John Baymore on wed 12 may 99
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Right on Tony, mel, et al=21 Good thoughts.
Firing is based on the clay, the glazes, the forming techniques, and the
desired results, not some arbitrary =22one size fits all=22 firing schedule.
You fire based on the needs of the ware. To fire well....you need to
understand those needs.
A person who makes 6=22 thick slab constructed sculptures that are 5 feet
tall out of a body containing a lot of fireclay is going to have a
different typical firing cycle from a person who throws 1/8=22 thick
porcelain tableware =3Cg=3E. If you want to build up ash, you gotta burn a =
lot
of wood. If you want microcrystals to develop you have to cool slow. And
so on.
A well designed kiln allows the potter to fire pretty much to whatever
cycle he/she wants to....and that is dictated by the work. If a basic one
chamber kiln only fires one way and that's it...........a one trick
pony........ then that is usually a sign of a design or operation problem
of some sort. If every time you try to fire in 8 hours instead of 12 the
bottom gets way hot, or if the converse happens, then you need to look at
the kiln and/or how you operate it with a serious eye and see what might be
the limitation. You want the =22passing power=22 on tap when you need it, =
as
Tony put it.
Yes..... there are some constraints on certain types of kilns....and those
ARE design limitations. For example, in a multichamber woodfired climbing
kiln.........if you heat up the front part (firebox and chamber one) too
fast and drive off too much H20 vapor (from byproducts of combustion,
moisture in the wood, moisture in the pots and the like) too fast, then the
huge temperature differential (below 212) in one of the latter chambers
causes it to =22rain=22 (condense out) all over the ware there.
That really IS a design defect in these kilns that can be overcome, and can
be changed...... you just continue the logical evolution of this type of
kiln design to the continious tunnel kiln.
Another example might be a =22hobby=22 type electric kiln. At the top end =
of a
high temperature cycle, if you decide to suddenly increase the rate of
climb to a very high value you might find that the heat input is not there
to allow that. At a certain point the rate of climb is dictated by the
ratio of heat losses to heat input and you are maxed out on available
wattage. So the kiln is firing it's OWN cycle at that point....not yours.
A design limitation again.
Knowing these design limitations is important too. So that you don't try
to do with your kiln something that it is not capable of doing. That's
mels', =22knowing your kiln=22. If that particular =22thing=22 is =
important to
you...... then you'd better get another kiln =3Cg=3E.
Industry certainly does have the ability to fire fast. They design their
bodies, glazes, and kilns for that purpose. Time is money. Energy is
expensive. Most of OUR equipment and techniques are based on technology
that is leftovers from the 19th century. Industry is headed for the 21st
=22with a bullet=22. You don't find too many pulse-fired high velocity =
burner
continious fiber kilns in potters studios =3Cg=3E. Most of us like a little
plastic CLAY in the body we work with. And we make glazes mostly out of
earthy raw materials and less out of things like frits.
And that is well and good....... for we are handcraft potters not
industrial potters =3Cs=3E.
Many of those REALLY fast firings are done in continious kilns where the
ware is moved on a conveyer arrangement through the heat zones that remain
stable. The appropriate temperature gradient is set up within the tunnel,
and the firing curve is based on how fast the conveyer =22conveys=22. Often
the kiln is built to fit the specific ware........ for example a little
over 20 mugs wide by just a little over one mug high. No large dense load
to penetrate, no wasteful kiln furniture to heat....... easy to get even,
efficient heat distribution. Those lines of mugs just keep getting loaded
by minimum wage laborers at one end and unloaded at the other 45 minutes
later =3Cg=3E as thay go marching through those heat zones. This type of
firing is not usually practical in a studio pot shop. What we do fits
periodic kilns better.
Large periodic industrial kilns that are designed to fire a =22fast=22 cycle
have burner technology on them that we typically don't use either (quite
costly to set up) that allows great heat penetration into the load.
Lacking this type of burner, they'd have trouble too. If you put the
burners we use (of the correct BTU input) on the same kiln....... it'd fire
quite unevenly.
The really fast firing thing comes with a =22technology penalty=22 attached.
If you REALLY want to do it, you gotta play by the rules. =3Cg=3E And in =
that
department.......industry rules.
Best,
....................john
PS: A firing cycle needs to be looked at =22cold to cold=22.....meaning =
that
the cooling has as much to do with things as the heating. Craft potters
seem to forget that.
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40compuserve.com
John.Baymore=40GSD-CO.COM (new second address.......website coming)
millie carpenter on thu 13 may 99
John & and others who understand this.
when I was taught to fire the electric kilns, it was the basic, a few
hours on low to really dry things, closing the lid and then the peeps,
an hour on medium, then blast on high until the cone sitter falls. now
I have learned why, when and how to fire down in my kiln. why do I
want/need to know the rate of rise of temperature? and how can I
determine this without a pyrometer or computer controls?
Millie in Md where the weather is so beautiful that I had a picnic
dinner out on the edge of the bike trail (our 12'wide 17 mile long
county park)
>
> Another example might be a "hobby" type electric kiln. At the top end of a
> high temperature cycle, if you decide to suddenly increase the rate of
> climb to a very high value you might find that the heat input is not there
> to allow that. At a certain point the rate of climb is dictated by the
> ratio of heat losses to heat input and you are maxed out on available
> wattage. So the kiln is firing it's OWN cycle at that point....not yours.
> A design limitation again.
>
> Knowing these design limitations is important too. So that you don't try
> to do with your kiln something that it is not capable of doing. That's
> mels', "knowing your kiln". If that particular "thing" is important to
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