Barbara Kobler on sun 25 jan 04
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, neither myself nor anyone else has any experience to speak of with 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.
The shelf is located only 4” from brick floor and we only fire 4-7 small pots at a time.
I recently noticed that the kiln floor is simply one layer of hard brick set into the ground with no soft brick or wall.
THE PROBLEMS: I have not been doing the firings but the raku kiln master reported the following: With a pyrometer reading of 1795 the pots were not even red when removed. The pyrometer was located directing across from the burners, in the top 1/3 of kiln.
Personally I did 4 saggar test fires, based upon the way I learned from Linda and Charlie Riggs, and even though pyrometer reading was 1700-1750 the pots never reached temperature and all were shades of grey. (sorry Linda). 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) Anyone in the Tucson area who could come and check this out for us.
Your suggestions for ways to solve this problem and also any simple information on constructing a raku kiln will be most appreciated.
Barbara
Barbara Kobler http://www.claywoman.net
Chris Schafale on mon 26 jan 04
Sounds to me like the pyrometer is bad! Your pots tell the truth -- the k=
iln is
not getting hot enough -- but you are mistakenly believing the pyrometer
about when to take them out. I'm no raku guru, but I have witnessed enoug=
h
inconsistencies with pyrometers in the few raku firings I've participated =
in. In
some cases, tapping the pyrometer with a finger caused the reading to go u=
p
by several hundred degrees.
Chris
On 25 Jan 2004 at 20:51, Barbara Kobler wrote:
> Okay guys. I=92m desperate. You=92ve 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, neither myself nor
> anyone else has any experience to speak of with raku and neither did
> anyone who built and set up the raku system.
>
> The raku kiln is approx. 4=92 dia 4=92 h. It has 2 venturi burners
> operating off of a large residential propane tank (about 6=92 long)
>
> There are 3 burner openings--6=94 sq., but only 2 are used and the
> middle one left open.
>
> The shelf is located only 4=94 from brick floor and we only fire 4-7
> small pots at a time.
>
> I recently noticed that the kiln floor is simply one layer of hard
> brick set into the ground with no soft brick or wall.
>
> THE PROBLEMS: I have not been doing the firings but the raku kiln
> master reported the following: With a pyrometer reading of 1795 the
> pots were not even red when removed. The pyrometer was located
> directing across from the burners, in the top 1/3 of kiln.
>
> Personally I did 4 saggar test fires, based upon the way I learned
> from Linda and Charlie Riggs, and even though pyrometer reading was
> 1700-1750 the pots never reached temperature and all were shades of
> grey. (sorry Linda). 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=92t 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=92t
> 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) Anyone in the Tucson area who could come and check this out for
> us.
>
> Your suggestions for ways to solve this problem and also any simple
> information on constructing a raku kiln will be most appreciated.
> Barbara
>
>
>
>
>
>
>
> 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.
--
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh, NC)
www.lightonecandle.com
NEW email: chris at lightonecandle dot com
(insert the @ and . as appropriate)
Laurie Kneppel on mon 26 jan 04
Hi Barbara,
It sounds like all your heat is going up to the top 1/3 of the kiln and =20=
the bottom where the shelf is located is too cool.
The pots should be glowing when they get to temperature.
First things to try are:
1. Plug up that extra unused burner port, too much air is probably =20
coming in and cooling the lower 2/3rds of the kiln.
2. Raise the shelf a couple more inches off the floor. The heat will =20
circulate under and around it more efficiently. Plus the pots will be =20=
raised up to a hotter part of the kiln.
3. Maybe the flue opening is too large? I think it's supposed to equal =20=
the size of the burner port openings. (not sure with two burners).
4. The floor of my raku kiln is hard brick with a layer of scrap fiber =20=
blanket over it. Maybe that helps with the heat retention? it fired =20
okay before I put that in, so maybe that's more psychological. The =20
floor of my raku kiln is cinderblocks to raise it off the bare ground =20=
with a layer of hard firebricks over that. There is a wall on top of =20
the hardbricks made out of softbricks and that is what the fiber shell =20=
sits on. The shelf sits on three hard firebrick soaps, so i guess =20
that's about 9 inches off the floor? Whatever the length of a standard =20=
brick is. The shelf has about 1 inch clearance all around from the =20
walls.
5. I have the psi at about 1.5 - 2 to start and gradually raise it to 5 =20=
when I'm finishing off the firing.
Don't know if that helps or not, but good luck with the next raku =20
firing! if you'd like to look at my raku kiln go to my website and =20
click on the Raku Firing link. I built it after reading every raku book =20=
I could find that had kiln photos, but got a lot of the best info from =20=
Stephen Branfman's Raku: A Practical Approach.
Laurie
Sacramento, CA
http://rockyraku.com
On Jan 25, 2004, at 12:51 PM, Barbara Kobler wrote:
> Okay guys. I=92m desperate. You=92ve all heard me talk about the very =
=20
> large retirement community studio (360 members) where I teach. Well =20=
> the raku firings have become a major problem.
