Bruce Girrell on mon 10 jul 06
mel wrote:
>i am becoming a fanatic on tight chimneys.
What brought you to this conclusion? What are you observing when firing with
a tight chimney compared to one with gaps? Are you talking about firing
control (kiln gets to temp better, atmosphere control is easier, fewer cold
spots, etc) or glaze results (color is better, higher percentage of good
pots, etc) or both?
I'm not asking you to defend anything, just what you have noticed that you
have linked to the tight chimney.
Bruce Girrell
David Hendley on tue 11 jul 06
Like many things I've learned the hard way, I found out
how important it is to have a chimney with no air leaks
by trying to fire a kiln with a crummy chimney.
This chimney was built with new bricks, but without mortar.
It worked fine for years, but, as you would expect, gaps
gradually opened up between the bricks. Ever so slowly, I
noticed that it was taking longer to fire, and I was getting
way too much reduction, especially early in the firings.
Finally, the kiln would not reach temperature, even after
hours of extra stoking time.
Finally, the light bulb went off in my brain. An hour with a
pipe clamp tightened things up and fixed the problem.
The best chimney, I think, is as close-as-possible to a monolithic
structure. Riser sleeves inside a steel pipe are good, as is
my current chimney, which is four sections of cast refractory.
It is possible to build a reasonably good brick chimney. If you
have hundreds of extra bricks laying around it only makes
sense to use them. If you carefully cement them together with
reliable store-bought mortar, they should hold tight, and a sheet
of high temp board on the outside adds a layer of insurance in
case gaps develop.
Whatever you use, pay as close of attention to the construction
of your chimney as the kiln itself.
David Hendley
Maydelle, Texas
david(at)farmpots(dot)com
"EXTRUDE IT! Getting the Most From
Your Clay Extruder" available at
http://www.farmpots.com
Ivor and Olive Lewis on wed 12 jul 06
Following on from Mel and David I would suggest the following ideas .
A Brick Stack is a living thing, almost organic. It is subject to the =
laws of Physics. So as it is heated the bricks will expand and then =
contract as they cool. There will also be temperature differentials from =
base to apex and from the interior to the exterior. The result will be =
relative motion between the courses and uneven movement between face =
brick and quoins. So it is inevitable that the joints will open. This =
leads to gaps which act as passive dampers by admitting cold air. This =
reduces the volume of air drawn in to promote combustion.
David suggests using a good Mortar. If this is just a mix of cement and =
sand It is almost useless. No adhesive properties. Smooth bed possibly =
but it is programmed to disintegrate which means that after a few =
firings it forms a layer of powder that can act as a dry lubricant =
allowing better sliding between bricks. If it is a Refractory adhesive =
then at the first firing the whole of the interior must be raised to =
maturity. Best thing to use is a thin slurry of ball clay. Make sure =
your bricks are wet before you apply it.
So you can use Mel's design, or follow David's instructions or monitor =
the situation and tighten the bricks once in a while and point the =
bricks with clay slurry.
Ivor Lewis.
Redhill,
South Australia.
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