Craig Dunn Clark on mon 26 jan 04
Barbara, first of all the kiln is a big waste of fuel, energy, time,
etc., if you are only using it to fire a few small pots. A raku kiln of that
size is built to fire big pots, not little ones. So the first thing to do is
to make yourself a much smaller kiln. Try and keep the dimentions at
2ftx2ftx2ft or less. Do you know anything about building a raku kiln? If not
contact me off list, or better yet call me, and I'll tell you what has
worked for me and what I've seen a number of others do. I would also suggest
reading both Pipenburgs books and those of Steve Branfam. Each has
information that describes the raku kiln building process as do a number of
older Ceramics Monthly's, Clay Times and Pottery Making Illustrated may have
also done a piece one though I'm not sure of the latter.
If you want to use the kiln you have described, which just happens to be
the size of the envelope kiln that I will hopefully be plumbing in before
the end of winter, then I suggest that you invest in at least one more inch,
preferably two or three, of fiber blanket.I have 4 inchs on mine and will
most likely add another inch or two as soon as the thing pays for itself.
Get at least a 6lb density blanket with a continuous duty rating of 2250 F
or higher for the hot face. The back up layers can be lower and the blanket
less dense. Having only one inch of blanket on a kiln that size is less than
ideal.
As to the descrepancy between what the pyrometer is indicating and what
is happening with the pots I'd probably have to see what is going on for
myself to give you a definitive answer. The best that I can speculate is
that the pyrometer may be in a "hot" part of the kiln and the pots may be in
a "cold" part of the kiln.
Generally speaking I do not use a pyrometer at all in my raku firings. I
fire by the color of the atmosphere and by looking for a reflection on the
side of one of the pots in the kiln. This is where a strategically
positioned peep hole is of great importance. Just make it about an inch or
so in diameter and plug it up with blanket until you want to take a peek.
Plug it back up when finished. You can do a rough monitoring of the
atmosphere by looking at the top of the kiln. When you start getting a nice
bright orange it's usually time to pull the pots. The temperature of 1795F
is a bit low for all of my glazes. None of them mature until atleast 1850F
or so.
When firing the kiln try and get a balance of heat in the kiln just as
you would in any reduction firing. Plug up the unused burner port before you
even begin firing. The size of the burner ports and the exhaust is
important. An open burner port without a burner is a source of considerable
air and will make the control of the atmoshpere difficult at best. Does the
kiln climb any higher than 1795F? If so just fire it up intil you see the
glazes melt and then pull them for post firing reduction. If not you either
have a problem with the burner port sizes, the exhaust size,, the proximity
of the burners to the ports (keep them within about a half an inch), the
size of the burners (they may not have enough umph to fire a kiln that large
with such poor insulation), or they aren't being adjusted properly.
It has always been easiest for me to use side loading kilns up on metal
stands for raku (this is not the set up I now have though. The big kiln is a
large heavy 11 gauge cylinder that is is conterweighted and lifts up off the
stack. I also use a coupla old electrics.) With a side load kiln the user is
not subjected to anywhere near the heat stress and the pots are easier, in
my opinion, to remove. The stands have tended to be about three feet tall.
THat way there is plenty of room underneath to hang a coupla small venturi
burners. A really good source of information and materials for this is a guy
in Dandridge Tennesse. He knows most of what there is to know about burners,
kilns, etc. His name is Marc Ward and he has a business which I believe is
called Ward Burners. Just follow this link http://www.wardburner.com/. The
man is very forthcoming with information and will set you up with anything
you need.
Hope this helps you out a bit
Call me if you would like and I'll try and answer the questions in more
detail
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Barbara Kobler"
To:
Sent: Monday, January 26, 2004 10:27 AM
Subject: Raku Big Firing Problems HELP long long
> This post may be a repeat. I was having trouble receiving and posting
yesterday. If it is I'm sorry. Also I added a couple of questions.
>
> Okay guys. I'm desperate. You've all heard me talk about the very large
retirement community studio (360 members) where I teach. Well the raku
firings have become a major problem. While I have 35 years experience
developing and using oxidation glazes and firing, neither myself nor any
other members at the studio have any experience to speak of firing raku and
neither did anyone who built and set up the raku system.
>
> The raku kiln is approx. 4' dia 4' h. It has 2 venturi burners operating
off of a large residential propane tank (about 6' long)
>
> There are 3 burner openings--6" sq., but only 2 are used and the middle
one left open. With a top hole about 8" dia. There is no peephole, only
the one on top and the empty hole along floor.
