David Martin Hershey on sat 27 may 06
Hi Charlie & All,
I've been reading through some old posts, and I
just wanted to write a short note to say thanks for all
your great posting on ClayArt recently.
A while back Mel said that he thought that some day
we might all move to around cone 1-3, and you have
surely proved what can be done at that range.
One of these days I am going to start experimenting
with your body recipe and glazes. They look great!
Also, I'm a big fan of your web site. I think it's one of
the best clay/artist/gallery websites that I've seen yet.
On the subject of bisque firing, I think it's the one
area where potters seem to have the least scientific
knowledge, and the most voodoo abounds.
Of course we all know that bisque should be slow,
but after that all bets seem to be off.
I've been trying to put together a bisque schedule with
scientific reasoning behind the ramps, temps, holds, etc.
You would not believe all the different schedules I've
found, and the differing reasoning for the ramps.
If you happen to still have Brad Schwieger's ramps
for his 48 hour bisque schedule, I would be very curious
indeed to see them.
Best, DMH
Beautiful Hermosa Beach CA USA
where our team
ocean swim season
opened today at 8am
with wind, whitecaps
6 foot waves
and a 5 mph drift...
on 052206 Charlie wrote:
> Many folks just don't understand the art and science of firing.
You can destroy paper thin
pots if you fire carelessly, or successfully fire solid monoliths if
you employ a slow firing schedule.<
>Last fall I hosted a workshop with Fong Choo, Julia Galloway,
and Brad Schwieger. When Brad said
his bisque firing schedule was 48 hours long, he wasn't kidding. I
tried to shorten the firing schedule by a day. I turned half of the
pots he made in the workshop to rubble.<
www.claylink.com
Vince Pitelka on sun 28 may 06
David Martin Hershey wrote:
> I've been trying to put together a bisque schedule with
> scientific reasoning behind the ramps, temps, holds, etc.
> You would not believe all the different schedules I've
> found, and the differing reasoning for the ramps.
> If you happen to still have Brad Schwieger's ramps
> for his 48 hour bisque schedule, I would be very curious
> indeed to see them.
DMH -
I don't have Brad Schwieger's bisque firing schedule, but I am familiar with
his work - very thick, faceted sculptural vessels - so I would assume that
he is just doing a long, slow preheat up to 180F to drive off all free
water, and then a slow ramp through the water-smoking period (900F to 1600F)
to allow the escape of all volatiles and chemically-combined water. In
addition, I expect that he places large flat-bottom pieces on wads or small
posts in order to allow better circulation of heat and air. As you know,
you must ensure adequate circulation of oxygen through the kiln during the
water-smoking period in order to avoid carbon and sulfur-coring in the clay
that can cause bloating at midrange or high-fire temperatures. I have never
gone to exta measures to ensure the necessary air circulation - virtually
all electric kilns seem leaky enough to allow proper volitilization - we
never have problems with carbon or sulfur coring as long as we follow a
reasonably slow bisque ramp. However. I think that one of the contributions
of the standard downdraft kiln-vent systems is that they do ensure a ready
supply of oxygen through this critical stage.
Through 35 years in clay I have always used the same bisque-practices. I
rarely ever loose anything in the bisque, and my students loose work only
when they fail to follow these practices, or when they fire work containing
significant trapped air spaces. We always set our programmable L&L EasyFire
kilns to preheat at 180 degrees F for 4 to 12-hours depending on the
thickness of the wares (even longer preheat if they are very thick) and then
do the standard slow-bisque ramp. I don't know the specifics of that ramp,
but you could find out from L&L. During the preheat we keep all spy plugs
out and keep the lid propped up a few inches on a piece of softbrick (never
hardbrick or kiln posts, because it will errode the refractory surfaces of
the kiln). We just replaced the last of our old L&Ls with the manual
controls, but on those ones we preheated overnight with the bottom switch on
low, all spy plugs out, and the lid raised a few inches. In the morning we
replaced the spy plugs, closed the lid, turned all switches to low, two
hours later all switches to medium, two hours later all switches to high,
and the kiln would generally shut off after about four more hours. For very
thick work we would lengthen the preheat time to sixteen hours and wait
three or four hours between turn-ups after the preheat.
We do a lot of bisque-firing in our updraft gas kilns, and the the pilots
are not enough to accumulate any significant heat for preheating. We
preheat overnight with a small candle-flame at each of the eight small
vertical venturi burners, with the door ajar and the damper open. After the
preheat, we raise the temperature approximately 250 to 300 degrees F per
hour, finishing the firing six to eight hours after the preheat. As above,
the firing would be considerably slower and longer if it contained very
thick work.
We always bisque to cone 08.
Most of the time when we have problems with things shattering or cracking in
the bisque firing, it is because the wares were improperly placed, or
because they contained significant closed air spaces. Long ago, I used to
bisque fire full sets of mixing bowls or serving bowls (6" to 16" by 2"
increments) nested inside each other (never with any weight or pressure
against the rim - just on the foot) , and I would sometimes loose one or two
bowls in the stack - either the base of the bowl would shatter, or else
there would be a crack from the rim towards the foot. I started stacking
the bowls on small softbrick shims - three shims per bowl, equally spaced
around the outer edge of the foot, and didn't have any more problems with
cracking or shattering. Some people advocate placing a bed of sand or grog
under a flat-bottom piece in the bisque firing, but that is about the worst
thing you can do, because it further insulates the bottom from temperature
changes and atmospheric circulation. I often place such pieces up on
wadding. But I digress.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
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