David Finkelnburg on wed 28 sep 11
I think Mel has made an excellent point about speed of firing. David
Hendley gets the results he likes from a slower firing than his kiln is
capable of. Mel gets the results he likes while firing at the top speed for
his kiln. David's slower firing and Mel's fast firing are both right becaus=
e
they produce the desired results.
Industry fires as fast as possible, routinely, but not mindlessly, and
neither should anyone else. Glaze recipes, clay bodies, thickness of ware
are just some features that have to be changed to let one fire exceptionall=
y
fast. The entire clay process, from design to forming to glazing to firing
is involved in producing good work. Most high fire work is done at cone
10. Notice, though, that Mel fast fires to cone 11. He has adjusted his
process to produce great results. David's process uses slower
firing...because his methods and materials work better that way.
If you want to fire faster, by all means ignore the old, outdated
firing "speed limits." However, also consider your process and do your own
tests so you can make your own adjustments, if necessary. In other words,
fire as fast as you can only when that lets you produce the results you
want.
Dave Finkelnburg
http://www.mattanddavesclays.com
ivor and olive lewis on thu 29 sep 11
One point that has not been discussed is the effect of rapid heating and
rapid cooling on the behaviour and service life of the fabric a kiln is mad=
e
from.
The second relates to energy absorption especially in kilns that consume
fuel. For example how long in time does it take for cold clay to adsorb all
of the energy from flames at plus 2000 deg C. In a full bore fast fire
situation is there a guarantee that the hot exhaust gas from fully consumed
fuel deliver all of its energy and cools down to a temperature equal to
that of the maturing clay before it is exhausted up the flue?
A third relates to the accuracy of pyrometric cones and their behaviour whe=
n
they are not processed under conditions recommended by their manufacturers.
Does anyone have information or thoughts about these aspects of kiln
management ?
Regards,
Ivor Lewis,
REDHILL,
South Australia
David Finkelnburg on sat 1 oct 11
Ivor,
Sure, fast firing is harder on a kiln than slow firing. Fiber kilns,
though, don't suffer much. Soft brick kilns show accelerated wear, but
unless firing is daily year round it's still not so bad. Hard brick is a
different story and slower firing may be essential to a long and happy hard
brick kiln's life.
Clay absorbs energy at a decreasing rate as the temperature difference
between the clay and the kiln vapors becomes progressively less during the
heating cycle. It is reasonable to assume that the hot kiln gas approaches
but never reaches the ware temperature. The faster the firing the larger th=
e
approach or temperature difference between the kiln gases and the ware. In
his research, Dr. Carty at Alfred U. uses a 3-hour soak at peak firing
temperature (no temperature change for 3 hours) to assure equilibrium
conditions are reached within fired samples.
Even when cones are used precisely as recommended they only approximat=
e
the firing temperature. Cones slump at or near peak firing temperature,
which is when heat transfer is almost entirely radiant. Kiln stacking and
cone placement are variables that affect how much radiant heat reaches the
cone. To be as consistent as possible it is helpful to place cones in the
same spot and with similar ware nearby each firing. Still they are no
substitute for an accurate pyrometer for knowing peak firing temperature.
Good firing,
Dave Finkelnburg
http://www.mattanddavesclays.com
-----------------------------
Date: Thu, 29 Sep 2011 15:55:15 +0930
From: ivor and olive lewis
One point that has not been discussed is the effect of rapid heating and
rapid cooling on the behaviour and service life of the fabric a kiln is mad=
e
from.
The second relates to energy absorption especially in kilns that consume
fuel. For example how long in time does it take for cold clay to adsorb all
of the energy from flames at plus 2000 deg C. In a full bore fast fire
situation is there a guarantee that the hot exhaust gas from fully consumed
fuel deliver all of its energy and cools down to a temperature equal to
that of the maturing clay before it is exhausted up the flue?
A third relates to the accuracy of pyrometric cones and their behaviour whe=
n
they are not processed under conditions recommended by their manufacturers.
Does anyone have information or thoughts about these aspects of kiln
management ?
David Finkelnburg on sat 1 oct 11
Taylor,
Take two aspirin and read more Clayart! :-) You do have it right, btw.
Ivor asked, "In a full bore fast fire situation is there a guarantee
that the hot exhaust gas from fully consumed fuel deliver all of its energ=
y
and cools down to a temperature equal to that of the maturing clay before i=
t
is exhausted up the flue?"
I responded, "Clay absorbs energy at a decreasing rate as the
temperature difference between the clay and the kiln vapors becomes
progressively less during the heating cycle. It is reasonable to assume tha=
t
the hot kiln gas approaches but never reaches the ware temperature...." In
other words, no, the hot exhaust gas temperature never equals that of the
maturing clay...unless and until one reaches peak temperature and soaks the
kiln to equilibrium, or begins cooling it by firing down.
It is equally correct to say, as you wrote Taylor, "...the ware
approaches but never reaches the kiln gas temperature..." It is simply
another way of saying precisely the same thing. The ware and vapor
temperatures are approaching each other. The kiln gas is cooling as the war=
e
is heating. However, in a kiln of rising temperature the kiln gas
temperature is always greater than the ware temperature.
In the context of a heating fuel gas-fired (natural gas, propane),
single-chamber kiln this is correct. The flame temperature is so hot we tak=
e
steps to keep the gas flame from impinging directly on and damaging the
ware. The gas is cooling rapidly because of the much cooler ware, kiln
furniture and kiln fabric. However, the flue gas temperature in the chamber
can never be equal to the ware temperature or the ware would no longer be
heating and the kiln temperature would cease rising.
All this is a very long-winded way to state a very basic physical law.
Heat flows from hot to cold. The hot gas heats the cold kiln and its load.
Good firing!
Dave Finkelnburg
http://www.mattanddavesclays.com
On Sat, Oct 1, 2011 at 12:44 PM, Taylor Hendrix wrot=
e:
> Did you mean the ware approaches but never reaches the kiln gas
> temperature rather than "the hot kiln gas approaches
> > but never reaches the ware temperature" as you did below? Or, is it
> possible that you are talking about something much more complex than my
> understanding that the kiln gasses give up their heat to the kiln and the
> ware?
> My head hurts.
>
| |
|
