Marcia Selsor on tue 10 dec 02
In my opinion three layers of hard brick OR 2 layers of hard brick and
one of soft brick.I also raise the height of the kiln onto cinder blocks
to reach a comfortable height for my body to load. Build it to fit you.
Marcia in Montana
Tuscany in 2003
http://home.attbi.com/~m.selsor/Tuscany2003.html
Lynne Kay on tue 10 dec 02
Hello Again
Should a gas kiln have two or three layers of brick on the floor? Should =
the floor be hard brick, soft brick or a mixture?
Thanks again
Lynne Kay
Norns@cox.net
John Baymore on thu 12 dec 02
My $0.02 on kiln floor insulation issues........
Heat energy moves from areas of higher concentration to areas of lower
concentration without consideration for the "direction" imposed by gravit=
y.
If the hot face of a FLOOR brick is at 2300F and the cold face is at 70F=
,
the heat transmission through it is the same as if it were a WALL brick
with the same temperature differential. Our "common sense" idea that
"heat rises" is not true. Hot gases rise when suspended in colder gases
due to density differences .......but heat energy is not synonomous with
hot gases. While there certainly are hot gases in a kiln chamber........=
what we are mainly insulating for is the transmission of heat energy from=
a
(hopefully ) equally heated chamber.
There are some minor factors that modify the total heat loss per square
foot of a wall/floor/arch such as the effects of air convection off the
cold face of a vertical wall section, and the likelyhood that the cold fa=
ce
temperature of the floor's cold face is higher due to a lack of such air
circulation.....but the efects are quite minor. There is also the impact=
on overall heat loss figures caused by the possible leakage of hot gases
out of the kiln (through cracks or porous refractory materials) in areas =
of
positive pressure, and the infiltration of cold air in areas of low
pressure..... but again these issues are pretty minor.
If you decide that the appropriate level of insulation for the walls for
your kiln should have some particular value, the floor should have a pret=
ty
similar value. Otherwise.....you are losing more heat through the floor
than you are through the other parts of the structure..... which will ten=
d
to make the hot face surface there lag slightly behind the hot face of th=
e
rest of the chamber surfaces.... which promotes unevenness. The floor is=
not a great opportunity to "save money" by using less brickwork.
Lots and lots of studio type kilns have underinsulated floors .....then t=
he
potters wonder why they have to adjust the bag walls or compensate in som=
e
other manner to get the bottom hot . Or just accept that the bottom
fires cold and use different glazes there. Or accept that their glazes
look different there...and it's OK. And so on. The Rhodes kiln book sho=
ws
lots of kilns with 5 inch thick hardbrick floors......which helped to fue=
l
the concept that the floor insulation is not very important.
For cone 10 type kilns I frequently use high duty hard brick for the
working surface of the chamber floor, followed by a layer of 2300 IFB,
followed by two layers of 2000 IFB.....total thickness 10 inches........ =
on
a kiln in which 9 inch walls were selected as appropriate. Sometimes I'l=
l
drop the last layer of 2000 IFB.....making it only 7.5 inches thick. =
Another approch is to lay the bricks on their sides.......... so that the=
top floor working surface is the 2 1/2 x 9 inch surface ....and use a 280=
0
F IFB for the first layer backed with a 2000 IFB for the second layer....=
.
for a 9 inch thickness. (Not my preferred choice....too expensive, not a=
s
durable a working surface, and repairs will cost more). Yet another
combination is a layer of 1 inch high duty thin splits (hardbrick) for th=
e
working floor surface, then a layer of 2600 IFB, a layer of 2300 IFB, and=
a
layer of 2000 IFB. In these layouts, the interface temperatures do not
exceed the use rating of the various bricks.
Multiple brick ratings used in the construction of a wall section are
usually referred to as "graded" construction. It maximizes the insulatin=
g
value of the wall section and also decreases the cost of construction.
Best,
..............................john
PS: See the archives..... this discussion has gone around a lot before.
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
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