John Baymore on fri 25 sep 98
------------------
Marc,
=3Csnip=3E
I've thrown vases, lifted them off the wheel, walked to the kiln, put 'em
in, turned on
the burners and finished the bisque in 9 hours. That's a bisque and it
doesn't
need preheating if you know your clay and pay attention=21
=3Cclip=3E
This ties into another thread that was asking about this idea.
I have done this little trick also with students to prove it can be done.
Blows a lot of myths out of the water fast. On occasion I have done it due
to a deadline which I was getting sorely pressed by =3Cg=3E. One reason it
works is that the clay is pretty evenly wet all the way through. So there
is no dried out =22skin=22 of clay platelets that are close together to =
inhibit
the flow of water from the more interior sections of the walls migrating to
the surface. Additionally the kiln acts as a humidity controlled dryer (if
you do it right), and because of the continually high partial pressure of
water vapor (moisture content of the air) due to a LOT of water evaporating
rapidly inside the kiln, stuff dries pretty evenly.
By using the kiln controls (balance of burners and damper) you control the
humidity of the air as the temperature goes up, and the ware dries right
out..... very evenly and rapidly. Once that is done..... up you go as
normal. That =22normal=22 is the same as whatever your clay body and =
forming
process will tolerate for degrees per hour climb rate in any firing.
If you let the pieces get leather hard in the =22traditional=22 drying
environment (sitting out on ware shelves) and then try to do this, it gets
a lot trickier. But it still can be done faster than most would believe,
with the kiln still acting like a dryer unit.
Anyone who is in doubt if this works can ask people who were in my
noborigama workshop this summer........ we had a run of humid rainy wet
weather during the making portion of the workshop. Up against a tight
deadline with no options, I fired two VERY wet bisques (stacked the second
one of mostly leather hard stuff....and some thick ones too) took it up
fast, and we lost only four pieces...one in the first and three in the
second. (Linda...Chris..... Patrick....Bonnie....Debbie.....you reading
this one?)
Uneven heat flow patterns in kilns contribute huge parts of the problems
with blowing up green pieces (see pre-heating below).
BTW...... in industry, humidity controlled drying units circulating large
volumes of air over pieces have been around for years. Newer technology
often incorporates this function into the continious tunnel kilns now.
Turnarounds of a less than an hour from formed to fired and cold happen in
the industrial world. Specialized stuff though.
This wet firing idea certainly can be done...... but it does help to know
exactly what you are doing, as I am sure you do. Very few potters have the
background you do in the firing end of the process.
=3Csnip=3E
I quit doing preheats 15 years ago. I think folks only do them because
their teachers did them. Their teacher only did them because they read
somewhere that they were supposed to.
I feel it serves no purpose other than to waste gas. ...........=3Csnip=3E
Why in the world would anyone preheat a glaze firing?
=3Cclip=3E
Hum. Categorically denying the function of a slow temperature increase in
ANY firing situation seems like a little overkill in the generalizations
department. I have always contended that the universal disclaimer to
questions about anything in ceramics has to be prefaced with .......=22It
depends.=22 =3Cg=3E
Out in the wide world there are many different materials situations,
forming processes, levels of technical understanding, and types of kilns.
In fact, on CLAYART we actually reach a wide segment of the world. I think
the reality is that you need to take the science that underlies the firing
of clay and glazes, mix in some combustion theory, couple that with the
vagarities of the application of that understanding or lack thereof, and
factor in the practical engineering level that is in place in the kilns of
the craft potter community.
Doing that, you may have some differing opinions on that sweeping
generalization.
Probably at issue is how you are defining =22pre-heat=22. If =22pre-heat=22=
in
this context means turning on the kiln for the heck of it, just cause
everyone else does it (not knowing why......just that is what you think you
are supposed to do) then YUP..... we're in agreement. But if preheat means
bringing up the kiln at a slower than usual rate that solves some real
firing consideration, then it is not wasted.
So if you KNOW (with some hard background facts) that you have to go slow
in the early firing with your particular situation, then it is not wasted.
I agree that all too many people fire kilns with no real knowledge of what
it is that they are really trying to accomplish (other than get =22nice=22
pots), and little technical background knowledge about the firing of
ceramic materials. An un-necessary =22pre-heat=22 is certainly =
possible.....
just as un-necessary levels of reduction are possible. Also soaks at
specific times that really make no difference in fired appearnce. All that
stuff. Ad nauseum.
However, I still think there are numerous situations where a slow rate of
increase in the early stages of a firing might be appropriate. Certainly
most revolve around bringing clay up from the green state. Don't forget
that some people DON'T BISQUE.... they green glaze. Or they salt fire
unglazed. Or wood fire unglazed.
Some kilns heat VERY unevenly in the early stages. Some of these are
supposedly =22modern=22 units but simply poorly designed. Some of these are
well designed kilns that are designed for certain effects and the uneven
heating is a trade off for those effects. To get these kilns as even as
possible it often is a benefit to take the first 400-500 degrees quite
slow. It is also often the only way to bring the chamber up evenly enough
to avoid sudden heat increases in sections of the chamber that could cause
problems with green clay in the 212F range.
