URL Krueger on sun 17 apr 05
A while back John H. said you could reduce the working
temperature of a glaze one cone by adding in 0.07 Boron.
This got me to thinking and I have a question regarding the
Si:Al ratio.
I understand that the stiffness of a glaze at a specific
cone will vary from runny to stiff as the Si:Al ratio is
decreased. But if you keep the Si:Al ratio the same does
the stiffness remain the same as you vary the cone?
Lets say I want to take Leach's 4:3:2:1 cone 10 recipie that
is high in CaO (0.76) but otherwise falls within the cone 6
limits from MC6G and add Boron to bring it down to cone 6.
If I replace some silica with 0.28 B203 so the SiB:Al ratio
is the same as the original Si:Al ratio will the resultant
glaze have the same "stiffness" at cone 6 as the original
did at cone 10?
What if I used zinc instead of boron. Would keeping the
Si:Al ratio the same result in the same stiffness?
--
Earl K...
Bothell WA, USA
John Hesselberth on sun 17 apr 05
On Sunday, April 17, 2005, at 03:27 PM, URL Krueger wrote:
> If I replace some silica with 0.28 B203 so the SiB:Al ratio
> is the same as the original Si:Al ratio will the resultant
> glaze have the same "stiffness" at cone 6 as the original
> did at cone 10?
Hi Earl,
Maybe, maybe not. Glaze can often be adjusted a cone or 2 fairly
easity. Sometimes they can be adjusted further, but not always. The
guideline I suggested will probably work 75% of the time over a 2 cone
range, but maybe only a third of the time over a 4 cone range. Glazes
are just too complex totally reduce to numbers given our current
knowledge. The numbers are very helpful in many cases, but not always.
That is part of what makes them so fascinating.
Regards,
John
Ron Roy on wed 20 apr 05
Hi Earl,
Take a look under surface tension and viscosity in the Hamer book to see
which oxides lower or raise viscocity.
If you keep the ratio (SiO2/Al2O3) but change cones - you will not have the
same viscosity - ratio is not a factor in viscosity and surface tension.
You will find a number of lists in the hamer book - even a periodic table
which shows which oxides have high and low viscosities.
To stop running you can add alumina for instance - it is at the top of the
viscosity list - in that it has a high viscosity. So we are advised to
increase it to cure running. Some times that is not what we need to we need
then to look at the fluxes - MgO stiffens a glaze for instance - subbing in
MgO for KNaO and Li20 for instance will help stop running.
Silica is neutral in both lists by the way - so you can see why ratio is
not an important factor in all this.
Boron is on the running side of neutral in both lists by the way - so I
would expect you would decrease stiffness if that is all you do and keep
the ratio the same.
Hope I answered the right question - RR
>A while back John H. said you could reduce the working
>temperature of a glaze one cone by adding in 0.07 Boron.
>This got me to thinking and I have a question regarding the
>Si:Al ratio.
>
>I understand that the stiffness of a glaze at a specific
>cone will vary from runny to stiff as the Si:Al ratio is
>decreased. But if you keep the Si:Al ratio the same does
>the stiffness remain the same as you vary the cone?
>
>Lets say I want to take Leach's 4:3:2:1 cone 10 recipie that
>is high in CaO (0.76) but otherwise falls within the cone 6
>limits from MC6G and add Boron to bring it down to cone 6.
>If I replace some silica with 0.28 B203 so the SiB:Al ratio
>is the same as the original Si:Al ratio will the resultant
>glaze have the same "stiffness" at cone 6 as the original
>did at cone 10?
>
>What if I used zinc instead of boron. Would keeping the
>Si:Al ratio the same result in the same stiffness?
>
>--
>Earl K...
>Bothell WA, USA
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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