Bruce Girrell on sat 1 nov 08
So...
when stacking a kiln, is there any particular rationale for placing larger =
pieces on the bottom layer, graduating toward smaller pieces at the top or =
perhaps smaller pieces at the bottom graduating toward larger at the top?
I see two things at issue: 1) stability of the shelving and 2) heat flow. A=
re there others?
Stability of the stack:
Kiln posts aren't very wide and, as a result, when they are tall, they can =
be wobbly. Anything stacked above that point would only increase wobbliness=
. So it would seem that placing taller pieces toward the bottom could creat=
e a somewhat unstable stack, whereas placing shorter pieces toward the bott=
om would ted to create a more stable stack.
Heat flow:
Just when I think I understand something about how heat moves in a kiln, I =
realize that I don't really understand it at all. So I'm a bit out on a lim=
b here. But somehow, it seems like a bad idea to be placing smaller items t=
oward the bottom of a stack. Having the shelves closer together toward the =
bottom of the kiln doesn't seem like a good idea, but since a lot of what h=
appens in kilns has more to do with radiation than convection or conduction=
, I'm not really sure.
In looking at kiln stacks that others have done, it seems that most people =
stack larger items toward the bottom.
A) Is my perception correct? With the exception of the top shelf, which mig=
ht have some unusually tall pieces, do most people stack larger items towar=
d the bottom of the kiln?
B) Is there a good reason to stack either larger or smaller items at the bo=
ttom?
What I'm actually trying to do is to plan the placement of spy holes. Our o=
ld kiln would often have a stack like
-----------------
| 4" space
----------------
| 6" space
----------------
| 8" space
----------------
The new kiln will have room for at least 4 layers instead of 3. So I'm tryi=
ng to come up with a "typical" shelf placement to help plan the location of=
the spy holes. I like to use draw rings, so the spy holes need to be close=
enough to the shelf height that I can pull the rings without banging them =
into other things like cone packs, pottery being fired, the next higher she=
lf, etc.
Thanks for your input.
Bruce Girrell
in gorgeously sunny northern Michigan
where the fall colors still abound and it's time to get out and buy some ap=
ples
Lee Love on mon 3 nov 08
I guess tall is relative. My tallest pieces are taller than my
props, so they have to go on the top of the load.
--
Lee Love in Minneapolis
http://togeika.multiply.com/journal
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Dale Neese on mon 3 nov 08
Yes there are other factors that you need to supply... updraft or downdraft.
Forced air or natural draft? You would have to take these things into
consideration as well as dimensions of your stacking space, bag wall or no
bag wall.
Dale Tex
"across the alley from the Alamo"
Helotes, Texas USA
www.daleneese.com
Bruce Girrell on mon 3 nov 08
Dale Tex wrote
...there are other factors that you need to supply... updraft or downdraft.
Forced air or natural draft? You would have to take these things into
consideration as well as dimensions of your stacking space, bag wall or no
bag wall.
Dale,
Thanks for helping to clarify some things
The kiln interior volume is about 28 cu ft. It is a downdraft sprung arch w=
ith two Ransome B3 atmospheric burners. No bag wall per se, but the flame i=
s confined to troughs along the base of each wall. The height of the trough=
could be increased should I find that the lowest shelf is being scorched. =
Alternatively, I could place a row or two of bricks to help screen the lowe=
st shelf from direct contact with the flames. I expect to have to do some t=
uning like that after I get the thing in operation. But is there a wall tha=
t the flame smacks into and turns a right angle upward? No. The flame runs =
parallel with the trough.
Bruce Girrell
William & Susan Schran User on tue 4 nov 08
On 11/1/08 1:40 PM, "Bruce Girrell" wrote:
> So...
> when stacking a kiln, is there any particular rationale for placing larger
> pieces on the bottom layer, graduating toward smaller pieces at the top or
> perhaps smaller pieces at the bottom graduating toward larger at the top?
Much depends on the kiln, but I will assume you are asking regarding a
downdraft, gas fired kiln.
I have found having the bottom layer stacked at least as high as the height
of the flue seems to produce more even temperatures top to bottom.
I also place most short pots towards the middle.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Dale Neese on tue 4 nov 08
Bruce,
I see.
The stacking heights for shelving would be critical in the lower part in the
back of the kiln as not to impede the flow of gases to the exit flue. The
Ransome Burners, coming from the rear wall of the kiln will shoot a pretty
long flame towards the front wall before turning upwards at the front wall.
