Jonathan Pennington on mon 17 dec 01
This is a question to which I'm hoping I will find a definitive
answer. It is also titled so that, hopefully, anyone doing research
before buying an electric kiln will see this thread and learn what I
wish I had learned.
When I bought my electric kiln, we talked with the people from the
electric kiln company (Does it even matter which one really? they all
sell essentially the same thing), and the people from a pottery supply
company which shall remain nameless, from whom we bought it. Both
assured us that getting the 3 inch thick walls was basically a waste
of money, because- in their words, though not a quote: A great deal of
the energy goes into heating the walls of the kiln, instead of heating
the pots. They said it would save money by buying the thinner walled
kiln because we want to heat the pots, not the kiln walls.
Now, of course, we were naive- I knew nothing about kilns (hell, I
knew nothing about pots, having bought the wheel for my wife-
:-). Now, having read many books, seen kilns, and taken it upon myself
to build my own gas kiln, I know that the entire reason that we have
IFB- instead of the old hardbrick- is that the insulating fire
brick... well dammit... IT INSULATES (If you can't tell, I'm actually
quite mad about this, having dealt with this anger for about two
years). When the *outside* of my kiln gets hot enough to melt the
metal fittings after simply draping a blanket over it- I have to
question the statement about the thin walled kiln's "efficency." Nils
Lou, in The Art of Firing, mentions a number of times that he can
place his hand on the outside of his firing kilns. I did that
once. Once.
So, I'd like to ask outright. What wall thickness would be optimal for a
5-10 Cu. ft. kiln that goes to cone 10? Ours is 2.4in, but considering
the inset for the elements, it's actually a lot thinner- especially
since the elements themselves are *in* that inset, and therefore the
hottest part of the kiln is closest to the outside where the
heat radiates warming, as it were, my *studio* walls, not my pots.
If the kiln gurus here were going to build a kiln this size, what's the
*minimal* thickness they would have the walls? The thinnest I've seen
so far is Western Oregon University's Oregon Flat Top, with a single
layer of IFB for a three inch wall thickness (gas fired, so no inset
to thin the walls further)- but it should be noted that Nils Lou in
his book states that this is possible because ITC is used to improve
effeciency dramatically.
As for the nameless electric kiln manufacturer- as for any electric
kiln manufacturer- well, they should $#@%*&^ know better. Especially
with this many potters complaining about the thickness of electric
kiln walls (do they do *no* market research?). On that subject, let me
just say that I'll build my own kilns from now on thank you very much.
As always, thanks.
-Jonathan
(contemplating starting an electric kiln company with the money he's
saved from the liable suit by removing all the names from this email
before hitting send :-)
--
Jonathan Pennington | jwpennin@bellsouth.net
"There are no pots, there is only clay." -original
"It's hard to take life too seriously
when you realize yours is a joke." -also original
David Hendley on sat 22 dec 01
Jonathan, I hope you haven't bought a used car that was
'only used by a grandma to go to church'.
Your supplier and the representative from the kiln company
obviously sold you a big lie.
I have been disgusted for years that they continue to sell these
kilns that are so woefully under-insulated. That said, the kiln
companies are starting to offer more reasonably insulated kilns.
As for the 'optimal' wall thickness for a small electric kiln, well,
you have to take other things into account, such as being able
to fit it through a door, how much space it takes up, and, of
course, cost, and decide for yourself.
I think a 4 1/2 inch thick IFB wall backed with 1 inch of board
or blanket insulation would be pretty good and still be a reasonable
size and price. I am as concerned about slow cooling as I am
about energy use and unwanted heat in the kiln room.
If you wanted to buy a kiln with these specs, it would be very
significantly higher priced than your kiln, maybe even double.
If you are building a kiln, the extra cost of more firebricks
would be significant, but not that great compared to the whole
kiln. A kiln company, however, has to figure in the cost of
carrying extra materials inventory, quite a bit more material for
the metal skin, and of course, more mark-up to make a profit.
