May Luk on mon 9 aug 04
Hi all;
I have this matte white glaze I need some advice on;
Cone 8 Ox
Matt white [Original]
Potash Spar______60
Whiting__________7
Dolomite_________13
China Clay_______20
Na2O______0.091
K2O_______0.238
MgO_______0.213
CaO_______0.457
Al2O3_____0.576
Fe2O3_____0.003
SiO2______2.537
TiO2______0.001
Si:Al_____4.40
COE_______75.52
I wanted this glaze to be more stable and made some changes in the Si:Al
ratio and added some boron for melt. Slow cooled: 80C/hr from 1080 - 800
Matt white [V.1]
Potash Spar__________37
Whiting______________4
Dolomite_____________12
China Clay___________13
Standard Borax Frit__17
Quartz_______________17
Na2O______0.137
K2O_______0.159
MgO_______0.215
CaO_______0.489
Al2O3_____0.426
B2O3______0.138
Fe2O3_____0.002
SiO2______3.037
TiO2______0.000
Si:Al_____7.12
COE_______68.91
I just got the glaze test and I found the tiles glossy. I would like to know
what my next testing should be? Line blend of upping the alumina and varying
the borax frit?
Another version of the test I'd like to do is using Zinc oxide in place of
Boron to see the differences in their properties [and colour responses when
I add colours later on]. I'm unclear of the percentage to use. Let's say I
have 0.138 of B2O3 in Version 1. Should I start with 0.138 of ZnO as a
starting point?
Thank you in advance for all your suggestions
May
London, UK
Ivor and Olive Lewis on tue 10 aug 04
Dear May,
You say this glaze opaque is unstable. Do you mean that it is leaching
or have you some other vice in mind when you say "unstable".
In other words, what are you trying o achieve?.
I would need more information from you before commenting upon your
mixtures and your unity analyses.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
May Luk on tue 10 aug 04
Dear Ivor;
I wasn't very clear in stating my intention. Now that I slowed down and
think about it - I'd like it to be more within limits.
The original glaze has this rough dry texture and it doesn't appear to be a
true matte glaze. I see that the silica is a bit low for the limits and I
like it to melt better and be a bit smoother, (satiny would be good). For my
tests, I went by John H's advice to start with a Si:Al ratio of 6 to 7 and
add some boron or zinc for the melt. With Version 1, I gone too far and came
out with a more transparent glossy glaze.
What you reckon?
Thanks
May
London, UK
John Hesselberth on tue 10 aug 04
Hi May,
Some thoughts on your 2 glazes. The first one is matte because of the
very low Si/Al ratio. I've never been able to make a stable matte glaze
this low. So you correctly raised the ratio in your modified glaze, but
you probably went a bit too far. Stopping between 6 and 7 might have
been better, although doing that by itself will still not get you a
matte glaze, because there is another problem. The low silica/alumina
ratio gives you what is often called an alumina matte. I sometimes call
them unmelted mattes. Whatever the name, it is probably a crystal
containing aluminum that is precipitating out (or not melting in the
first place -- a more likely scenario in my view) that is giving the
matte surface.
When you raise the Si/Al ratio you can no longer count on having the
matte surface formed via an aluminum containing crystal. You have to
look elsewhere for something that is present in high enough
concentration to crystallize out on cooling. The best way to do this,
in my experience, is to boost up the concentration of one of the
alkaline earths -- calcium, magnesium, strontium, or barium. You can
also do it with zinc. Glazes made this way I have found to be stable if
they are fully melted during firing and not overloaded with colorants.
In your modified glaze you don't have a high enough concentration of
any of the alkaline earths to get this effect. Calcium needs to be .8
or above. Mg maybe .35 or .4 or above. I don't have experience with
barium, but I think you can get the matte surfaces at about 0.3 or
above. Strontium is a little more complex and seems to be highly
dependent on the colorants you use. So as a next experiment, I would
suggest taking MgO up to 0.4-0.45 by giving up some calcium. I think
you will get your matte surface back, although it may look different
than the original. By now you have developed an entirely new glaze--not
just tweaked an existing one.
