mel jacobson on fri 16 mar 12
there have been some very nasty theories about
building kilns that started in the 50's.
the most awful was the huge flue exit
to the chimney.
i have seen them over 90 square inches
built into a 30 cubic foot kiln.
if the stack is big and tall, there is no way
you can store enough energy into the kiln itself.
the kiln stalls at about 2000F. and, stays there.
fuel in, let it all out. it just makes sense to keep
the heat in the kiln, not the atmosphere.
in that case, more heat is coming up and out the
chimney then is heating the pots. it will never catch up.
so, the solution was to give it more fuel, and more fuel.
the nils/mel/ theory would be to make the flue
exit smaller. keep the heat in the kiln. nils does
it twice. like 35 square inches at the flue exit, and
he does it again where the stack connects to the chimney stack.
it creates of venturi, a swirl
that lets the fumes fly up the stack, but the heat stays in the kiln.
his book explains it all. what i do know is, i have always had
a smaller flue exit. my first kiln had this feature and it never stalls.
the second feature is free stacking brick for the chimney.
most potters use bad brick that is broken, rough and uneven.
that sort of stack will have many gaps and open places. take
the total square inches of gaps and add them up...you would
have like a hole 20 inches round in the middle of your stack.
that makes for a passive damper. the kilns chimney does not
work. it will not draw. so, keep firing, over and over a hundred
times and let the kiln stall...add more gas and it still stalls.
think of drilling a thousand quarter inch holes in your house.
`hey, they are just little holes.` add them up and you have
an open window to the outside.
our solution is to make a metal stack, like spiral pipe, add kaowool
flue liners that are `welded` together with itc 100. one solid
pipe, lined with a solid liner. no leaks. the draw is perfect.
nils always has said...`if the stack is too tall, it will not work,
if the stack is too short, it may not work. add or subtract until
it works perfectly. your location, your wind, your weather, your
obstructions to your flow of air are going to be unique to you.
find out what they are. work with them. adjust to them.
but, there is the rub. many people build and have kilns that do
not work well. and, they just fire it over and over.
if my kiln stalled i would not fire it again until i knew why.
i would change the stack, tighten the kiln, change the flue exit size,
use more gas, use less gas. i would chart everything, one change at
a time. see what happens. that is what i did for the first five years
i had a kiln.
\fire with one burner, fire with two burners, damper in, damper out,
high gas pressure, low gas pressure. fast cool, slow cool. i wanted to
know. if i lost pots, i would make more. i was always looking for a
100 percent perfect firing. i started to get that....all the pots looking
good. that is the system i use now.
32 square inch flue exit, tight stack...tight kiln. two burners on full
at the early stage, turn the kiln down as energy builds. the lower the
gas pressure the faster the heat/energy rises. and do
not make smoke and soot, soft reduction rules. it is the opposite
of a race car. take your foot off the gas.
as i have said. professionals like hank murrow have a different system.
he fires 28 hours in a very wonderful kiln. he down fires for many hours.
it is a fantastic system for hank. he wants a certain look in his work an=
d
he has found out how to get it. if i did that with our kilns at the farm
it would cost me $200 to fire one kiln...hundreds of gallons of propane.
it would not work. we fire that small kiln with 14 gallons of propane with
great results. sell one pot and you pay for the firing. hank has, perhaps
the most sophisticated kiln i have ever seen. to build it, it would cost
thousands and thousands of dollars. it is a really fantastic kiln. but, f=
or
the average person, building a kiln in their back yard...impossible.
i encourage people to read nils' book...the art of firing...in many ways
it should be called `the science of firing`. it goes into real detail abou=
t
the how and why. i can hardly do justice in notes to clayart. but, what
i do is encourage the learning of what heat is, does and how to control
it. doing it the same, over and over without perfect or quality
results is the
complete definition of stupid.
folks have been firing pots for thousands of years. it is not the hardest
thing to do. but, it does take thought, experimentation and intelligence.
blind faith in old theory smacks at modern technology. there are right
and wrong ways to do things. find the right way.
