DJ Brewer on mon 15 nov 10
I just got my new kiln hooked up to electricity (directly, with a
breaker by it as advised by many here) and I am firing my very own kiln
for a bisque for the very first time.
I began the firing at 8 pm. I set the initial soak for 195 degrees F but
by 1 am when I got up to check it, the thermocoupler measured 208, and I
saw it increase in a matter of a minute to 215 -- which to me meant that
any piece inisde with moisture still left in it would likely explode
from vapor turned to steam. I checked it again a few minutes later, and
the temperature was back down to 197.
My question is this: Is 195 too close to the boiling temp to set one's
kiln for a bisque soak?
This is a critical question for me because I make monster mugs and jugs
with much thicker clay on the faces than on the normal walls of a
vessel. I let them dry for a month sometimes before firing to avoid
having the faces crack -- I live on a river in a VERY VERY humid climate.
for those who want more details -- here they are below.
I set four dry pieces on the bottom of the kiln -- no shelf, because I
am not strong enough to lower the shelves in that low and I was too
excited to go get my 16 year old son to help do that for me.
Here are the ramps
Ramp 1 -- 75 an hour to 195 -- hold 8 hours (for the current small
load, this was an overly long soak)
(Note: For a face jug load, my increase rate would be 30 degrees an hour
until it reaches 195.)
Ramp 2 -- 150 per hour to 450 -- no hold
Ramp 3 -- 200 per hour to 950 -- hold 30 min
Ramp 4 -- 150 an hour to 1250
Ramp 5 -- 9999 an hour to 1600
Ramp 6 -- 100 per hour to Cone 08 (1736 according to the Skutt
programmable cone chart)
I started the firing at 8 pm and I got up to check the firing at 1.
Much to my surprise and dismay, the temperature inside the kiln read
208. As I watched, I saw it rise to 211, then to 215.
I hit review to make sure I had not misprogrammed it, but the
programming is correct.
So then I realized that for moisture removal -- its much better to set
the kiln at a temperature MUCH lower than 195 because that is still too
close to the boiling point. The heat from the firebricks and elements
cause the temperature inside the kiln to exceed 212 -- at least where
the thermocoupler is measuring from.
I am not sure if I am being overly cautious, but at any rate, next time
I'm going to set the soak for 150 or maybe 170 degrees.
SInce I am usually firing thicker pieces with sculpted faces on mugs and
jugs, I think the slower and more cautious approach would be wiser for me.
If this seems unnecessary to the more experienced among you who read
this far, please let me know.
thanks
DJ
Snail Scott on mon 15 nov 10
On Nov 15, 2010, at 1:46 AM, DJ Brewer wrote:
> My question is this: Is 195 too close to the boiling temp to set one's
> kiln for a bisque soak?
Personally, I'd go lower, but that is the standard set in
the program of the school's computerized kiln when I
was hired, and since it worked before, I stuck with it;
no problems. In theory, 195F is a fine temperature,
but I am just not that trusting of thermocouple accuracy
over the long term. I wouldn't worry too much about
it spiking higher now and then, though. The system
with a computerized controller requires that the kiln
alternate between 'full ON' and 'full OFF' at various
rates to control the temperature and rate of increase.
The thermocouple is close to the elements and responds
in real time to temperature changes, but the clay is
slower to respond. IT didn't reach 215F, the air around
it did, just for a brief interval. Heating up the clay takes
longer.
> I let them dry for a month sometimes before firing to avoid
> having the faces crack -- I live on a river in a VERY VERY humid
> climate.
Forget letting them dry for a month; they got as dry as
they were gonna get after a week or so in the air. Sitting
longer won't get them any drier. If concerned, use a longer
candling phase when you fire.
> ...I set four dry pieces on the bottom of the kiln -- no shelf...
I get the excitement of wanting to get the new kiln up and
running, but don't put stuff on the floor of the kiln of you can
avoid it. It will not heat up evenly with the rest of the piece,
since it is against the insulating brick and can only be
heated through the clay, from above, and the floor of most
kilns is a cold zone. Yes, you can usually get away with this,
but best not to. Besides, you definitely want to get a shelf in
there before your first glaze firing. Put it up on a few 1" posts
or some such, up off the floor.
In general, when firing half-empty, you will get the best results
by putting the work up in the middle, near the thermocouple
or the kiln sitter. Just leave the bottom empty, and just load
all your stuff on another shelf starting higher up in the kiln.
If you do put stuff near the bottom, put some witness cones up
higher anyway, as a check on the accuracy of the thermocouple.
Another cone pack at the bottom will let you check how far
the range of temperatures runs.
-Snail
Bonnie Staffel on tue 16 nov 10
DJ wrote:
" I set four dry pieces on the bottom of the kiln -- no shelf, because I =
=3D
am
not strong enough to lower the shelves in that low and I was too excited =
=3D
to
go get my 16 year old son to help do that for me.
DJ, I am rather concerned about placing the heavy pots on the floor of =3D
the
kiln. There is no way for the heat to get under your vessels and may =3D
cause
cracks or some other tragic cracking or even explosion. I did this once =3D
and
really learned my lesson. All my pieces that were on the floor of the =3D
kiln
were ruined by that action. Heat must be distributed over the entire =3D
piece
so that it can be changed from clay to bisque. If you put wads under the
pieces to lift them from the bottom of the kiln, that would allow the =3D
heat
to get under the work.
Definitely cross fingers on this firing no matter how slow or carefully =3D
it
is done.
Bonnie Staffel
http://webpages.charter.net/bstaffel/
http://vasefinder.com/bstaffelgallery1.html
DVD=3DA0 Throwing with Coils and Slabs
DVD=3DA0 Introduction to Wheel Work
Charter Member Potters Council
John Rodgers on wed 17 nov 10
Bonnie,
I place two inch posts on the floor of my kilns, then a full round shelf
on those, to make a false floor in my kilns. This allows heat to
circulate around and under pottery at the bottom of the kiln.
John
John Rodgers
Clayartist and Moldmaker
88'GL VW Bus Driver
Chelsea, AL
Http://www.moldhaus.com
On 11/16/2010 6:21 PM, Bonnie Staffel wrote:
> DJ wrote:
> " I set four dry pieces on the bottom of the kiln -- no shelf, because I =
am
> not strong enough to lower the shelves in that low and I was too excited =
to
> go get my 16 year old son to help do that for me.
>
> DJ, I am rather concerned about placing the heavy pots on the floor of th=
e
> kiln. There is no way for the heat to get under your vessels and may caus=
e
> cracks or some other tragic cracking or even explosion. I did this once a=
nd
> really learned my lesson. All my pieces that were on the floor of the kil=
n
> were ruined by that action. Heat must be distributed over the entire piec=
e
> so that it can be changed from clay to bisque. If you put wads under the
> pieces to lift them from the bottom of the kiln, that would allow the hea=
t
> to get under the work.
>
> Definitely cross fingers on this firing no matter how slow or carefully i=
t
> is done.
>
> Bonnie Staffel
>
> http://webpages.charter.net/bstaffel/
> http://vasefinder.com/bstaffelgallery1.html
> DVD Throwing with Coils and Slabs
> DVD Introduction to Wheel Work
> Charter Member Potters Council
>
>
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