I hope this helps--it is rather complex to explain and I may not have
hit on a way of saying it that is understandable. If not, ask again.
Regards,
John
On Monday, August 9, 2004, at 05:18 AM, May Luk wrote:
> Hi all;
>
> I have this matte white glaze I need some advice on;
> Cone 8 Ox
> Matt white [Original]
> Potash Spar______60
> Whiting__________7
> Dolomite_________13
> China Clay_______20
>
> Na2O______0.091
> K2O_______0.238
> MgO_______0.213
> CaO_______0.457
> Al2O3_____0.576
> Fe2O3_____0.003
> SiO2______2.537
> TiO2______0.001
> Si:Al_____4.40
> COE_______75.52
>
> I wanted this glaze to be more stable and made some changes in the
> Si:Al
> ratio and added some boron for melt. Slow cooled: 80C/hr from 1080 -
> 800
>
> Matt white [V.1]
> Potash Spar__________37
> Whiting______________4
> Dolomite_____________12
> China Clay___________13
> Standard Borax Frit__17
> Quartz_______________17
>
> Na2O______0.137
> K2O_______0.159
> MgO_______0.215
> CaO_______0.489
> Al2O3_____0.426
> B2O3______0.138
> Fe2O3_____0.002
> SiO2______3.037
> TiO2______0.000
> Si:Al_____7.12
> COE_______68.91
>
> I just got the glaze test and I found the tiles glossy. I would like
> to know
> what my next testing should be? Line blend of upping the alumina and
> varying
> the borax frit?
>
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on wed 11 aug 04
Dear May Luk,
I am sending you a private message with attachments that cannot be
opened on Clayart. The intellectual content of what I have to say
cannot be appreciated without these illustrations.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
.
David Hewitt on wed 11 aug 04
May,
As others have said the Si:Al ratio appears to be too high for what you
want.
Also I see no need for adding a borax frit to a cone 8 glaze.
By adding, say, 20 parts silica to your original recipe the Si:Al ratio
becomes 5.9, which may be quite suitable for a shiny glaze.
I would try this and then do a progression blend with alumina hydrate to
get an alumina matt of your choice or a similar progression blend with
magnesium carbonate to get an alk. earth matt.
If any of this appeals to you o test I would for my own information and
education be pleased to know hoe it works out.
David
In message , May Luk writes
>Hi all;
>
>I have this matte white glaze I need some advice on;
>Cone 8 Ox
>Matt white [Original]
>Potash Spar______60
>Whiting__________7
>Dolomite_________13
>China Clay_______20
>
>Na2O______0.091
>K2O_______0.238
>MgO_______0.213
>CaO_______0.457
>Al2O3_____0.576
>Fe2O3_____0.003
>SiO2______2.537
>TiO2______0.001
>Si:Al_____4.40
>COE_______75.52
>
>I wanted this glaze to be more stable and made some changes in the Si:Al
>ratio and added some boron for melt. Slow cooled: 80C/hr from 1080 - 800
>
>Matt white [V.1]
>Potash Spar__________37
>Whiting______________4
>Dolomite_____________12
>China Clay___________13
>Standard Borax Frit__17
>Quartz_______________17
>
>Na2O______0.137
>K2O_______0.159
>MgO_______0.215
>CaO_______0.489
>Al2O3_____0.426
>B2O3______0.138
>Fe2O3_____0.002
>SiO2______3.037
>TiO2______0.000
>Si:Al_____7.12
>COE_______68.91
>
>I just got the glaze test and I found the tiles glossy. I would like to know
>what my next testing should be? Line blend of upping the alumina and varying
>the borax frit?
>
>Another version of the test I'd like to do is using Zinc oxide in place of
>Boron to see the differences in their properties [and colour responses when
>I add colours later on]. I'm unclear of the percentage to use. Let's say I
>have 0.138 of B2O3 in Version 1. Should I start with 0.138 of ZnO as a
>starting point?
>
>Thank you in advance for all your suggestions
>
>May
>London, UK
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
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