Bruce Davis on sun 23 nov 03
I do a lot of raku and have been using both commercial and home-made glazes.
I have a Randy Broadnax raku kiln which is front-loading and has a viewing
port in the door. So, I have a good view of what is going on at various
temperatures. One home-made glaze in particular has me curious.
Lemke Luster is a great glaze but at melting temperature it foams up to about
1/2" bubbles and if I have other pots sitting nearby, it smears them with
glaze. Two adjacent pots which both have this glaze actually join together for a
time and then the glazes separate and settle down into and nice smooth
coating. Any ideas as to what is causing this???
Lemke Luster
Frit 3110 31.38
Gerstley Borate 52.63
Ky. ball clay 5.26
Lithium Carb 10.53
Black Copper Ox 0.53
Cobalt Carb. 2.11
Copper Carb. 3.16
Bentonite 5.26
(I round these off)
Best regards,
Bruce Davis
Mud Run Pottery
Craig Martell on sun 23 nov 03
Hi:
Bruce wanted to know why his raku glaze was boiling.
It's the gerstley borate. There's a lot of chemically bound H2O that is
being released as gas. Gerstley Borate is an extremely variable substance
and the LOI (loss on ignition) values go from about 25 to 30%, which is a
fair amount. Raku pots are fired fairly fast and this creates a violent
boiling of the glaze when the H2O becomes airborne. I've seen this happen
many times when I've fired gerstley and colemanite glazes in raku. Slow
the fire and drive off the gas slowly. I've fired glazes with 80% GB that
haven't boiled at all if the temp advance is slow enough. When you get to
the point where the glaze starts to get "glossy", you have passed the boil
point and you can then crank up the heat a bit if you want. You will have
to do some trial and error with temp rise to get this right. Once you've
done it successfully, you have the answer.
regards, Craig Martell Hopewell, Oregon
Snail Scott on sun 23 nov 03
At 07:36 AM 11/23/03 EST, you wrote:
>...at melting temperature it foams up to about
>1/2" bubbles...Any ideas as to what is causing this???
I have noticed that high-frit glazes often seal over
at lower temperatures than other glazes, and trap
escaping gasses. Even when not doing raku, I have
more blister problems with high-frit glazes than
with others.
-Snail
Ababi on sun 23 nov 03
The foam is part of the story. After a while it "cools down".
Ababi Sharon
Glaze addict
Kibbutz Shoval Israel
ababisha@shoval.org.il
http://ababi.active.co.il
http://www.matrix2000.co.nz/Matrix%20Demo/Ababi.htm
.
| |
|