You could place a target brick at some point within the trough to deflect
the flame upwards sooner. The exact distance could be determined by
monitoring several firings with cone plaques placed around the stack. I've
seen this kiln layout in the Bailey downdraft kilns of similar size at it
seems to me that the lower shelves were not any shorter or higher than the
exit flue was tall. The bottom shelf was up off the floor with one inch,
maybe 1/2" posts. Then the first shelf's bottom came right to the top of the
exit flue in height. Probably no more than 8 inches. I agree that the taller
normal stilts in the bottom would tend to make the entire stack unstable as
you go up but eight inches might be the limit you want. You could substitute
9 inch hard brick soaps, (9 in. x 2-1/4 in. x 2-1/2in) for the bottom stilts
which would give you more stability and another inch in height. I think the
height of this first shelf, 8 or 9 inches is where your spy holes, cones
should be. Any lower and you would have an opening into a critical area of
the draft towards the exit flue. The downdraft I built had a flue channel in
the floor under the first shelf so the heat was drawn down under the first
shelf before exiting the rear. Something I've only seen incorportated in a
kiln a few times.
Draw rings? Are you planning to introduce some soda or salt? The size of
spy holes needed to pull rings is going to let a lot of cold air into the
kiln around your cones.
Another critical area is the arch. Too much ware is sometimes stacked into
that area to fill the kiln and the gases get held up there like a dam making
an even firing sometimes impossible. You would have to have a strong draft
to get that heat back down to the bottom. Too strong of a draft and the
flame get sucked out sooner leaving colder spots in the front. I don't like
to stack any kiln overly tight in the arch area. If you are going to have 4
layers I suggest the top spy be on the 3rd layer.
------------------top
4"-3"?
---X-------------
4"
------------------
6"
----------------X-
9"
-------------------bottom
1/2"-1"
------------------- kiln floor
Also the positioning of the stack itself within the kiln, distance to the
front and back walls. I have to try and imagine how the flame will act
entering the kiln, where it hits, bends and gets back to the exit flue. Is
there plenty of space in the front to allow gases to go before being drawn
back thru the stack and down into the exit flue. Spacing between shelves
side to side, is there space for circulation? I would consider more in the
front and less space in the rear between the shelves and the back wall.
Gad, where did the time go? It's late and those are some of my suggestions
to ponder.
Hope you get er done. Trails.
Dale Tex
"across the alley from the Alamo"
Helotes, Texas USA
www.daleneese.com
jonathan byler on tue 4 nov 08
as long as the posts are clean on the ends, I have had no trouble
stacking our gas car kilns on 12" posts. it looks a bit shaky
depending on the layer of kiln wash, but it takes a lot more to knock
them down than one might think. also, the weight bearing down from
the upper shelves actually seems to add to the stability. I find
packing tighter at the top of our bailey down draughts seems to help
with even firing.
jon byler
3-D Building Coordinator
Art Department
Auburn University, AL 36849
On Nov 4, 2008, at 1:12 AM, Dale Neese wrote:
> Bruce,
> I see.
> The stacking heights for shelving would be critical in the lower
> part in the
> back of the kiln as not to impede the flow of gases to the exit
> flue. The
> Ransome Burners, coming from the rear wall of the kiln will shoot a
> pretty
> long flame towards the front wall before turning upwards at the
> front wall.
> You could place a target brick at some point within the trough to
> deflect
> the flame upwards sooner. The exact distance could be determined by
> monitoring several firings with cone plaques placed around the
> stack. I've
> seen this kiln layout in the Bailey downdraft kilns of similar size
> at it
> seems to me that the lower shelves were not any shorter or higher
> than the
> exit flue was tall. The bottom shelf was up off the floor with one
> inch,
> maybe 1/2" posts. Then the first shelf's bottom came right to the
> top of the
> exit flue in height. Probably no more than 8 inches. I agree that
> the taller
> normal stilts in the bottom would tend to make the entire stack
> unstable as
> you go up but eight inches might be the limit you want. You could
> substitute
> 9 inch hard brick soaps, (9 in. x 2-1/4 in. x 2-1/2in) for the
> bottom stilts
> which would give you more stability and another inch in height. I
> think the
> height of this first shelf, 8 or 9 inches is where your spy holes,
> cones
> should be. Any lower and you would have an opening into a critical
> area of
> the draft towards the exit flue. The downdraft I built had a flue
> channel in
> the floor under the first shelf so the heat was drawn down under
> the first
> shelf before exiting the rear. Something I've only seen
> incorportated in a
> kiln a few times.
> Draw rings? Are you planning to introduce some soda or salt? The
> size of
> spy holes needed to pull rings is going to let a lot of cold air
> into the
> kiln around your cones.
> Another critical area is the arch. Too much ware is sometimes
> stacked into
> that area to fill the kiln and the gases get held up there like a
> dam making
> an even firing sometimes impossible. You would have to have a
> strong draft
> to get that heat back down to the bottom. Too strong of a draft and
> the
> flame get sucked out sooner leaving colder spots in the front. I
> don't like
> to stack any kiln overly tight in the arch area. If you are going
> to have 4
> layers I suggest the top spy be on the 3rd layer.
> ------------------top
> 4"-3"?
