Donn Buchfinck on mon 11 oct 99
Hi
I have a friend who is building a kiln and they would like to know the theory
behind stretcher courses.
how many courses between them
do you need them
any and all info would be appreciated
thanks
Donn Buchfinck
San Francisco
John K Dellow on tue 12 oct 99
Don ,
I wonder if you confusing "header course" with "stretcher
course ". i.e. a stretcher is the normal way to lay a brick on
its 41/2" x 9" side and a header course is used to strengthen a
wall be being lade across two stretcher courses.
If your friend is going to use header bricks then he needs
to be careful with shrinkage rates ,if the courses are made up of
different brick types ,for example K26 & hard brick .
The 120 cu.ft kiln I recently pulled down had a cheap K20's
on the inside & diatoms on the outside and a header course every
5 courses ,with diatoms as the header. The K20's shrank more then
the diatoms and caused the inner wall to work loose . No big
drama in a terra cotta kiln but would have been messy in a glaze
kiln .Lots of spalling :).
Donn Buchfinck wrote:
>
> ----------------------------Original message----------------------------
> Hi
> I have a friend who is building a kiln and they would like to know the theory
> behind stretcher courses.
> how many courses between them
> do you need them
>
> any and all info would be appreciated
>
> thanks
>
> Donn Buchfinck
> San Francisco
--
John Dellow "the flower pot man"
Home Page http://www.welcome.to/jkdellow
Craig Martell on tue 12 oct 99
Donn wrote:
>I have a friend who is building a kiln and they would like to know the theory
>behind stretcher courses.how many courses between them do you need them
Hey:
I don't know about theory but the one thing I think is very important in
regard to stretchers is the fact that you are using two rows of bricks for
a 9" wall and you can stagger the joints so that you don't have a direct
joint line through the wall into the firebox. You also have a 4.5 inch lap
on either side of the joint in relation to the course above and below,
unless one or the other is a header course. You can lay stretchers a bit
loose too and take care of expansion stuff with that method. I leave about
a sixteenth inch between bricks and the outer row covers the joint. I use
4 courses of stretchers and then use a header course. This makes a very
stable wall. Some folks alternate stretchers and headers. It depends on
the person and their preference. I use as few header courses as possible
because you have to lay them tight to close the joints that go clear
through the wall. The headers tie the two stretcher courses together and
prevent separation.
later, Craig Martell in Oregon
Mike Gordon on wed 13 oct 99
Hi Gang!
Thought I'd jump in here with a question of my own since the topic is
kiln building. I'm doing an old 70's era AD Alpine 20 cu foot updraft.
When I tore the arch out of it I forgot to measure the flue opening. Any
suggestions? Thanks in advance, Mike
John Baymore on wed 13 oct 99
------------------
(clip)
I have a friend who is building a kiln and they would like to know the
theory behind stretcher courses.how many courses between them do you need
them.
(snip)
I'll assume from the context that we might actually be talking about HEADER
courses between STRECHER courses.
The function of a header course is to stabilize the two thinner layers of
brick in a wall, unifying them into one wall. Otherwise the inner layer
floats free from the outer layer.... and due to differences in temperature
and (reversable) thermal expansion, the two layers can move around a lot
when firing. Not something you want =3Cg=3E.
There is no exact =22God-given=22, never to be broken =22RULE=22...... =
however
practical application says that every 5th to 6th course is a good pattern
to tie the inner and outer walls together for walls that are the
sizes/heights used in most pottery studio type kilns. The joints of course
should be staggered off all of the courses above and below by using
something like soaps to offset the joints. They should be set tight to one
another, and any expansion joints that need to be made are enclosed in the
structure at the corner wall angles so that the cracks running from the
interior to the exterior are all as tight as possible.
The cracks running straight through a header course from hot face to cold
face are a possible means for radiant energy to escape and also allow the
possibility of air infiltration. So a header course probably has a
slightly lower =22heat effectiveness factor=22 than a pairing of bricks set
strecher fashion. One reason not to use more than you need.
There is a POSSIBLE additional reason not to use headers too often,
depending on your kiln construction method. I'll give a little background
to help explain this:
Optimum kiln wall construction utilizes a graded refractory construction
method. Simply, that means that you use a higher duty rating brick on the
hot face... and then a lower duty rating brick on the cold face. It is
possible to make this grading even more complex and have multiple layers in
the wall. In the typical 9=22 wall of studio fuel-fired kilns, this would
usually add undesireable comlplication to the construction. In most
cases, this grading on our type of kilns involves a lining, or hot face
layer, of 4 1/2 inches of one rating of brick and a backup layer, or cold
face layer, of 4 1/2 inches of another rating brick. The exact location of
the brick in the kiln structure determines what particular combination of
materials might be used to meet specific design needs.
On aspect of selecting the best materials to use involves calculating the
heat loss through the wall and seeing what the interface temperatures (the
point where one refractory meets another grade) will be for various
combinations. For example typically a K or G (or whatever=3Cg=3E) 20 has an
insulating value per inch higher than the insulating valuer for a K/G/? 26.
If put behind 4 1/2 inches of G/K/? 26, you need to know what the
temperature of the hot face side of the OUTER, lower rating brick will be
under any worst case conditions.... so that you don't subject the outer
layer brick's hot face side to a temperature that will exceed the use
rating. Continually doing this will degrade the outer face bricks
insulating value, and can eventually comprimise it's structural integrity.
BTW....... there is some margin for error in the IFB temperature ratings...
since they are rated for continious duty and we typically fire periodic
kilns. Heavy reduction and slagging effects from things like wood ash and
soda/salt are not taken into account in the IFB temperature use ratings
however. There are some more specialized brick that are designed for such
environments... and the brick manufacturers info will tell you which brick
work best in which =22harsh=22 environments. Oops... I REALLY digress.....
=3Cg=3E.
The reason for the grading of the refractories is to obtain maximum
insulating value in the wall section. The higher duty rating brick may be
able to withstand higher temperatures, but the insulating value per inch
of that material is typically lower than the insulating value per inch of a
lower duty rating brick. You don't need the high duty rating on the
outside surface, and using it will lower the insulating value of the wall.
Technically, a 9 inch wall made all of one single rating of brick is
probably not the best construction method for a permanant pottery kiln.
What is IS, is simple to keep track of. You don't have to deal with things
like keeping track of which brick is which, nor deal with slight size
differences between different bricks.
So..... now back to the =22number of headers in a wall =22 point. IF you
construct walls utilizing graded refractories, then every header course
lowers the insulating value of the wall in that layer, because it is the
higher duty but lower insulation brick all the way through the whole wall,
from hot face to cold face.
The more header courses... the lower the insulating value of the overall
kiln. So you try to balance the trade off of good wall stability with
insulating value.
So.... some more thoughts on headers/strechers. More than you wanted to
know =3Cg=3E.
BEst,
......................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40compuserve.com
John.Baymore=40GSD-CO.COM
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