i love this metaphor. `every time i fire my kiln, all the pots break
into three pieces. i have hundreds of pots that are broken in three
pieces. i think i am being punished for my sins...so, i will keep firing
that clay body and see if it corrects itself.`
if you fire your kiln and every glaze runs onto the shelf and sticks
the pots there forever. would you change the glaze? perhaps you
may turn off the kiln sooner, or something...but, would you continue
to use that glaze, that way? no, you modify. you change, you correct.
being a clay person means that you are always a quarter this, a quarter
that...crafts person, artist, techno-geek, heat expert, gas expert,
electrical expert. if you leave a quarter out...you have failure.
that is why we call it `CRAFT`.
mel
http://www.visi.com/~melpots/
clayart page below:
http://www.visi.com/~melpots/clayart.html
Ben Morrison on fri 16 mar 12
One of my instructors in college has two round holes he drilled through the=
=3D
IFB's with a hole saw for his exit flues. He connected them to a 30 foot s=
=3D
teel pipe and strapped it to the side of his house with brackets. It works =
=3D
well. I'm not a fan of the huge flue/chimney theory either Mel.=3D0A=3D0A-B=
en=3D
=3D0A=3D0A=3D0A________________________________=3D0A From: mel jacobson lpots2@V=3D
ISI.COM>=3D0ATo: Clayart@LSV.CERAMICS.ORG =3D0ASent: Friday, March 16, 2012=
2:5=3D
8 AM=3D0ASubject: ivor, kiln theory=3D0A =3D0Athere have been some very nas=
ty the=3D
ories about=3D0Abuilding kilns that started in the 50's.=3D0A=3D0Athe most =
awful =3D
was the huge flue exit=3D0Ato the chimney.=3D0Ai have seen them over 90 squ=
are =3D
inches=3D0Abuilt into a 30 cubic foot kiln.=3D0A=3D0Aif the stack is big an=
d tall=3D
, there is no way=3D0Ayou can store enough energy into the kiln itself.=3D0=
Athe=3D
kiln stalls at about 2000F.=3DA0 and, stays there.=3D0Afuel in, let it all=
out=3D
.=3DA0 it just makes sense to keep=3D0Athe heat in the kiln, not the atmosp=
here=3D
.=3D0A=3D0Ain that case, more heat is coming up and out the=3D0Achimney the=
n is h=3D
eating the pots.=3DA0 it will never catch up.=3D0Aso, the solution was to g=
ive =3D
it more fuel, and more fuel.=3D0A=3D0Athe nils/mel/ theory would be to make=
the=3D
flue=3D0Aexit smaller.=3DA0 keep the heat in the kiln.=3DA0 nils does=3D0A=
it twice=3D
.=3DA0 like 35 square inches at the flue exit, and=3D0Ahe does it again whe=
re t=3D
he stack connects to the chimney stack.=3D0Ait creates of venturi, a swirl=
=3D0A=3D
that lets the fumes fly up the stack, but the heat stays in the kiln.=3D0Ah=
is=3D
book explains it all.=3DA0 what i do know is, i have always had=3D0Aa sma=
ller=3D
flue exit. my first kiln had this feature and it never stalls.=3D0A=3D0Ath=
e se=3D
cond feature is free stacking brick for the chimney.=3D0Amost potters use b=
ad=3D
brick that is broken, rough and uneven.=3D0Athat sort of stack will have m=
an=3D
y gaps and open places.=3DA0 take=3D0Athe total square inches of gaps and a=
dd t=3D
hem up...you would=3D0Ahave like a hole 20 inches round in the middle of yo=
ur=3D
stack.=3D0Athat makes for a passive damper.=3DA0 the kilns chimney does no=
t=3D0A=3D
work.=3DA0 it will not draw.=3DA0 so, keep firing, over and over a hundred=
=3D0Ati=3D
mes and let the kiln stall...add more gas and it still stalls.=3D0Athink of=
d=3D
rilling a thousand quarter inch holes in your house.=3D0A`hey, they are jus=
t =3D
little holes.`=3DA0 add them up and you have=3D0Aan open window to the outs=
ide.=3D
=3D0A=3D0Aour solution is to make a metal stack, like spiral pipe, add kaow=