>
> The shelf is located only 4" from brick floor and we only fire 4-7 small
pots at a time. (usually 4" h and 8"dia or 12"h and 4"d)
>
> I recently noticed that the kiln floor is simply one layer of hard brick
set into the ground with no soft brick or wall. Therefore the flame is only
2" from the floor and just barely under the shelf, and the metal rim edge
along the bottom of the kiln wall sets directly on the hard brick floor.
>
> The kiln is made of hard metal mesh, with 1" fibre.
>
>
>
> THE PROBLEMS: I have not been doing the firings but have been present at
about 3 in the last month. Yesterday the people who fired the raku kiln
reported the following: With a pyrometer reading of 1795 the pots were
removed but they were not even red and did not ignite paper. The pyrometer
was located directing across from the burners, in the top 1/3 of kiln. P.S.
the pyrometer is a new unimax and has been tested. It does work properly.
>
> Last month I did 5 saggar test firings the way I learned from
> Linda and Charlie Riggs,at their workshop, and sucessfully do them in my
own studio. and even though pyrometer reading was 1700-1750 the pots never
reached temperature and all were shades of grey. I took them home and
refired in my own electric kiln successfully.
>
> QUESTIONS:
> (1) Is it possible that the psi on the large propane tank is set too low
and the flame is therefore not producing enough heat? What psi do you raku
people use.
>
> (2) Doesn't the hard brick floor cause a loss of heat? Would it be better
to lay in a medium or soft brick floor and short wall? How would you
recommend construction of a base so we older folks don't have be down on our
bellies. How do youse guys set it up.
>
> (3) Since the interior volume is so much larger than what we fire at the
studio, would it be better to have 2 smaller kilns instead of one so huge?
>
> (4) How large and where should a peephole be?
>
> (5) Wouldn't it be best to cover up the 3rd burner hole at bottom that is
not used.
>
> (6) Where should the pyrometer be located in relationship to the burners?
>
> (7) Any one know of a raku person in the Tucson area who could come over
for a consult both as a raku builder and/or artist? I'll contact SACA
(so.AZ caly artists) but one of you may also know someone.
>
> (8) Any other help you guys can come up with.
>
>
> Barbara Kobler http://www.claywoman.net
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Cate Loveland on mon 26 jan 04
Barbara,
While I don't profess to be an expert, I made my wife's raku kiln and have
fired it sucessfully for about 2 years....
In order of question:
The pressure needed for a venturi burner varies, but the info on our burner
states 10" of water pressure backpressure (while the valve is open). You didn't
state whether the line ran straight from the tank, or through regulators -
regulators are a requirement!
The hard brick floor will lose heat, but I don't believe that is a major
problem - more likely your firebox ( the area between the shelf and the floor is
way too small. I have one burner in an 18" diameter kiln, and have the shelf
8" from the floor. The additional space give more room for oxidation of the gas
while it burns - this increases the heat!
Two smaller kilns sounds good, but you can work with what you have - remember
that heat rises and you have your pots about as low as they can go without
actually being engulfed in flames. raise the bottom shelf, and for smaller
loads, raise it again - I keep our pots in about the top third of our kiln.
I would definitely close the unused burner hole. I might also restrict the
flue (hole at the top). While not enough air keeps the flame from burning
hottest, too much just cools everything down.
I don't worry about where the pyrometer is with regard to the burners, I want
to know the temperature where the pots are; hence I always place the
pyrometer so it's slightly above the middle of the group of pots.
I know that I might be available to monitor a firing - I do not gaurantee
results, but Cate has been pleased lately...
daniel on mon 26 jan 04
Hi Barbara,
I'm sure that others will offer advice from greater experience but here
goes.
I fire three different raku kilns. Two have pyrometers and one doesn't. Of
the two that do only one can be relied upon to any extent.
One of the kilns is relatively tall and are simply gas up through the
bottom and out through the top. The uneveness between bottom and top can be
substantial. In the larger kiln I will often put a stack of bricks under a
piece to get it higher up the kiln where the heat is more even. With tall
pieces getting it even from bottom to top is a bit harder in the larger
kiln. In your case this should not really be a problem.
I was thinking about the 1750-1795 temp range. If the pieces hit this raku
glazes should be feeling it. The way I generally fire is to have at least
one 80/20 (GB/nepheline syenite) piece in there or a copper luster. They
melt very obviously and you can fire by eye with them. Mattes are much
harder to read. I do check the temp as I go for fun but its not much of a
gauge except on the large kiln, and then with provisos above.
