Dave Finkelnburg on mon 18 mar 02
Brandon,
Jim Wunch said he preferred using wedges for an arch, also. He said the
9-inch thick arch is much stronger.
Dave Finkelnburg
From: Brandon Phillips
>I just built two sprung arch kilns these last two months and I used wedge
>bricks for the arches. They taper the long way so you have a full 9" arch
>without having to layer bricks. Our larger kiln is 45" wide (interior) and
>using all #1 wedge bricks gave us I think about a 9" rise. No straights
>needed. Just my two cents.
John Baymore on tue 19 mar 02
Dave,
I think that particular session ended up being bit confusing to many
people.
Jim Wunch said he preferred using wedges for an arch, also. He said the
9-inch thick arch is much stronger.
A 9 inch thick arch certainly is "stronger" than a 4 1/2 inch one if the
sole consideration is structural integrity. However, that makes the enti=
re
9 inches the same K value (insulating value per inch). So while physical=
ly
stronger, it is less insulating than a composite, graded set of
refractories. So........ a trade off. Necessary........ IF structural
strength is an over-riding design criteria. In most handcraft potter siz=
ed
kilns........ it isn't.
Most of the problems with arches that I have encountered over the years o=
n
potter's kilns relate to using too light steelwork to take the thrust
generated by the arch. (Or from using ring arch construction....another
story ) The steel bends, so the arch sags.... as the arch sags the
vector of the thrust goes more to the horizontal.... so the steel bends
some more.....and so on. Structural integrity of the brick layer is not
typically the causative issue.
You also should keep in mind that the thermal stress through the section =
of
9 inches of a solid refractory can tend to promote spalling of the hot fa=
ce
(cracking parallel to the hot face surface) due to uneven heating through=
the section of the material. This is not of as much concern in many
industrial kilns.....because many of them are not periodic (heated up and=
cooled down for each cycle)........ which definately exacerbates this
issue. The shorter the cycle the kiln is fired on, the more this becomes=
a
problem. So for a huge anagama that is fired only a couple times a year
(few cycles) and is fired over many days up and then many days down (slow=
heating + cooling) ......... it is less of a concern. =
Putting in an arch using arch brick (as opposed to wedge brick) allows yo=
u
to back up the 4 1/2 inches of a certain K value with different materials=
that have higher insulating value........ which is to me a VERY desireabl=
e
consideration. Particularly in the smaller size kilns that potters tend =
to
use when compared to traditional industrial brick-type kilns. =
Structurally....... we don't often stress the refractories all that much.=
=
We also don't tend to fire the lining to the point that a great deal of i=
t
is slagging off (simply gone) like many industrial processes do with bric=
k
constructon units. An exception to this thought might ba a soda or salt=
kiln that is fired until it self destructs.
For handcraft potters, a great combination in most cases is 4 1/2 inches =
of
arch brick combined with flexible ceramic fiber blanket in graded layers
over it, with the exterior coated with either sheet metal to contain any
loose fiber dust, or a thin coating of an air setting mortar troweled or
sprayed onto the fiber. Fiber is so light that the weight contributed to=
the lower arch structure is pretty negligible.
However..... puting a second 4 1/2 arch (say of 2000 F IFB) OVER an inner=
4
1/2 inch arch (say of 2600 F IFB) has to be done correctly. If you just
put one arch directly in contact with the other (when the kiln is
built)...... when it is at temperature the hot face layer expands more th=
an
the cold face layer. This causes the outer layer of arches to simply be
pushed upward so that they are no longer supported by the buttressing of
the outer arch....but are sort of just resting on the arch brick below. =
The innner arch is way hotter than the outer arch and so it expands more.=
=
To be done right, this configuration requires that the outer arch layer b=
e
constructed with a gap to the outer surface of the inside hot face arch. =
(Hard to describe that in words ). Makes two very distinct arches wit=
h
an air space between them to allow the inner arch to rise but still miss
the hot face of the outer arch. Pain in the butt to build........ and no=
t
necessary any longer given newer refractory materials available.
I haven't built a 9 inch thick all brick arch in about 20 years now. And=
there are plenty of 20+ year old 4 1/2 inch arches I have built still
operating out there in clayland . I think that a 9 inch thick, all-wed=
ge
arch is probably structural "overkill" for most handcraft potters
applications, and is potentially wasteful of valuable energy resources. =
Maybe still useful in specialized rare applications, but not for overall
general use.
As they say....... my 2 cents worth. All the usual disclaimers . =
BTW........ nice to see you in person at NCECA.
Best,
..............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
"DATES SET: Earth, Water, and Fire Noborigama Woodfiring Workshop Augu=
st
16-25, 2002"
vince pitelka on wed 20 mar 02
> Jim Wunch said he preferred using wedges for an arch, also. He said
the
> 9-inch thick arch is much stronger.
Dave -
The 9-inch arch made entirely of wedge bricks is of course stronger and more
stable, but in the context of studio kilns it is entirely unnecessary, and
far more expensive. A 4.5-inch arch made from a mix of arch bricks and
straights, with a 4-inch layer of homemade insulating castable on top gives
a very stable, long-term 9-inch arch.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
vince pitelka on thu 21 mar 02
> I have a pallet of wedge bricks and would NEVER build a 9" arch out of
> them. Too much wasted heat. A way to use them, it occurs to me, would be
> to cut them in half crossways, making them 4.5" long.
> Bu the way Vince, what is your mix for home made insulating castable?
Paul -
One recipe we have used is 20 lbs fireclay, 20 lbs cheap builder's sand, a
heaping 5-gallon bucket of medium coarse sawdust, and 10 lbs. of Portland
cement. Mix as thick as possible (so that it will still tumble-mix) in a
cement mixer. Obviously this is NOT a hotface insulation, but it will work
outside a 4.5 inch layer of hardbrick or IFB, and it air-sets rigid enough
to do a good job of reinforcing an arch.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Paul Herman on thu 21 mar 02
Greetings Vince and arch-builders all,
I have a pallet of wedge bricks and would NEVER build a 9" arch out of
them. Too much wasted heat. A way to use them, it occurs to me, would be
to cut them in half crossways, making them 4.5" long.
Bu the way Vince, what is your mix for home made insulating castable?
Hardway Herman
----------
>From: vince pitelka
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: NCECA Tech Notes -- 9" arch
>Date: Wed, Mar 20, 2002, 8:44 PM
>
> A 4.5-inch arch made from a mix of arch bricks and
> straights, with a 4-inch layer of homemade insulating castable on top gives
> a very stable, long-term 9-inch arch.
> Best wishes -
> - Vince
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