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2010 Ceramic Publication Company
10 Hot Kiln Tipsby David L. Gamble
I conduct a lot of workshops forK12 teachers around the
countryevery year and Im thrilled aboutthe enthusiasm educators
have forteaching ceramics to kids. Through-out these workshops I
answer manyquestions and hear a lot of storieson the subject of
firing, especially ofelectric kilns.
There are two main reasons forfiring clay, and in most cases
twodifferent firings are requires. One
is to prepare pieces for glazing byfiring what is called a
bisque. In thisfiring, the heat from the kiln changesthe molecular
structure of the clayand hardens it so it will no longerbreak down
in water to its originalmoist, pliable form. This also makespieces
less fragile, but still porousenough to absorb water so thatwhen
wet glaze is applied, it willstick to the surface. The second
fir-ing is to melt and fuse any appliedglaze to the surface and for
highertemperature firings, this firing furtherstrengthens the
pots.
TEN TIPS FOR
SUCCESSFUL FIRING
1Before you fire. When installingyour kiln, make sure its at
least18 inches away from any wall. Vac-uum the interior of the
kiln, especiallythe element grooves (figure 1), about
every 20 firings, and after every fir-ing when a piece blows up
in the kiln.Inspect hinges and handles for wear(figure 2). Check
the thermocouple(s),and replace if necessary (figure 3).Every six
months, unplug the kiln (if
your kiln has a plug) and inspect theprongs as well as the
insulation (fig-ure 4). Brown or black discolorationindicates a
worn plug or loose wires
and a potential fire hazard. Keep all
flammable, combustible and meltablematerials (cardboard,
wareboards,newspaper, fabric, vacuum hoses,plastic, etc., away from
the sidesand top of the kiln. These areas getextremely hot.
2Protect your shelves. Kiln washprotects your shelves from
glazedrips. Inspect shelves prior to firingand recoat any bare
spots or re-
Vacuum the interior of the kiln, espe-cially the element
grooves.
Inspect hinges and lid handles forwear. Tighten loose screws or
pins.
Check the thermocouple(s), for exces-sive oxidation.
Unplug the kiln and inspect theprongs and insulation.
Inspect kiln shelves and reapply kilnwash to any bare spots
before firing.
cently cleaned and scraped shelves
as needed (figure 5). Remove anyloose or chipped kiln wash
thatmake flake onto pots during the fir-ing. Store unused shelves
in a safeand low-traffic area.
3Always use cones. Pyrometriccones are formulated fromceramic
materials including clay,oxides, feldspars, and frits, andare
designed to bend at specific
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2010 Ceramic Publication Company
time/temperature combinations togive you an accurate reading
onthe heatwork created in your kiln.Cones measure the relationship
oftemperature absorbed by the wareover time. Tip: Use cones even
if
you are using an automatic kilncontroller. Cones verify the
ac-curacy of the controller and are a
backup if the controller fails.
4Clean up greenware. Signa-tures and decorations leaveburrs that
must be removed using adamp sponge while leather hard,or drywall
sanding screen for drierwork, before the bisque firing (fig-ure 6).
Once fired, the only way toremove these is by grinding with aDremel
tool, or sanding with wet/dry silicon carbide sandpaper.
5Handle greenware with care.Bone dry greenware is frag-ilemore
fragile than when itsleather hard. Never pick up piecesby any
appendage or handle.
6Fire dry pots. To see if a pot isdry, touch the pot to your
cheek(figure 7). If it is cold or damp, thereis still moisture in
it and you willneed to preheat the kiln to 180Fand leave it at that
temperature andvented until all moisture its gone.
Water boils at 212F (100C), andthats the temperature where
theresdanger of blowing up pieces. If themoisture is not driven out
and thetemperature rises to water boilinglevels, the rapid
expansion of thesteam thats created blows out thewalls of your
piece.
7Wipe your feet. Any glaze thattouches the shelf during a
firingsticks to it. Carefully sponge off anyglaze within inch of
the bottom ofthe foot. For pots with thick or runnyglazes, clean
off a bit higher thanthat. Do not rely on the kiln wash tosave the
pot or the shelf from beingdamaged by glaze drips (figure 8).
8Loading greenware or glaze-ware. Electric kilns heat from
theoutside walls, where the elementsare located, in towards the
center,
so stagger the shelves and placetaller pots in the middle of the
stackto promote better heat penetrationto the middle of the kiln.
Greenwarepieces can touch and can be stackedin some cases, but I
prefer to leavespace between them for even heatdistribution. When
placing a largeflat piece on the top shelf, allow
approximately 5 inches of clear-ance to the top. Extra
clearanceallows for heat from the sides of thekiln to travel up and
over, reachingthe middle of the piece so that allareas heat evenly.
If wide piecesare heated unevenly, the expansionrate of the side
may be considerablydifferent from that of the center of thepiece,
which will cause it to crack.In all firings, keep a the edge of
thestack at least 1 inch from sides of the
kiln (figure 9).
9Bisque fire slowly. Clay con-tains organic material thatneeds
time to burn out. If you raisethe temperature of the kiln toofast,
gases will become trapped inthe clay body. Organic materialsburn
off between 572F (300C)and 1472F (800C). Also, if notcompletely
burnt out in the bisque,organics may give you trouble in
the glaze firing as it as escapesas gas, pushing through the
glazeand creating pin holing, which canmar the glaze surface.
10Keep records. Keep a fir-ing record of firing times(lengths),
the cones you used andthe result of their melt (draw a quicksketch
of how they looked, or notewhether the target cone was at 1,3 or 5
oclock for example), andrecord the number of firings in a
particular kiln. These records cangive you indications on
elementwear (e.g., if firings take longer thanusual) and future
maintenance thatmay be needed. n
David L. Gamble is a frequent contributorto PMI. He holds
masters degrees from Ed-inboro and Indiana State Universities in
arteducation and fine arts, and currently servesas an education
consultant for Skutt.
Remove burrs using a damp spongeor drywall sanding screen before
thebisque firing.
A pot that feels cool to the touch still
has moisture in it and needs to bepreheated before firing.
Sponge off any glaze within inch of
the bottom of the foot.
Stagger the shelves and place tallerpots in the middle of the
kiln to pro-mote better heat penetration. Allow 5inches of
clearance when placing largeflat work on the top shelf.
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