> ---X-------------
> 4"
> ------------------
> 6"
> ----------------X-
> 9"
> -------------------bottom
> 1/2"-1"
> ------------------- kiln floor
> Also the positioning of the stack itself within the kiln, distance
> to the
> front and back walls. I have to try and imagine how the flame will act
> entering the kiln, where it hits, bends and gets back to the exit
> flue. Is
> there plenty of space in the front to allow gases to go before
> being drawn
> back thru the stack and down into the exit flue. Spacing between
> shelves
> side to side, is there space for circulation? I would consider
> more in the
> front and less space in the rear between the shelves and the back
> wall.
> Gad, where did the time go? It's late and those are some of my
> suggestions
> to ponder.
> Hope you get er done. Trails.
>
> Dale Tex
> "across the alley from the Alamo"
> Helotes, Texas USA
> www.daleneese.com
Ron Roy on mon 10 nov 08
Hi Bruce,
Much depends on your kiln - you will have to make adjustments as you go
along - they are all different. Some times you need to stack more open in
the bottom because to takes longer to heat up there - especially if you
have a hard brick floor - the extra mass takes longer to heat up.
Also the bottom will cool faster so those forms that have a tendency to
dunt should be stacked higher because they will cool slower.
No reason to not have lots of spy holes - you can plug and unplug them as
needed - I can remember drilling a few new ones where I needed them.
You can use hard bricks as posts near the bottom - I can remember stacking
large pots - that I had to slide in - the bricks I used for those pots were
in contact with the rear wall to help stabilize the stack,
The most important advice I ever got about making adjustments was to direct
flame to the cool spots - same for those areas where you don't get proper
reduction. You can use bricks to direct flame same as you see rocks in a
stream moving water around.
Take pictures of the kiln after each firing - you will be able to learn
from the different colours of the bricks - the pale areas will tell you
where you are not getting enough reduction.
It can be heartbreaking in the beginning but getting in there with the pots
will be a big help - remember to give the kiln a chance to even out -
firing fast will not be helpful.
RR
>So...
>when stacking a kiln, is there any particular rationale for placing larger
>pieces on the bottom layer, graduating toward smaller pieces at the top or
>perhaps smaller pieces at the bottom graduating toward larger at the top?
>
>I see two things at issue: 1) stability of the shelving and 2) heat flow.
>Are there others?
>
>Stability of the stack:
>Kiln posts aren't very wide and, as a result, when they are tall, they can
>be wobbly. Anything stacked above that point would only increase
>wobbliness. So it would seem that placing taller pieces toward the bottom
>could create a somewhat unstable stack, whereas placing shorter pieces
>toward the bottom would ted to create a more stable stack.
>
>Heat flow:
>Just when I think I understand something about how heat moves in a kiln, I
>realize that I don't really understand it at all. So I'm a bit out on a
>limb here. But somehow, it seems like a bad idea to be placing smaller
>items toward the bottom of a stack. Having the shelves closer together
>toward the bottom of the kiln doesn't seem like a good idea, but since a
>lot of what happens in kilns has more to do with radiation than convection
>or conduction, I'm not really sure.
>
>In looking at kiln stacks that others have done, it seems that most people
>stack larger items toward the bottom.
>A) Is my perception correct? With the exception of the top shelf, which
>might have some unusually tall pieces, do most people stack larger items
>toward the bottom of the kiln?
>B) Is there a good reason to stack either larger or smaller items at the
>bottom?
>
>What I'm actually trying to do is to plan the placement of spy holes. Our
>old kiln would often have a stack like
>
>-----------------
>| 4" space
>----------------
>| 6" space
>----------------
>| 8" space
>----------------
>
>The new kiln will have room for at least 4 layers instead of 3. So I'm
>trying to come up with a "typical" shelf placement to help plan the
>location of the spy holes. I like to use draw rings, so the spy holes need
>to be close enough to the shelf height that I can pull the rings without
>banging them into other things like cone packs, pottery being fired, the
>next higher shelf, etc.
>
>Thanks for your input.
>
>Bruce Girrell
>in gorgeously sunny northern Michigan
>where the fall colors still abound and it's time to get out and buy some apples
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on wed 12 nov 08
It is very helpful to have the flue the whole length of the floor - that
way you can adjust to bring flame to where it's needed.
Just provide for moveable bricks so the spaces (flue exit) can be adjusted.
RR
>Dale Tex wrote
>
>...there are other factors that you need to supply... updraft or downdraft.
>Forced air or natural draft? You would have to take these things into
>consideration as well as dimensions of your stacking space, bag wall or no
>bag wall.
>
>
>
>Dale,
>
>Thanks for helping to clarify some things
>
>The kiln interior volume is about 28 cu ft. It is a downdraft sprung arch
>with two Ransome B3 atmospheric burners. No bag wall per se, but the flame
>is confined to troughs along the base of each wall. The height of the
>trough could be increased should I find that the lowest shelf is being
>scorched. Alternatively, I could place a row or two of bricks to help
>screen the lowest shelf from direct contact with the flames. I expect to
>have to do some tuning like that after I get the thing in operation. But
>is there a wall that the flame smacks into and turns a right angle upward?
>No. The flame runs parallel with the trough.
>
>Bruce Girrell
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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