Pottery In Australia Vol 39 No 4 December 2000

flue Ihm is too sma ll has choked many a good kiln. As
with the burners, bigger is better. Ii is easy to reduce the
size of the flue with the damper, but impossible to
increase it (The damper is an adjuslable pia Ie, which
closes off the flue outlet)
To check your flue, the following tesl should be
carried out during a firing. When the kiln has reached
over I JOO"C and Ihe burners are at maximum pressure,
open the damper fully. Remove the spyhole hung, and
place a lighled candle or laper near the spyhole. The
flame should be pulled into Ihe spyhole, which hows a
negative pressure in the kiln, indicating a greater flow of
gases from Ihe kiln than into Ihe kiln. 111is indicates that
the flue is large enough. If you cannot create Ihis effect,
Ihe kiln has a design fau lt, whic h wil l need to be
corrected.
FIRING THE LPG KILN
During the early stages of firing, the damper musl be left
open and the burners regulaled back so as to comrol the
temperature rise. Even with a glaze firing , it is not
desirable to heat the kiln too quickly, as POl' or shelves
may crack if a hot flame impinges on one side unevenly.
When red heat is reached at about 600"C, the bumers are
rurned up to about 60/0 of maximum pressure and the
damper is closed off, until thert' is a very slight airflow
into the spy hole when opened (check with a lighted
taper, etc). This means there is a slight negative pressure
in Ihe kiln , and the kiln is in an oxidising condition. As a
final check, close the damper a little more, and the
airfl ow should reverse, i.e. a slight flow OUI of the
spyhoJe when it is opened. Restore the damper to the
previous position. This is the MOST EFFICIENT way to
fire Ihe kiln AT THAT PRESSURE SETTING, Ihe kiln is
getting the MAXIM UM heal from the gas. Ilowever, when
the pressure is adjusted, the PHOCED URE MUST BE
REPEATED.
This is continued umil about 1000"C, or when it is
decided to commence reduclion. Some pollers creale a
reducing almosphere by closing off Ihe primary air
adjustment on the burners. I do nol agree with this
practice because il crea tes an incorrect gas/air mixture
and wasles gas, and can create a lot of black smoke,
which is wasled fuel (money!). I consider that a more
efficient mel hod of creating a reduction atmosphere is to
close off the damper slightly, unlil a positive pressure
exists within the kiln. A taper or candle should be blown
out by Ihe airflow from the opened spyholc.
The lempef'dIUre rise should he watched closely du ring
Ihis operation - this is the li me th at a DIGI TAL
PYROMETER proves its wonh, as a small change can be
very easily seen. When the kiln has stabilised, a flame
may be noticed from the flu e -this is normal and
desirable during a reduction firing -this is the healed
unburned gas igniting when it reaches the air. The flame
should be no more than 6" long, and may also be seen
from the spyhole when the bung is removed. Obviously,
it is unwise to look closely into a reducing kiln, but the
inside atmosphere will appear cloudy and the contents
will be indistinct A mirror and a welding glass are good
tools 10 have on hand for peering into gas kilns'! tW
BPQ KILN SERVICE
PO BOX tB, BEACHMERE, QLD 4;10
TEL 07 ;49') 0735 FAX, 07 5498 334; ~IOIllI.E ();08 886 B;4
E ~MAll : dcrcoggins@one.net.au
WEB SITE, hup/!web.one.nel.au/-dercoggins
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39/4 DECEMBER 2000 + POTIERY IN A USTFW.lA 71