William Brouillard on fri 15 may 98
Fiber kilns and heat loss
One of the big differences between fiber kilns and brick
kilns is in the amount of material or mass (translate as total
weight of the materials used to build the kiln, for this
discussion) each has.
Many brick kilns use a combination of hard
brick and insulation brick in their construction. Fiber kilns
use steel, fiber, hard brick and some soft brick in their
construction. Fiber has volume but not much mass. Soft
bricks have the same volume as a like sized hard brick but
weigh less or have less mass. There do not seem to be
typical hard or soft brick kiln designs but I will generalize
and say that a fiber kiln would have considerably less mass than a
similar sized brick kiln.
When a kiln is fired the pots, kiln furniture and the kiln are heated. A
brick or masonry type kiln will absorb more heat into the
walls, floor and arch than a fiber construction. The amount
of heat absorbed will vary depending on the type of
brick/fiber and length of firing. Denser brick and longer
firings would mean that more heat is absorbed by the brick.
The more pottery and shelves in the load the greater the
mass. The brick kiln and the fired pottery and furniture
become a heat sink. After the kiln is turned off the kiln is
still red to white heat and will take days to cool down. The
greater the mass the longer it will take to dissipate the heat
of the firing. The heat will pass by radiation, convection
and conduction from the pots to the atmosphere inside the
kiln to the walls of the kiln , through the walls to the
outside air.
A kiln densely packed with tile will be slower to fire and
slower to cool than a kiln full of a few large and thinly
thrown pots. The weight or mass of the pottery would be
less than the tile. The kiln with the tile will cool slowly.
One of the advantages of fiber kilns is that they cost less to
fire. That lower cost is due to fuel savings. Less mass
means more of the heat goes to the pots and not to the
walls of the kiln. A fiber kiln may work well for some
types of work and not so well for others. Slow cooling may
be what you are used to but faster cooling may be OK for
your clays and glazes. Hall china single fires their C/11
porcelain, cold to cold, in six hours. They know their clays
and glazes and fire as fast as they can. Your clays and
glazes will be different and may not hold up under a fast
fire or a fast cooling. (test, test, test)
The addition of hard brick to the floors and bag wall areas
of a fiber kiln would slow the cooling but would also add
to the cost of the firing. A balance can be reached with
brick and fiber. Talk to people with fiber kilns and find out
if their cooling cycle is similar to yours and if they have
problems associated with cooling. What you fire may be
more important that how much hard brick you use in the
construction.
The thickness of the walls in fiber construction may make
a big difference. Some of the fiber kilns I have seen have
thin walls 3 to 5 inches. A nine inch thick fiber wall might
promote a slower cooling cycle and still be a fuel saver.
Talk to the manufactor and see what they recommend. Fiber
has some interesting insulation properties. Get some tech data
and work out a solution to your firing needs.
--
william brouillard
1011 literary road
cleveland,oh.44113
ch151@cleveland.freenet.edu
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