Gerard Carriere on tue 13 jun 00
=20
I rememer being concerned about very low expansion on a Cone 6 Randy's Re=
d
glaze submitted by one of the potters during the 1997 summer course with
Ron Roy at Canadore. It showed expansion of approx. 350 or 6.34 under new
system. I tried to fix it to
improve on high risk of compression for overware.
Was not so successful.
Randy's Red Recipe
Flint 30
EPK 5
G200 20
Talc 15
Gerstley Borate 30
RIO 15
I have noticed that several Northern potters are going mad about this gla=
ze
(the original) and I am concerned about safety since one of my ovenware
pieces cracked as
expected after several tests.
Does the high % of RIO affect the expansion in a special way?
Can someone suggest a safe alternative or a source of glazes close to
Randy's Red
in appearance?=20
G=E9rard on Bear Lake
Craig Martell on tue 13 jun 00
Gerrard sez:
> I rememer being concerned about very low expansion on a Cone 6 Randy's Red
>glaze submitted by one of the potters during the 1997 summer course with
>Ron Roy at Canadore. It showed expansion of approx. 350 or 6.34 under new
>system. I tried to fix it to improve on high risk of compression for overware.
>Was not so successful.
Hi:
I looked at the glaze with Insight 5. You asked what red iron oxide does
to the expansion. Red iron raises the expansion quite a bit at
15%. Without red iron ox you have a value of 6.18 which goes to 7.05 with
the addition of 15% red iron ox. My numbers are not the same as yours
probably due to different analyses plugged into our respective programs.
Randy's red is oversupplied with talc and gerstley borate and both of these
are lower expansion fluxes. Boron is at .76 moles and the limit is
.5. MgO is at .42 moles and the limit is .2. Plus, the spar is a bit low
at 20% and this material contributes the higher expansion oxides. So, you
might want to try adjusting the boron and talc to a lower value, probably
one at a time to avoid confusion as to the effect. Then, try raising the
spar. You may want to exchange the g-200 for soda spar which is a higher
expansion spar and will also favor the red color. But, do one thing at a
time and see where it takes you. I think you can arrive at your destination.
regards, Craig Martell in Oregon
GURUSHAKTI@AOL.COM on tue 13 jun 00
I don't know how well this will work. The expansion has been upped a bit to
6.61 from 6.01. The SiO2 Al2O3 ratio is down to 14.67 from over 16 (based on
my Insight calculation). I took out a lot of the magnesia from the talc and
replaced it with whiting to up the expansion. There's a hair more alumina
(was .18 and is now .20) and a bit less silica (was 2.96 and is now 2.86)
than the original. The boron is upped a bit as well, from .38 to .40. The
spar was also upped a bit.
Randy Red Revised
SILICA 30
G-200 FELDSPAR G200 22
GERSTLEY BORATE 30
EPK KAOLIN 5
TALC 7
WHITING 5
ADD: RED IRON OXIDE 15%
You might try the same recipe and up the whiting to 8 and lower the talc to
4. This will raise the expansion to 6.89. Then do a wet mix of a tablespoon
or more of each one to wind up at 6.75 expansion.
Let me know how it works if you try it. When I get the time I'll make a test
for myself.
Warm regards,
June
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