> While I have 35 years experience developing and using oxidation =20
> glazes, neither myself nor anyone else has any experience to speak =20=
> of with raku and neither did anyone who built and set up the raku =20
> system.
>
> The raku kiln is approx. 4=92 dia 4=92 h. It has 2 venturi burners =20=
> operating off of a large residential propane tank (about 6=92 long)
>
> There are 3 burner openings--6=94 sq., but only 2 are used and the =20=
> middle one left open.
>
> The shelf is located only 4=94 from brick floor and we only fire 4-7 =20=
> small pots at a time.
>
> I recently noticed that the kiln floor is simply one layer of hard =20
> brick set into the ground with no soft brick or wall.
>
> THE PROBLEMS: I have not been doing the firings but the raku kiln =20
> master reported the following: With a pyrometer reading of 1795 the =20=
> pots were not even red when removed. The pyrometer was located =20
> directing across from the burners, in the top 1/3 of kiln.
>
> Personally I did 4 saggar test fires, based upon the way I learned =20
> from Linda and Charlie Riggs, and even though pyrometer reading was =20=
> 1700-1750 the pots never reached temperature and all were shades of =20=
> grey. (sorry Linda). I took them home and refired in my own electric =
=20
> kiln successfully.
>
> QUESTIONS:
> (1) Is it possible that the psi on the large propane tank is set too =20=
> low and the flame is therefore not producing enough heat? What psi =20=
> do you raku people use.
>
> (2) Doesn=92t the hard brick floor cause a loss of heat ? Would it be =
=20
> better to lay in a medium or soft brick floor and short wall? How =20
> 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.
>
> (3) Since the interior volume is so much larger than what we fire at =20=
> the studio, would it be better to have 2 smaller kilns instead of one =20=
> so huge?
>
> (4) Anyone in the Tucson area who could come and check this out for =
us.
>
> Your suggestions for ways to solve this problem and also any simple =20=
> information on constructing a raku kiln will be most appreciated.
> Barbara
>
>
>
>
>
>
>
> Barbara Kobler http://www.claywoman.net
>
> =
_______________________________________________________________________=20=
> _______
> 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 =20
> melpots@pclink.com.
>
Bruce Girrell on mon 26 jan 04
> (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.
Too low of a pressure would simply mean that it would take longer to get to
temperature. 5 PSI should be plenty. What is the nominal BTU output of your
burners at 5 PSI?
> (2) Doesn’t the hard brick floor cause a loss of heat ?
The added thermal mass and the higher thermal loss due to the hard brick
will make it take longer to get to temperature but this is not at the heart
of your problem.
> Would it
> be better to lay in a medium or soft brick floor and short wall?
Yes. This will improve the efficiency of your kiln.
> 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.
Our kiln floor is set on two layers of cinder block. The floor is built from
three layers of insulating fire brick. so the inside height of the kiln
floor is 19 1/2 inches from the ground. Add to that another layer of IFB to
create a flame trough and four more inches of furniture and we have the
first row of pots sitting 26 inches from the ground. Very easy to reach.
> (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?
Probably. You don't give the third dimension of your kiln, but it sounds
very large for a raku kiln. 2 x 2 x 2 would be much better.
> Your suggestions for ways to solve this problem and also any
> simple information on constructing a raku kiln will be most appreciated.
You pretty much gave the answer in your intro:
> The pyrometer was
> located directing across from the burners, in the top 1/3 of kiln.
The pyrometer is not reflecting the conditions that the pots see. It is
being highly influenced by the flame. But why are you firing raku based on a
pyrometer?????? This is a most inappropriate use of a pyrometer.
Raku needs to be fired by glaze condition. The glaze needs to achieve a good
melt and will, therefore, appear shiny when it is ready. If the glazes are
such that it is difficult to tell when they have matured (mattes would be an
example) then you need to fire by color. An alternative is to always include
a pot with a glossy glaze in the firing so that you can judge maturity. Once
you have done a number of firings where you can see the glaze mature, then
you will have a basis on which to judge color. Remember, though, that your
perception of color can change. If you normally fire during the day and then
do a night firing you may judge the pots to be done too soon if you base the
firing on color.
Brick up that third burner port, too.
Bruce Girrell
RubiaM&M on mon 26 jan 04
Dear Barbara
I am in Brazil. Reading your message I tought that perhaps I could give
you a hand.
I just built a raku and high temperature kiln for a friend. And we made
a site about it.
We made several photos of the project and also the first firing. If you
visit
www.art3d.com.br
you must do these steps to see what we have done:
1. click on "forno" (kiln) in the right side of the screen
2. then click on "forno raku" (raku kiln)
3. then click on "fotos do forno" (raku photos) left side
4. after seen these photos went back click on "forno de raku" again and
after that
click on "fotos de teste" (test photos) where you will see the
result of the firing.
