13.21%w 14.83%m
MnO2 0.00 0.01 0.00
MgO 0.03* 0.35 0.54
K2O 0.03* 0.97 0.65
Na2O 0.21* 4.21 4.28
Fe2O3 0.00 0.15 0.06
TIO2 0.00 0.05 0.04
B2O3 0.22 5.00 4.52
AL2O3 0.34 11.14 6.88
SiO2 3.36 64.92 68.18
P2O5 0.00 0.02 0.01
COST/KG 1.51
Si:Al 9.91
SiB:Al 10.56
EXPAN447.20
As you can see the expansion of this glaze is 447.2 and that should be good
for most ^6 clays.
Since crazing is caused when the expansion of the glaze is too high for the
body you need to lower the expansion of the glaze. This can be done by just
adding more silica but that will make the glaze shinier. This glaze already
appears to be quite shiny. Adding more alumina will not only help lower the
expansion but also maintain the silica/alumina ratio which will maintain the
same gloss. There are several other ways to lower the expansion but if you
also use this glaze with colours this is the best way to maintain the same
colours.
Here is a revision you might want to try which lowers the expansion to
419.89
FRIT 3134........... 19.50
NEPHELINE SYENITE... 17.50
WOLLASTONITE........ 18.50
EPK KAOLIN.......... 17.50
SILICA.............. 27.00
========
100.00
CaO 0.73* 12.26%w 13.78%m
MnO2 0.00 0.01 0.00
MgO 0.03* 0.33 0.51
K2O 0.03* 0.90 0.60
Na2O 0.21* 3.90 3.97
Fe2O3 0.00 0.16 0.06
TIO2 0.00 0.05 0.04
B2O3 0.22 4.65 4.21
AL2O3 0.37 11.36 7.02
SiO2 3.70 66.37 69.78
P2O5 0.00 0.02 0.01
COST/KG 1.47
Si:Al 9.94
SiB:Al 10.54
EXPAN419.89
Since by doing this the glaze becomes more refractory it may not melt as
well. If this is the case we can increase the boron to help with the melt
If you decided to test this revision, please let me know the results and if
you still have problems, you may contact me off list and we can work on it.
To test it you might want to do a simple line blend with the original to see
where the crazing ends.
Just another observation, this glaze is quite high in Calcium which with a
slow cooling could cause the glaze to be less clear that is cause it to matt
somewhat.
Margaret
----- Original Message -----
From: "Arnolds Home Improvements"
To:
Sent: Tuesday, August 27, 2002 7:46 PM
Subject: Glaze Craze
> Hello All !!!!!!!!
>
> I have a clear cone 6 glaze that works and looks great except on bowls,
> where it crazes like crazy. All other shapes no problem.
>
> I bisque to cone 08 and fire to cone 6, use standards 119 and 181
stoneware
> clay. I have heard that you can add silica to a glaze to stop crazing, but
> if you add silica how much and when you add silica do you have to remove
> some of something else? If so what?
>
> Here is the recipe if someone could take a look and see if they can spot a
> problem I sure would appreciate it !!!
>
> Clear cone 6
> Frit 3134 21
> Nepheline Syenite 19
> Wollastonite 20
> EPK 16
> Silica 24
>
> Thanks in advance for any help anyone can offer!!!!!!!!!!!
>
>
>
> Gene Arnold
> mudduck@advi.net
>
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