It adds up in a hurry. I think the kiln companies do indeed
try to give customers what they want, and it is a pretty tough
sell to have a kiln priced at twice as much as the competition,
even if it will pay for itself with energy savings over the life of
the kiln.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
----- Original Message -----
From: "Jonathan Pennington"
To:
Sent: Monday, December 17, 2001 6:26 AM
Subject: Electric kiln wall thickness (long)
> This is a question to which I'm hoping I will find a definitive
> answer. It is also titled so that, hopefully, anyone doing research
> before buying an electric kiln will see this thread and learn what I
> wish I had learned.
>
> When I bought my electric kiln, we talked with the people from the
> electric kiln company (Does it even matter which one really? they all
> sell essentially the same thing), and the people from a pottery supply
> company which shall remain nameless, from whom we bought it. Both
> assured us that getting the 3 inch thick walls was basically a waste
> of money, because- in their words, though not a quote: A great deal of
> the energy goes into heating the walls of the kiln, instead of heating
> the pots. They said it would save money by buying the thinner walled
> kiln because we want to heat the pots, not the kiln walls.
>
> Now, of course, we were naive- I knew nothing about kilns (hell, I
> knew nothing about pots, having bought the wheel for my wife-
> :-). Now, having read many books, seen kilns, and taken it upon myself
> to build my own gas kiln, I know that the entire reason that we have
> IFB- instead of the old hardbrick- is that the insulating fire
> brick... well dammit... IT INSULATES (If you can't tell, I'm actually
> quite mad about this, having dealt with this anger for about two
> years). When the *outside* of my kiln gets hot enough to melt the
> metal fittings after simply draping a blanket over it- I have to
> question the statement about the thin walled kiln's "efficency." Nils
> Lou, in The Art of Firing, mentions a number of times that he can
> place his hand on the outside of his firing kilns. I did that
> once. Once.
>
> So, I'd like to ask outright. What wall thickness would be optimal for a
> 5-10 Cu. ft. kiln that goes to cone 10? Ours is 2.4in, but considering
> the inset for the elements, it's actually a lot thinner- especially
> since the elements themselves are *in* that inset, and therefore the
> hottest part of the kiln is closest to the outside where the
> heat radiates warming, as it were, my *studio* walls, not my pots.
>
> If the kiln gurus here were going to build a kiln this size, what's the
> *minimal* thickness they would have the walls? The thinnest I've seen
> so far is Western Oregon University's Oregon Flat Top, with a single
> layer of IFB for a three inch wall thickness (gas fired, so no inset
> to thin the walls further)- but it should be noted that Nils Lou in
> his book states that this is possible because ITC is used to improve
> effeciency dramatically.
>
> As for the nameless electric kiln manufacturer- as for any electric
> kiln manufacturer- well, they should $#@%*&^ know better. Especially
> with this many potters complaining about the thickness of electric
> kiln walls (do they do *no* market research?). On that subject, let me
> just say that I'll build my own kilns from now on thank you very much.
>
> As always, thanks.
> -Jonathan
> (contemplating starting an electric kiln company with the money he's
> saved from the liable suit by removing all the names from this email
> before hitting send :-)
> --
> Jonathan Pennington | jwpennin@bellsouth.net
> "There are no pots, there is only clay." -original
> "It's hard to take life too seriously
> when you realize yours is a joke." -also original
Craig Martell on sun 23 dec 01
Hi:
Speaking of my own experience using electrics for glaze fireing, an
addition of one inch of AP Green Insblok 19 to the outer wall worked
wonders. It's not a real expensive addition either.
My goal was to add some insulation to save energy and to also slow the cool
rate. I had an Olympic 16 cu ft oval kiln that was a poorly engineered
unit and wasn't even good to cone 6. I wanted to cover the seams between
rings so I cut the board to run top to bottom on it's longest length. I
used an old radial arm saw to bevel the sides of the board to fit around
the kiln. There's a tremendous amount of heat loss thru the lids too so I
made ceramic buttons and mortared and buttoned 2600 degree fiber to the
inside of the lid.
