Barbrox@aol.com on sat 25 jan 97
I am really looking forward to getting my new kiln which I hope will
alleviate these problems- however until the I do not want to have to hammer
another whole load of glazed pieces. I am firing cone 6 oxidation in a small
Duncan that is about 10 years old. I am working with Miller#50 clay and
Masterglaze AS8 (hobbit blue) while I am testing several new recipes. I
started getting some blisters on the pieces loaded on the top shelf of the
kiln. I have tried sieving my glaze, slowing down the firing, plugging the
lid peep during cooling and the problem has only gotten worse. I put witness
cones in on the last firing, put a cone 7 in the sitter (I had been only
using a 6 in the sitter before). The bottom shelf appears to be the cold spot
(the cone 5 didn't even go down). I suspect the kiln is cooling too fast and
I suspect I may have an element out. I really don't understand why the
blisters are so much worse on the top. Oh by the way, the glaze that is
giving me the worst problems is the commercial glaze. I would appreciate any
advice from anyone out in clayville. TIA Barbara in Lyme,Ct.
Patsy Catsos on sun 26 jan 97
When I have trouble with the top shelf cooling too fast, a half hour "soak"
at the peak temperature usually helps a lot.
Patsy
Jeremy/Bonnie Hellman on sun 26 jan 97
Barbara- you can tell if an element is out by putting paper matches (the
kind you use to get in restaurants) in the heating elements when they are
completely cool. You then turn on the kiln at high power, with the lid
open, while you watch. Very quickly you will see the matches ignite, if
the heating elements are working. If some of the matches ignite and
others don't you know that those elements are not working as well as the
others. Bonnie in Pittsburgh
>----------------------------Original message----------------------------
>I am really looking forward to getting my new kiln which I hope will
>alleviate these problems- however until the I do not want to have to hammer
>another whole load of glazed pieces. I am firing cone 6 oxidation in a small
>Duncan that is about 10 years old. I am working with Miller#50 clay and
>Masterglaze AS8 (hobbit blue) while I am testing several new recipes. I
>started getting some blisters on the pieces loaded on the top shelf of the
>kiln. I have tried sieving my glaze, slowing down the firing, plugging the
>lid peep during cooling and the problem has only gotten worse. I put witness
>cones in on the last firing, put a cone 7 in the sitter (I had been only
>using a 6 in the sitter before). The bottom shelf appears to be the cold spot
>(the cone 5 didn't even go down). I suspect the kiln is cooling too fast and
>I suspect I may have an element out. I really don't understand why the
>blisters are so much worse on the top. Oh by the way, the glaze that is
>giving me the worst problems is the commercial glaze. I would appreciate any
>advice from anyone out in clayville. TIA Barbara in Lyme,Ct.
"Outside a dog, a book is a man's best friend. Inside a dog, it's too
dark to read" Groucho Marx
" " Harpo Marx
"Time flies like an arrow. Fruit flies like an avocado" Att. to GM
vpitelka@Dekalb.Net on sun 26 jan 97
>I started getting some blisters on the pieces loaded on the top shelf of the
>kiln. I have tried sieving my glaze, slowing down the firing, plugging the
>lid peep during cooling and the problem has only gotten worse. I put witness
>cones in on the last firing, put a cone 7 in the sitter (I had been only
>using a 6 in the sitter before). The bottom shelf appears to be the cold spot
>(the cone 5 didn't even go down). I suspect the kiln is cooling too fast and
>I suspect I may have an element out. I really don't understand why the
>blisters are so much worse on the top. Oh by the way, the glaze that is
>giving me the worst problems is the commercial glaze. I would appreciate any
>advice from anyone out in clayville. TIA Barbara in Lyme,Ct.
Barbara -
How close is your top shelf to the lid of the kiln?? If it is too close,
with a layer of flat pieces, then the lid will really cool the wares on the
top, freezing bubbles in place. I have seen that happen a lot, and if those
same wares are re-fired lower down or elsewhere in the kiln they come out
fine. Bubbles always occur in the maturing glaze, as outgassing is an
inevitable part of a newly active melt. If the wares are soaked briefly at
maturing temperature, the volatization runs its course, and the glaze
usually settles down. In most kilns this is accomplished simply by adequate
maturing temperature and slow-enough cooling, but older electric kilns are
notorious for cooling too fast. Until you replace this kiln, you may have
to fire it by a witness cone rather than with the cone sitter, and when
maturing temperature is reached, leave the elements turned on to low for an
hour or so (or to medium if the elements are old) before shutting it off.
If your top shelf is not too close to the kiln lid, then you need to test
your top coils and make sure they are working OK. Just turn them on high
with the kiln lid open and make sure they get hot within a minute or two.
If your shelf is not too close to the lid AND your top coils are working
fine, then chances are that you are overfiring this glaze. It is a downside
of these toploaders, especially the older ones, that there is considerable
discrepancy in temperature from bottom to top. Your kiln sounds like a
pretty extreme case. Some people accomodate this problem by firing slightly
lower-maturing glazes near the bottom, and higher-maturing glazes near the
top. The newer toploaders with thicker insulation fire more much evenly.
- Vince
Vince Pitelka - vpitelka@Dekalb.Net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
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