All of the above said, if the pyro is reading 1700+ and your using cone
06-04 range glazes and their not even sintering then either the pyro is
optimistic or the heat gradient is higher than I'm guessing at. If the pyro
is right then I don't think gas supply is really your problem. Get the pots
up into the heat. Choke off the flow through of heat a bit and fire a little
more slowly ( I don't know how fast you are firing actually so take this
with a grain of salt :) ) so its more even and see how that goes. One
interesting test would be to run some tests with tiles with 80/20 on them at
different heights in the kiln and see which ones melt and at what pyro
reading. Then you'd know more exactly what's going on.
Finally on gas pressure, I'm no expert at all but the propane tanks we have
at one place usually read 10-12psi and they can supply the big gas kilns
while I'm raku'ing with no problems.
HTH
D
Barbara Kobler on mon 26 jan 04
This post may be a repeat. I was having trouble receiving and posting yesterday. If it is I'm sorry. Also I added a couple of questions.
Okay guys. I’m desperate. You’ve all heard me talk about the very large retirement community studio (360 members) where I teach. Well the raku firings have become a major problem. While I have 35 years experience developing and using oxidation glazes and firing, neither myself nor any other members at the studio have any experience to speak of firing raku and neither did anyone who built and set up the raku system.
The raku kiln is approx. 4’ dia 4’ h. It has 2 venturi burners operating off of a large residential propane tank (about 6’ long)
There are 3 burner openings--6” sq., but only 2 are used and the middle one left open. With a top hole about 8" dia. There is no peephole, only the one on top and the empty hole along floor.
The shelf is located only 4” from brick floor and we only fire 4-7 small pots at a time. (usually 4" h and 8"dia or 12"h and 4"d)
I recently noticed that the kiln floor is simply one layer of hard brick set into the ground with no soft brick or wall. Therefore the flame is only 2" from the floor and just barely under the shelf, and the metal rim edge along the bottom of the kiln wall sets directly on the hard brick floor.
The kiln is made of hard metal mesh, with 1" fibre.
THE PROBLEMS: I have not been doing the firings but have been present at about 3 in the last month. Yesterday the people who fired the raku kiln reported the following: With a pyrometer reading of 1795 the pots were removed but they were not even red and did not ignite paper. The pyrometer was located directing across from the burners, in the top 1/3 of kiln. P.S. the pyrometer is a new unimax and has been tested. It does work properly.
Last month I did 5 saggar test firings the way I learned from
Linda and Charlie Riggs,at their workshop, and sucessfully do them in my own studio. and even though pyrometer reading was 1700-1750 the pots never reached temperature and all were shades of grey. I took them home and refired in my own electric kiln successfully.
QUESTIONS:
(1) Is it possible that the psi on the large propane tank is set too low and the flame is therefore not producing enough heat? What psi do you raku people use.
(2) Doesn’t the hard brick floor cause a loss of heat? Would it be better to lay in a medium or soft brick floor and short wall? How would you recommend construction of a base so we older folks don’t have be down on our bellies. How do youse guys set it up.
(3) Since the interior volume is so much larger than what we fire at the studio, would it be better to have 2 smaller kilns instead of one so huge?
(4) How large and where should a peephole be?
(5) Wouldn't it be best to cover up the 3rd burner hole at bottom that is not used.
(6) Where should the pyrometer be located in relationship to the burners?
(7) Any one know of a raku person in the Tucson area who could come over for a consult both as a raku builder and/or artist? I'll contact SACA (so.AZ caly artists) but one of you may also know someone.
(8) Any other help you guys can come up with.
Barbara Kobler http://www.claywoman.net
Marcia Selsor on tue 27 jan 04
Barbara Kobler wrote:
> QUESTIONS:
> (1) Is it possible that the psi on the large propane tank is set too lo=
w and the flame=20
is therefore not producing enough heat? What psi do you raku people use.
I start with 1.5-2 psi and work up to 2.5-3 I go to 3.5 for the bigger=20
kiln.
(I have 3 small propane tanks with two burners coming from them. I can=20
fire my big raku kiln with them OR two smaller ones using one burner each=
..
>=20
> (2) Doesn=92t the hard brick floor cause a loss of heat? Would it be b=
etter to lay=20
in a medium or soft brick floor and short wall?
I use a blanket of fiber on the floor. It heats quickly.
How would you recommend construction of a base so we older folks don=92t=20
have be down
on our bellies. How do youse guys set it up.