Sometimes this practice is as simple as buying some =22free time=22 for the
potter...... (s)he feels safe running the kiln on pilot while sleeping, and
gains maybe 400 degrees or so of =22free of babysitting time=22 heat
accumulation at the point the actual firing is started. That gas used is
the cost of getting a nights sleep and not babysitting the kiln as long the
next day. 400 degrees F at 200 / hr climb rate is 2 hours of life =
=22saved=22
from babysitting a kiln. For some, well worth the price of the gas used in
excess of the =22optimum=22 situation of gas usage that could have occured =
if
the kiln was taken straight up faster the next day under close supervision.
There are also specialized kiln types out there that have some special
considerations that make a slow =22pre-heat=22 necessary. Here is an =
unusual
example that I deal with all the time which is outside the experience of
most potters:
In a glaze load in a large woodfired noborigama of many chambers, a lot of
water is absorbed into the wares (there's a lot of ware to absorb it, to
start with). Additionally, wood contains a lot of water in it's cell
structures even when supposedly dry. And a byproduct of combustion is
water vapor. So in the early stage of a firing a LOT of water vapor is
given off into the effluent circulating through the kiln as the heat
exchange medium.
If the firing is progressed rapidly at the main firemouth, a lot of water
vapor is generated quickly (sum total of drying wet ware, wood moisture,
and byproducts of combustion) and the humidity of the gases circulating out
of the first chamber is very high and the volume of those gases produced is
evolving at a high rate. Were this a single chamber kiln this would be
insignificant and of no concern. It would just go up the chimney and into
the atmosphere.
Because of the segmentation and length of the (more than 2 chamber)
climbing kiln, there is a huge temperature differential along the length of
the kiln from the main firebox to the entrance to the chimney. This
situation is similar in an anagama, but more pronounced in the noborigama
due to the improved heat extraction of the segmentation. On my kiln, the
main firebox can be at about 2000F and the fourth chamber is still at about
450F.
(BTW.....These ancient things are pretty fuel efficient designs.)
As the temperature of the effluent DROPS in circulating through each
sucessive chamber (with its' effective heat exchanger composed of ever
cooler stackings of wares and shelves), the amount of water vapor the kiln
atmosphere can hold in the vaporous form DECREASES. (Think of dew on the
grass on a cool summer morning.) Additionally, it is picking up water
vapor from the drying wares in each sucessive chamber. If the amount of
water saturated effluent passing through the relatively cold chamber four
(or three, for that matter) is allowed to increase too fast, a large amount
of water vapor condenses out all over the (cold) interior chamber walls and
on the (cold) ware. You in effect create a mini rainstorm or heavy dew in
the chamber. (I am reminded of the =22sweating=22 thread of the summer here
=3Cg=3E.) This ruins the glaze coating on the pots. And wreaks havoc on =
any
green ware in the chamber.
This is a reality of these types of kilns. (Not as much of a problem on a
two chamber...... not enough temperature differential.) I have been warned
about this by other chamber kiln firers. I have seen it happen in others
kilns. I have (unfortunately) done it myself on the first noborigama I
built and fired. Anyone who has taken my summer noborigama workshop has
had the experience of sticking their hand into chamber four and feeling the
wetness of the flow of gases out of chamber three into four...... it is a
prime guage as to the pacing of the firing. Touch the walls early on and
they are damp. When chamber four stops having =22wet wind=22 flowing =
through
it, and is above 212F, then the firing can be pushed.... but not before.
Generally speaking, the larger the number of chambers on a climbing kiln,
the more significant the temperature differential and the longer the slow
=22pre-heat=22 part of the firing.
While I was in Mashiko, Hamada Shinsaku (Shoji's first son in pottery)
explained to me that he (and crew, including Tomoo, his son) now fires his
noborigama up to barely dull red heat in the first chamber for a whole day
just to dry it out. He then shuts the whole thing down, seals the kiln,
goes to bed, and gets a good rest. Then a day later, the kiln is restarted
and actually fired off at a faster climb rate. He says it is easier on the
firers. This is very different from the traditional long slow pre-heat and
then increasing rate of climb bringing it to completion.
One of these days I will try this sequence and see if I like it better=21
(BTW...... Hamada Shinsaku is a quite a departure from his dad. His house
is quite contemporary as opposed to traditional, his climbing kiln is very
=22modern=22 in design and construction, and his studio layout is very =
modern
too. He makes nice pots which are certainly derivative of Shoji's, as it
should be. Yet different.)
So a slow =22pre-heat=22 on this type of kiln really IS a necessity for
functional reasons. There are many ways to accomplish this slow preheat
(ala' Shinsaku-sensei), but it has to happen somehow.
My real point here is that there is no such thing as a =22one size fits =
all=22
prescription for firings. It is a blend of scientific theory, applied
engineering, and with a little practicality thrown in for good measure.
(Sometimes I'd swear that there is a magic factor too =3Cg=3E.)
Hopefully a little food for thought.
Best,
...........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Judith Enright on sun 27 sep 98
Something else to keep in mind for those of us who use electric kilns with the
Enviro-Vent: the moisture being sucked from wet ware will rust the fan blades
on the E-vent motor, so repeated wet-firings could cause early failure of the
motor.
Judith Enright @ Black Leopard Clayware
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