TCOLSON@raychem.com on tue 21 may 96
Can you stand another geeky kiln venting post?...if not, hit delete!
Don:
You are right about the general effects of venting kilns. In a nutshell, what
you get when you vent is:
1. Much better temperature uniformity at low temps. Below red heat, radiant
heat isn't doing much for you, so the only way to get heat to move around in the
kiln is to move the air. The air in the kiln will move around a bit on its own
(hot air rises, mixing things up), but nowhere near as much as if you force it
with a vent. Within reason, the more you vent, the better the temperature
uniformity (of course, your electric bill goes up as well as you pay to heat up
the air!) Above red heat, the dominant way heat moves is by radiation (thermal,
not nuclear!). The venting probably doesn't gain you as much after about
600-700C, temp-wise, but without it you are relying on radiant heat to fix the
bad uniformity that was created in the low temp part of the firing.
2. Better atmosphere uniformity. As with the temp, venting continuously takes
whatever's in the kiln out and replaces it with fresh air. As the firing
proceeds, all sorts of stuff is burned/boiled/bubbled out of the materials and
the atmosphere could be very different at differnet times and places in the
kiln. Replacing whatever is in there with fresh air means that the atmosphere
is more even within a firing and much more consistent firing-to-firing. If you
are looking for lots of variety in glaze effects and such, this might not be
what you want, but if consistency is important, a vent is a good thing.
All this volitalized crud is what's eating the metal parts of your kiln. And, of
course a vent takes all that crud out of the studio so the atmosphere you
breathe is a lot more consistent (and healthy!) as well.
The folks at Orton have a short video on firing that is worth a watch and covers
this subject. Although they have a commercial interest in pitching venting, I
sure can't find any fault with the technical basis for their pitch. They stress
a slow rise to red heat and venting.
As far as bleeding a reducing gas like CO or H2 into a hot electric kiln, you
are also right. You'd get a great strong reduction. Both gases have their
little(?) safety problems so I wouldn't try it! Combustion of a primary fuel to
heat the kiln isn't necessary. CO would much rather be CO2 and H2 would much
rather be H2O.
Climbing down off the geek soap box...
Tom
Tom Colson
tcolson@aimnet.com or tcolson@raychem.com
Tiles On The Web: http://www.aimnet.com/~tcolson/webtiles.htm
Check out Daniel Oberti's work on the site. Very cool!
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