Paul Vernier on thu 21 oct 04
I have been firing a Copper Red recipe in a kiln that is difficult to get
an even reduction. So, I have had many opportunities to observe something
that I have been unable to find an explanation for in my glaze books.
Where I get insufficient reduction the glaze is green with significant
crazing. Where I get good reduction and a good red color, there is no
crazing that I can see with the naked eye or a basic magnifying glass.
I would like to determine if the reduced area has crazing that I can not
see, suggestions?. Much easier to see on light color glazes. I use a liner
glaze for functional ware. But it makes me wonder if we are inappropriately
using glazes that visually appear uncrazed? I guess if they pass the leach
test they are likely safe but of questionable durability.
So, back to my original subject. If it was crazed it could be a very fine
pattern and mean a higher expansion that the oxidized area. Or, if in fact
there was no crazing a lower expansion. I assume it has to do with the
reduction change from a copper oxide to a copper metal. Or are they other
changes that could cause this? Do other glazes exhibit this behavior under
uneven reduction?
Paul Vernier "Not quite ready for winter" in Santa Cruz
John Britt on thu 21 oct 04
Paul,
The copper reds are still crazing when red, although the opacity makes it
difficult to see. Try rubbing a pot/tile with a black magic marker and it
will reveal the craze lines. Also sandblasting the surface will reveal the
crazing.
John Britt
www.johnbrittpottery.com
Bonnie Staffel on fri 22 oct 04
When I was able to do a lot of Cone 04 copper reds in the special Reduction
Production kiln, to overcome the crazing, I would fire my porcelain bisque
to its maturation point, then heat the pot and dip it into the basic low
fire white glaze. I applied the copper carbonate by spraying or brushing a
decoration. I would then dip it again in a cover coat of the base glaze.
They came through very nice and tight, waterproof, and with no crazing. The
red color was rich and bright.
Warm regards,
Bonnie Staffel
http://webpages.charter.net/bstaffel
http://www.vasefinder.com/
Potters Council member
Ivor and Olive Lewis on fri 22 oct 04
Dear Paul Vernier,
Crazing can be exposed by using a disclosing fluid. this is a coloured
liquid with a very low viscosity which flows freely into minute
fissures. A red spirit based ink will do the trick. Paint it on, allow
it time to soak in then wipe the excess away. Use a X10 hand lens
Without knowledge of the composition of your glaze it would be
difficult to form a meaningful answer to your other questions.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Paul wrote, in part
> So, back to my original subject. If it was crazed it could be a very
fine
> pattern and mean a higher expansion that the oxidized area. Or, if
in fact
> there was no crazing a lower expansion. I assume it has to do with
the
> reduction change from a copper oxide to a copper metal. Or are they
other
> changes that could cause this? Do other glazes exhibit this behavior
under
> uneven reduction?
______________________________________________________________________
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Ron Roy on sun 24 oct 04
Hi Paul,
Just steam it - the water will evaporate from the craze lines last.
I discovered this with some mashed potatoes on a dark crazed glaze.
RR
>I would like to determine if the reduced area has crazing that I can not
>see, suggestions?. Much easier to see on light color glazes. I use a liner
>glaze for functional ware. But it makes me wonder if we are inappropriately
>using glazes that visually appear uncrazed? I guess if they pass the leach
>test they are likely safe but of questionable durability.
>Paul Vernier
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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