ool=3D
=3D0Aflue liners that are `welded` together with itc 100.=3DA0 one solid=3D=
0Apipe=3D
, lined with a solid liner.=3DA0 no leaks.=3DA0 the draw is perfect.=3D0Ani=
ls alw=3D
ays has said...`if the stack is too tall, it will not work,=3D0Aif the stac=
k =3D
is too short, it may not work.=3DA0 add or subtract until=3D0Ait works perf=
ectl=3D
y.=3DA0 your location, your wind, your weather, your=3D0Aobstructions to yo=
ur f=3D
low of air are going to be unique to you.=3D0Afind out what they are.=3DA0 =
work=3D
with them.=3DA0 adjust to them.=3D0A=3D0Abut, there is the rub.=3DA0 many =
people b=3D
uild and have kilns that do=3D0Anot work well.=3DA0 and, they just fire it =
over=3D
and over.=3D0Aif my kiln stalled i would not fire it again until i knew wh=
y.=3D
=3D0Ai would change the stack, tighten the kiln, change the flue exit size,=
=3D
=3D0Ause more gas, use less gas.=3DA0 i would chart everything, one change =
at=3D
=3D0Aa time.=3DA0 see what happens.=3DA0 that is what i did for the first =
five y=3D
ears=3D0Ai had a kiln.=3D0A\fire with one burner, fire with two burners, da=
mper=3D
in, damper out,=3D0Ahigh gas pressure, low gas pressure.=3DA0 fast cool, s=
low =3D
cool.=3DA0 i wanted to=3D0Aknow.=3DA0 if i lost pots, i would make more.=3D=
A0 i was=3D
always looking for a=3D0A100 percent perfect firing.=3DA0 i started to get=
tha=3D
t....all the pots looking=3D0Agood.=3DA0 that is the system i use now.=3D0A=
=3D0A32 =3D
square inch flue exit, tight stack...tight kiln.=3DA0 two burners on full=
=3D0Aa=3D
t the early stage, turn the kiln down as energy builds.=3DA0 the lower the=
=3D0A=3D
gas pressure the faster the heat/energy rises. and do=3D0Anot make smoke an=
d =3D
soot, soft reduction rules.=3DA0 it is the opposite=3D0Aof a race car.=3DA0=
take =3D
your foot off the gas.=3D0A=3D0Aas i have said.=3DA0 professionals like han=
k murr=3D
ow have a different system.=3D0Ahe fires 28 hours in a very wonderful kiln.=
=3D
=3DA0 he down fires for many hours.=3D0Ait is a fantastic system for hank.=
=3DA0 =3D
he wants a certain look in his work and=3D0Ahe has found out how to get it.=
=3D
=3DA0 if i did that with our kilns at the farm=3D0Ait would cost me $200 to=
fir=3D
e one kiln...hundreds of gallons of propane.=3D0Ait would not work.=3DA0 we=
fir=3D
e that small kiln with 14 gallons of propane with=3D0Agreat results.=3DA0 s=
ell =3D
one pot and you pay for the firing.=3DA0 hank has, perhaps=3D0Athe most sop=
hist=3D
icated kiln i have ever seen.=3DA0 to build it, it would cost=3D0Athousands=
and=3D
thousands of dollars.=3DA0 it is a really fantastic kiln.=3DA0 but, for=3D=
0Athe =3D
average person, building a kiln in their back yard...impossible.=3D0A=3D0Ai=
enc=3D
ourage people to read nils' book...the art of firing...in many ways=3D0Ait =
sh=3D
ould be called `the science of firing`.=3DA0 it goes into real detail about=
=3D
=3D0Athe how and why.=3DA0 i can hardly do justice in notes to clayart.=3DA=
0 but,=3D
what=3D0Ai do is encourage the learning of what heat is, does and how to c=
on=3D
trol=3D0Ait.=3DA0 doing it the same, over and over without perfect or quali=
ty=3D
=3D0Aresults is the=3D0Acomplete definition of stupid.=3D0A=3D0Afolks have =
been fir=3D
ing pots for thousands of years.=3DA0 it is not the hardest=3D0Athing to do=
.=3DA0=3D
but, it does take thought, experimentation and intelligence.=3D0Ablind fai=
th=3D
in old theory smacks at modern technology.=3DA0 there are right=3D0Aand wr=
ong =3D
ways to do things.=3DA0 find the right way.=3D0A=3D0Ai love this metaphor.=