- To solve temperature problems I prefer to use cone 07 (985 C).
I expect this site may be useful for you. Any way this is a chance to
show a bit of what we are doing here in Brazil. If you have any doubts
about my kiln, please fell free to ask.
Best regards
Rubia Moraes
http://fotos.terra.com.br/album.cgi/475962
Tony Ferguson on mon 26 jan 04
Barbara,
>
> 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.
>
Sounds like your kiln is not adequately insulated. The floor is the least
of your problems as you need to insulate your walls. Ask your self: "Does
it look like there is enough flame coming out?" If there isn't, then you
need to find out exactly why? Check your gages, makes sure everything is
working properly.
> (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.
>
Your suggestions are sound. Pictures of the kiln would help me better make
recommendations as I am not sure what you mean by a wall--or if there is
one, how thick, etc.
> (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?
>
Actually, this is a very good idea as you will have more control over the
firing, reduction, and use less fuel. Email me off list for more
information I could offer you and raku kilns. It would be nice, however, if
you could slavage what you have.
Thank you.
Tony Ferguson
On Lake Superior, where the sky meets the Lake
Custom & Manufactured Kiln Design
Stoneware, Porcelain, Raku and more
by Coleman, Ferguson, Winchester...
http://www.aquariusartgallery.com
218-727-6339
315 N. Lake Ave
Apt 312
Duluth, MN 55806
logan johnson on mon 26 jan 04
Barbara,
While I am not an expert I have built four raku kilns and they all work fine. Here's my take on your problem.
Yes, the volume of you raku kiln is huge (16 cu ft) and you simply aren't getting enough BTU's heat into it. Most raku kilns I have seen are less than four cubic feet in volume and as far as I'm concerned, smaller fires better. Logan has three different sizes (4 cu ft, 2.5 cu ft, and 1.5 cu ft) and we always use the smallest one possible for the pieces being fired. I think your kiln is just way to big.
If I knew more about your situation I might be able to better advise you. I could either advise you on a cheapo do-it-yourself kiln or what to buy commercially.
Dennis ( Logan's Husband)
Barbara Kobler wrote:
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, neither myself nor anyone else has any experience to speak of with raku and neither did anyone who built and set up the raku system.
Logan Johnson Audeo Studios
www.audeostudios.com
"Carpe Argillam!!"
Wally on tue 27 jan 04
Barbara,
Just a few thoughts.
a) Might be a good idea to close the middle burner hole. It might
suck up too much secondary air.
b) I don't think that this kind of giant kiln is really appropriate
for Raku. The difference between "cold spots" and "hot spots" could
be just too big for those quick raku firings. As you suggested, 2
smaller kilns may be better.
c) The problem might also be very simple : a defective
pyrometer....Maybe you could double-check this pyrometer by firing
it in a small kiln, together with a pyrometer that you experienced
to be previously accurate.
d) If above is OK, and you want to continue with this monster-kiln,
suggest to put a layer of fibre on the brick floor.
e) Most willing to come and check it out..... Just send me a return
ticket...... ))---ooo
Good luck !
Wally, from freezing Flanders.
< In clayart@yahoogroups.com, Barbara Kobler wrote:
> 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, neither myself nor anyone else has any experience to speak
of with 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.
>
> The shelf is located only 4" from brick floor and we only fire 4-7
small pots at a time.
>
> I recently noticed that the kiln floor is simply one layer of hard
brick set into the ground with no soft brick or wall.
>
> THE PROBLEMS: I have not been doing the firings but the raku kiln
master reported the following: With a pyrometer reading of 1795 the
pots were not even red when removed. The pyrometer was located
directing across from the burners, in the top 1/3 of kiln.
>
> Personally I did 4 saggar test fires, based upon the way I learned
from Linda and Charlie Riggs, and even though pyrometer reading was
1700-1750 the pots never reached temperature and all were shades of
grey. (sorry Linda). 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) Anyone in the Tucson area who could come and check this out
for us.
>
> Your suggestions for ways to solve this problem and also any
simple information on constructing a raku kiln will be most
appreciated.
> Barbara
>
>
>
>
>
>
>
> Barbara Kobler http://www.claywoman.net
>
>
_____________________________________________________________________
_________
> Send postings to clayart@l...
>
> 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@p...
Rare - Earth - Design on tue 27 jan 04
----- Original Message -----
From: "Bruce Girrell"
To:
Sent: 26 January 2004 17:50 PM
Subject: Re: Raku Firing Much Big Problems HELP
Bruce,
I gathered that the kiln was circular, the measurements given were
4' Dia x 4' high which gives approx. 50 cu. ft., a monster in which
a few small pots are being fired.
Surely there must be some mistake in the given measurements?
If not, the problem is obvious, as is the solution, "definately a smaller
kiln".
Regards,
Bob Hollis
>
> Probably. You don't give the third dimension of your kiln, but it sounds
> very large for a raku kiln. 2 x 2 x 2 would be much better.
>
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