Mark W. Potter, Jr. on wed 2 jun 04
First I want to say that I feel like I know all of you, and
yet I have met not one of you. . . . What a miracle the
internet is . . and what a fabulous community Clayart has
become.
This post is to all you very knowledgeable clay chemists. I
had a problematic bisque firing recently.
I'm was an old Vulcan Kiln small round kiln, and shortly
after I began the firing two out of the five elements went
completely. I know this because I heard them go, and the
firing was just beginning and I opened up and peeked. I
decided to go forward and see if the three elements would
make it on their own.
The kiln would not make temperature. It stalled at around 840
degrees Centigrade. Its an old kiln. I tried everything to
make it hit stoneware bisque temperature. I put another layer
of insulating brick on the lid . . ect. etc. . . . no rise in
temp.
Here are my questions:
a) Can bisqued ware be re-fired through any stall-
point? Ie. is there a point at which a kiln can stall/fail
during which the ware can be ruined by the stall or failure?
Or can you always take something back through any temperature
point. Ie. if failure happened right in the middle of losing
chemical water, would that make the ware blow up if you
brought it up past that point again??
b) Then miraculously after opening the kiln - the
ware appears sufficiently bisqued to be able to glaze. I'm
assuming that my chart is correct, ie. Cone 06 is around 980
C. How the heck? I did let it cook at 840 C for a LONG TIME!!
Many hours! Could my thermocouple be giving me incorrect
info, or is the bisque temperature range really that broad?
I guess I'm asking for guidance of what to do under
such circumstances so as not to lose a load of work. Is there
a critical 'altitude' which must be reached to safely handle
work? Or is it just a matter of degree?
Dave Finkelnburg on wed 2 jun 04
Mark,
You will got lots of good responses on this.
Basically, a cone measures time exposed to temperature, sometimes called
"heat work." The longer you leave the ware at 840C, the higher the cone it
will be fired to, within reason. You won't get cone 10 down that way, but
instead of cone 014, you probably fired to at least cone 012, maybe 010.
You probably have what is called "soft bisque." So...if you handle the
pieces carefully...you're good to glaze...and fix the elements. :-(
If you look at charts of temperature versus cone, they are always stated
in degrees temperature rise per hour. At 300-C/hour temperature rise, 840C
is not as high a cone as at 150-C/hour rise. There has been more time for
the ware to accumulate heat work with the latter, slower firing rate.
Regards,
Dave Finkelnburg
----- Original Message -----
From: "Mark W. Potter, Jr."
Sent: Wednesday, June 02, 2004 8:34 AM
....> b) Then miraculously after opening the kiln - the
> ware appears sufficiently bisqued to be able to glaze. I'm
> assuming that my chart is correct, ie. Cone 06 is around 980
> C. How the heck? I did let it cook at 840 C for a LONG TIME!!
> Many hours! Could my thermocouple be giving me incorrect
> info, or is the bisque temperature range really that broad?
Cynthia Bracker on wed 2 jun 04
If I did my math right, you did get past quartz inversion, so that's a
plus. It seems that you reached approximately a Cone 014, but since
cones measure heat work over time, it is possible that you achieved as
hot as a Cone 012 since you said you stayed at 840C for a long time.
That's still an awfully cool bisque firing, even if you are using
stoneware and glazing and firing to Cone 10. You can probably re-bisque
the pieces without problems, but you'll need to replace your elements.
Vulcan kilns are no longer being made and I don't know of any other kiln
company that has picked up the parts, so you'll probably have to go to
Duralite or Euclid and have elements custom wound for you. On your
refire, I would make sure to go through Quartz Inversion (537-593C,
again if I did my math correctly) slowly. You might also check with a
Kiln repair technician about your kiln and what other problems may be
happening or about to happen before you sink a lot of money into your
kiln, especially since they are no longer made. If you don't already
have a Kiln tech that you trust, I would highly recommend David Sturm,
although, admittedly, I am biased. He's also my husband. But he's been
doing this for 10 years and is very good, even at diagnosing over the
phone. If you'd like his number or the numbers for Duralite and/or
Euclid, please e-mail me off-list and I'd be happy to provide them to
you! Good Luck!
Cindy
Mark W. Potter, Jr. wrote:
>First I want to say that I feel like I know all of you, and
>yet I have met not one of you. . . . What a miracle the
>internet is . . and what a fabulous community Clayart has
>become.
>
>This post is to all you very knowledgeable clay chemists. I
>had a problematic bisque firing recently.
>
>I'm was an old Vulcan Kiln small round kiln, and shortly
>after I began the firing two out of the five elements went
>completely. I know this because I heard them go, and the
>firing was just beginning and I opened up and peeked. I
>decided to go forward and see if the three elements would
>make it on their own.
>
>The kiln would not make temperature. It stalled at around 840
>degrees Centigrade. Its an old kiln. I tried everything to
>make it hit stoneware bisque temperature. I put another layer
>of insulating brick on the lid . . ect. etc. . . . no rise in
>temp.
>
>Here are my questions:
> a) Can bisqued ware be re-fired through any stall-
>point? Ie. is there a point at which a kiln can stall/fail
>during which the ware can be ruined by the stall or failure?
>Or can you always take something back through any temperature
>point. Ie. if failure happened right in the middle of losing
>chemical water, would that make the ware blow up if you
>brought it up past that point again??
>
> b) Then miraculously after opening the kiln - the
>ware appears sufficiently bisqued to be able to glaze. I'm
>assuming that my chart is correct, ie. Cone 06 is around 980
>C. How the heck? I did let it cook at 840 C for a LONG TIME!!
>Many hours! Could my thermocouple be giving me incorrect
>info, or is the bisque temperature range really that broad?
>
> I guess I'm asking for guidance of what to do under
>such circumstances so as not to lose a load of work. Is there
>a critical 'altitude' which must be reached to safely handle
>work? Or is it just a matter of degree?
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
>
>
>
| |
|