=3DA0 `ev=3D
ery time i fire my kiln, all the pots break=3D0Ainto three pieces.=3DA0 i h=
ave =3D
hundreds of pots that are broken in three=3D0Apieces.=3DA0 i think i am bei=
ng p=3D
unished for my sins...so, i will keep firing=3D0Athat clay body and see if =
it=3D
corrects itself.`=3D0A=3D0Aif you fire your kiln and every glaze runs onto=
the=3D
shelf and sticks=3D0Athe pots there forever.=3DA0 would you change the gla=
ze?=3D
=3DA0 perhaps you=3D0Amay turn off the kiln sooner, or something...but, wou=
ld y=3D
ou continue=3D0Ato use that glaze, that way?=3DA0 no, you modify.=3DA0 you =
change=3D
, you correct.=3D0A=3D0Abeing a clay person means that=3DA0 you are always =
a quar=3D
ter this, a quarter=3D0Athat...crafts person, artist, techno-geek, heat exp=
er=3D
t, gas expert,=3D0Aelectrical expert.=3DA0 if you leave a quarter out...you=
hav=3D
e failure.=3D0Athat is why we call it `CRAFT`.=3D0Amel=3D0A=3D0Ahttp://www.=
visi.com=3D
/~melpots/=3D0Aclayart page below:=3D0Ahttp://www.visi.com/~melpots/clayart=
.htm=3D
l
ivor and olive lewis on sun 18 mar 12
Mel,
Thanks for that exposition..
I am still puzzled by the auxiliary expansion chamber that Nils builds into
the base of his stacks. In a true venturi diminution of the cross sectional
area of a duct increases the velocity of the gas flow and reduces the
pressure at that point. The classic example is the Auto Carburettor.
The thing to remember, as it applies to stalling, is that the rate of
conduction of heat into the system is greatest when the temperature
difference between the load ( pottery, refractories and furniture) and the
flame front is highest. In some systems, the cause of stall is the need for
secondary air control to burn the extra fuel when insufficient air is
injected through the burners. Unburnt gas has no kinetic energy, until it
burns it is latent energy.
Sincere regards,
Ivor Lewis,
REDHILL,
South Australia
John Britt on sun 18 mar 12
Ivor,
Finally someone has realized that it is not a venturi.=3D20
You write: "I am still puzzled by the auxiliary expansion chamber that N=
=3D
ils builds=3D20
into the base of his stacks. In a true venturi diminution of the cross se=
=3D
ctional
area of a duct increases the velocity of the gas flow and reduces the
pressure at that point. The classic example is the Auto Carburettor."
Nor is merely having a one brick constriction in the chimney. Otherwise a=
=3D
damper=3D20
could also be called a venturi. According to the definition it is a tape=
=3D
ring=3D20
constriction not merely a constriction.
John Britt Pottery
Fredrick Paget on sun 18 mar 12
>Ivor,
>
>Finally someone has realized that it is not a venturi.=3D20
>
>snip.....
>John Britt Pottery
It is not a flat top either. It is a dome with a rise of about 2
inches at the middle due to being jacked up in the middle during
construction. A partial dome like this has a large horizontal
component of force so hence the constricting threaded rods which are
tightened to support that force. Then the whole thing sits on the
walls of the kiln as unit. Very clever design.
A completly flat top would have no support except friction and
adhesion of the clay in the cracks, This dome shape redirects the
downward force of gravity to a horizontal force and it is stable. It
is of course very heavy and that vertical component of force is borne
by the kiln walls.
Fred Paget
--
Twin Dragon Studio
Mill Valley, CA, USA
John Britt on mon 19 mar 12
Exactly. But why bother with words and those pesky meanings?
John Britt Pottery
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