I was a bit dissatisfied with the elecrical components of the kiln too and
changed a lot of that stuff. With the added wall thickness I had to redo
the stainless steel banding so all the electrical stuff had to come off
anyway. I welded stainless steel cable to the ends of the banding and
attached turnbuckles so I could refit. I had new elements of 13 top and
bottom and 14 guage center zones made. I dumped the interbox
connectors(pugs between rings) and hard wired all the innards. I also hard
wired the power cord to a 100 amp Bulldog disconnect.
The kiln worked great after this was finished. Cut 2.5 hours off the climb
to cone 10 and if I shut off around 6 or 7pm there would still be red heat
early the next morning. My glazes looked a lot deeper and richer with
fewer firing faults and I was a happy guy.
hope this is of some help, Craig Martell in Oregon
Michael Wendt on sun 23 dec 01
Jonathan asked about wall thickness and I now have test data to report. I
finished the installation of the "C" brick top ring in my Olympic style
updraft gas kiln. The "C" brick is 7.625" wide by 7.5" tall by 1" thick (the
top and bottom legs of the "C" extend outward 4.5" and are also 1" thick).
The hollow part of the "C" is filled with 6 lb density Kaowool and the outer
skin is stainless steel.
Results:
1. Firing time has been reduced by more than 1 hour with just the addition
of 1 ring in a 40" tall kiln!
2. Surface temperature of the ring at cone 10 is 203 degrees F while the
surface temperature of the adjacent ring made of 2600 degree IFB stacked
flat for a 4.5" wall thickness was nearly 400 degrees F.
3. The cooling rate was so much slower that we have had to resort to leaving
the damper open 1/8" to speed cooling enough to stay on a 24 hour cold to
cold schedule.
4. MOST IMPORTANT: The cone difference from top to bottom used to be cone 8
top, cone 12 middle, cone 10 bottom. Now we can get to cone 9 at the top
without overfiring the middle or bottom. Lots of the colors I use work best
at cone 9 and start to fade and blur past that so I am happy.
Conclusion: Remove the old jacket and add even 1" of kaowool around the
outside of your electric kiln. Then re-jacket it with a new larger stainless
skin. You will be surprised at the difference. Moreover, the thinner the
original kiln, the more dramatic the effect will be and as an added bonus,
the bricks last longer because they see less thermal stress which is caused
by the huge temperature difference between the inside and outside surfaces
of an essentially rigid brick structure.
Regards and Holiday wishes to all,
Michael Wendt wendtpot@lewiston.com
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
wendtpottery.com
OWLPOTTER@AOL.COM on mon 24 dec 01
When my old Crusader kept taking longer and longer to reach cone 9, my
husband and I decided to to take it apart to see why. The lid hinge had
fallen off after 6 firings, somewhere around 50 firings the handles fell onto
the floor, and after awhile even the logo plate on the side had fallen off.
After a couple of years the stainless steel skin was being held in place by
only 2 of the strap-type clamps at the rear - the rest had popped loose when
I tried to tighten them.
When we removed all of the metal parts, like the kiln sitter, switches,
wiring and cage etc, the kaowool blanket the manufacturer had originally
wrapped around the fire brick underneath the stainless steel jacket, fell
tinkling to the floor. It had almost entirely crystalized. That meant that
about 1 inch of firebrick was all that had been insulating our kiln.
We used aircraft stainless steel pop rivets and replaced the strap-clamps
with high temp muffler clamps. Then replaced any cracked or broken brick,
wrapped the kiln in a new 2 inch blanket, replaced the stainless steel jacket
and kiln sitter, electrical parts (which we also beefed up) including all new
elements and this kiln then went to cone 9 in less than 5 hours with a double
lid on top- but took forever to cool down! The glazes, however, were
gorgeous!
Better yet, we had just purchased our fifth Crusader, and it far outlasted
the brand new one. We have done the "replace everything" on all of our
electric kilns and they fire quickly and evenly. We discovered that even the
2 inch blanket replacement we put on that first 'remodel' crystalized over
time and needed to be replaced eventually.
If you think about it, it makes sense, even potter-built gas kilns need to be
rebuilt after continuous high firing.
-Carolynn Palmer, Somerset Center, Michigan
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