Corinne P. Null on sat 7 aug 99
Ron, Tom, and other glaze guru's,
I've been using the 138 White glaze listed here as a base for cone 10
oxydation (or reduction) but find that it is crazing. It's really a lovely
crazing pattern that I would love if it weren't on the inside of functional
work, or the base for second glazes. I also use the same glaze, with the
addition of 4% RIO, and 4% milled rutile, which creates a mottled slightly
runny mustardy-gold, which has not the slightest intention of crazing.
That tells me that the fluxes are having quite an effect on this glaze.
So, what can I do to the 138 White to stop the crazing? I've heard "Just
add more silica" but my glaze calculation program is not helping me with
this one!
138 White
===========
CORNWALL STONE...... 50.00 50.00%
BALL CLAY........... 15.00 15.00%
DOLOMITE............ 20.00 20.00%
BONE ASH............ 10.00 10.00%
FLINT............... 5.00 5.00%
========
100.00
CaO 0.58* 14.27%
MgO 0.29* 5.06%
K2O 0.06* 2.59%
Na2O 0.07* 1.93%
TiO2 0.01 0.34%
Al2O3 0.33 14.60%
P2O5 0.08 5.18%
SiO2 2.12 55.78%
Fe2O3 0.00 0.25%
Cost/kg 2.00
Si:Al 6.49
SiB:Al 6.49
Expan 6.80
Appreciate your help,
Corinne Null
Bedford, New Hampshire
null@mediaone.net
David Hewitt on mon 9 aug 99
Corrine,
I find your experience interesting in that it is line with a point that
Mike Bailey and I raised on Clayart a few months back when we were
experiencing a reduction in crazing with just adding colouring oxides to
a particular base glaze.
To help with your problem though. Yes I think that adding more silica
will reduce the tendency to craze. If, for example, you add 20 parts
instead of the 5 in your recipe the glaze coefficient of expansion
reduces from 5.79 to 5.03 English & Turner x10-6/oC or from 7.11 to 6.30
x10-6/oC using Appen's coefficients.
This would suggest that, trying a progression blend of adding silica in
additions of 5 parts up to say a total of 25 parts to your glaze, would
be a sensible test. If one of these shows a point where the crazing has
disappeared and the glaze has not changed its character in some other
way, then you have an answer.
David
In message , Corinne P. Null writes
>----------------------------Original message----------------------------
>Ron, Tom, and other glaze guru's,
>
>I've been using the 138 White glaze listed here as a base for cone 10
>oxydation (or reduction) but find that it is crazing. It's really a lovely
>crazing pattern that I would love if it weren't on the inside of functional
>work, or the base for second glazes. I also use the same glaze, with the
>addition of 4% RIO, and 4% milled rutile, which creates a mottled slightly
>runny mustardy-gold, which has not the slightest intention of crazing.
>That tells me that the fluxes are having quite an effect on this glaze.
>
>So, what can I do to the 138 White to stop the crazing? I've heard "Just
>add more silica" but my glaze calculation program is not helping me with
>this one!
>
>138 White
>
>===========
> CORNWALL STONE...... 50.00 50.00%
> BALL CLAY........... 15.00 15.00%
> DOLOMITE............ 20.00 20.00%
> BONE ASH............ 10.00 10.00%
> FLINT............... 5.00 5.00%
> ========
> 100.00
>
> CaO 0.58* 14.27%
> MgO 0.29* 5.06%
> K2O 0.06* 2.59%
> Na2O 0.07* 1.93%
> TiO2 0.01 0.34%
> Al2O3 0.33 14.60%
> P2O5 0.08 5.18%
> SiO2 2.12 55.78%
> Fe2O3 0.00 0.25%
>
> Cost/kg 2.00
> Si:Al 6.49
> SiB:Al 6.49
> Expan 6.80
>Appreciate your help,
>
>Corinne Null
>Bedford, New Hampshire
>null@mediaone.net
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, 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
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