I don't quite understand this question but I load from the top on two=20
smaller kilns and lift the walls on pulleys for the big kiln.
>=20
> (3) Since the interior volume is so much larger than what we fire at th=
e studio,=20
would it be better to have 2 smaller kilns instead of one so huge?
If you are not using the kiln to capacity, then yes you should use=20
smaller kilns.
>=20
> (4) How large and where should a peephole be?
I use a peephole 1" in diameter placed where I can see the surface of=20
the pots.
>=20
> (5) Wouldn't it be best to cover up the 3rd burner hole at bottom that=
is not used.
Yes, plug it with fiber.
>=20
> (6) Where should the pyrometer be located in relationship to the burner=
s?
The pyrometer should be placed near the pots to read the temperature of=20
the glaze.
>=20
> (7) Any one know of a raku person in the Tucson area who could come ove=
r for a consult
both as a raku builder and/or artist? I'll contact SACA (so.AZ caly=20
artists) but
one of you may also know someone.
Can't help there.
>=20
> (8) Any other help you guys can come up with.
My two small kilns were built from one roll of two inch fiber. One is=20
24" in diameter and the other is 18". I use the small one for individual=20
firing of horsehair pots. My big kiln is 35" x 27" x 40" with the=20
chamber on pulleys.
I think you should raise the shelf. I have some great perforated shelves=20
I got from Euclids in Canada. You might be able to find some closer to=20
home. They have one inch holes in the. Great for circulation, but these=20
shelves are not necessary for successful raku.
Always use proper safety measures when handling fiber. I spray with ITC o=
n
the inside and a rigidizer on the outside. Wear mask and cover skin and
goggles for the eyes.
Happy Raku firing.
Marcia Selsor
>=20
>=20
Odin Maxwell on wed 28 jan 04
On Mon, 26 Jan 2004 16:27:44 GMT, Barbara Kobler
wrote:
> I recently noticed that the kiln floor is simply one layer of hard brick
> set into the ground with no soft brick or wall. Therefore the flame is
> only 2" from the floor and just barely under the shelf, and the metal
> rim edge along the bottom of the kiln wall sets directly on the hard
> brick floor.
I built a raku kiln a couple years ago. At first, I had it resting on
hardbricks on the ground. I could barely get red heat even firing it for
an hour. At that point I wasn't sure whether it was the bricks, the
thickness of my fiber, or the power of my burner. Easiest to check was
the bricks. I bought a piece of insulating board about an inch thick. I
placed it over the bricks (inside kiln brick faces covered) and it fired
right up. As these boards aren't very durable, I picked up a piece of
steel plate at a scrap yard, then put down a layer of 3" thick insulating
firebricks. I braced them with angle iron to the steel plate. The plate
makes leveling the kiln on uneven ground easy - it isn't a necessary thing
though. So, before deconstructing the kiln, do a test by puting down some
fiber or other insulating refractory over the bricks and see if it helps.
If it does, you know where the problem is.
I used 2" of fibre, My kiln is a tube about 2'x 2' 3" (insualting brick
ring adds 3" to height). I connect a 115k BTU venturi burner to a 30 lb
propane tank. We've fired to temperature in as fast as 10 minutes:
preheated kiln, regulator removed (very unadvisable and I don't recommend
this). Interestingly, procelain seems to survive this kind of abuse much
better than stoneware or "raku" clay - usually we take 25-30 minutes.
As for a pyrometer, the most sure-fire way to pull the pieces at the right
time is to peek. When standing by my kiln, you can look right down the
exhaust hole into the kiln (looking at an angle of course, too close to
directly overhead and you singe your eyebrows - no billed hats allowed).
When the surface of the glaze is shiny and smooth, the pots are ready.
Although you may want to experiment with the cratered surface of
underfired glaze. It's interesting to watch, the glaze boils, then heals
over leaving warts, then finally smoothes out completely. Fun (bad for
the eyes).
Last thing, the hottest part of a flame is near it's tip. So if you see
flames rising from the exhaust port, you are wasting energy. I like to
adjust the flame by turning it on till flames come out the exhaust, and
then back off till they just dissapear inside. Works like a charm.
Last, and this is only for you pyromaniacs out there. Get a 3lb coffee
tin. Get the kiln hot. Turn off the gas. Put the coffe can over the
exhaust port upside down. Turn on the gas. POOF - the gas hits the hot
kiln, pops, and the can will catch about 2-3' or air. Again, this
practice is not advisable and I'm not reccomending it. ;-)
| |
|
