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Reduction iring
Reductionisfiringwherethekilnatmospherehas
insufficientoxygenforcompletecombustion(morefuel
thanairtoburnit).Athightemperatures,thisproduces
carbonmonoxidegas,whichwillsteallooselybonded
oxygenfromothermaterials,inceramicsmainlyironand
copper.
Thischangesthemolecularformofthematerialand
producescolorchanges.Inoxidation,irongenerally
producestan,brown,orrustcolors.Inreduction,small
amountsofirongiveceladoncolors:green,blue,olive,or
greygreencolors.Copperinoxidationisgreenorblue
green.Smallamountsofcopperintheappropriatebase
glazeinreductionproducescopperredsinshadesknown
assang
de
boeuf
(oxblood
red
toward
aplum
red),
peachbloom(pinktopeachyreds),orflamb(orangish
red).Chinesehistoricceramicsareparticularlyfamousfor
thesecolors.Whilereductioncanbedoneatother
temperatures thanhighfire(cones811), itisheldby
manyclayartiststhathighfirereductionhasthebestcolor
response,withrichertones,thanreductionatlower
temperatures.
Bisquingworkbeforeglazing
Workmayberawglazedandoncefired.Thisisnotthemostcommonprocedurebecausebonedrygreenwareis
fragile,maysplitorcrackfrombeingwettedbyglaze,andcontainscarbonaceousmatterthatwillburnoutin
firing,possibly
causing
pin
holing
in
the
glaze.
Greenware
is
also
prone
to
blowing
up
iffired
quickly
past
the
point
wherewaterturnstosteam(212F),andpiecesofexplodedwarewouldbetrappedinglaze.Glazesforoncefiring
mustbebalancedforshrinkagewiththerawpot,andusuallycontainamuchlarger%ofclaythanglazesfor
bisqueapplication.
Industrysometimesbisquefirespotterytomaturitysothatitmaybesupportedduringthefiring(e.g.translucent
porcelains).Thisvitrifiedwareisnolongerporousandspecialbinderandapplicationmethodsmustbeusedto
applytheglaze.
Studiopottersusuallybisquetoatemperaturebelowthematuringpointoftheclay,thenapplyglazetothe
porousbisqueandglazefiretomaturity.
Pyrometersmeasuretemperature,usuallythroughaprobeinsertedintothekiln.Thistellsusthetemperatureat
theplace
of
the
probe
and
the
time
read.
This
may
not
indicate
whether
the
materials
in
the
kiln
have
sufficiently
melts.
Pyrometricconesmeasureworkheat(theeffectoftimeandtemperatureonmaterials)andaretheusual
indicatorsforfiring.ConesweredevelopedbySeger.AmericanconesaremanufacturedbyOrton.Conesarea
combinationofclayandglazematerials,calibratedtomeltataspecifictemperature.Theintervalbetweenconesis
about32degreesF.Conesaremadeinlargeandsmallformat.Meltingtemperatureissomewhatdifferent
betweenlargeandsmallconesofthesamenumber.Smallconesareoftenusedinelectrickilnsforvisualcones
becausepeepsaresmallandit'shardtoseeseverallargecones.Smallconesareusedinelectrickilnkilnsitters.
SAFETYNOTE:carbonmonoxidegasisTOXIC.Donotremainina
closedkilnroomduringreduction.
China.Qingdynastyceladoncups
China.Qingdynastysangdeboeufvases
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SeeAppendix2inHandsInClayforcolorandtemperatureequivalents.Lowerfiringconesarenumberedwitha
"0"infrontofthenumber,andreadlikeanegativenumber(e.g.022iscoolerthan06).Thereisnoconezero.
Numberinggoesfromcone01tocone1.Conesnumbersw/oa"0"infrontarehotterinascendingnumerical
order:cone1iscoolerthancone6.
Conepacksshouldbemadetoputinfrontofbothtopandbottom
peepsofthegaskilns.Claywadsareusedasabasetosupporta
seriesofcones:theoretically,aguidecone(tellsyouwhenyouare
gettingtoward
temperature
desired),
afiring
cone
(the
desired
temperature),andaguardcone(togaugethelimitsyoumayfire
to).
Conesshouldbetiltedataslight(8degree)anglesothatyouwill
seethemsoftenandbendbeforemelting.Placeconescloseto
eachothersothegroupmaybeeasilyseenthroughthepeep,flat
sidefacingforward,numberfacingyou,conethatmeltssoonest
first(ifconeorderisreversed,thefirstconetomeltwillknockthe
othersover).Itisdifficulttoget2wideconepacksonasupportin
frontofthepeep,sobalancetheneedtocatchthedripswiththe
logisticsofplacing2conepacksnexttoeachother.Notethatthe
conesin
the
bisque
illustration
are
junior
sized
lowfire
cones
fired
closetotheirmeltingpoint.Inthecone10reductionconepack,
youwillbefiringwaybeyondthelowerconepack,andthecones
willmeltintoaliquidandrun.Makeabigboatonthefrontofthe
conepacks.Makeconepacksaheadoftimeanddrythem.Glaze
kilnsareusuallyheatedfasterthanbisque(gasburnersareoften
hardertocontrolatlowtemperatures),andwetconepacksmay
blowup,scatteringbitsofclayontoglazedware.
Forhighfireyouwillneed2setsof2conepacks:
1. Lowerconesfordeterminingwhentobeginreduction.
Usuallowconesare011,010,09,4orsimilar.MAKEABIG
BOATONTHEENDOFTHELOWCONEPACK.THESECONESWILLMELTDOWNTOAPUDDLEINFIRING
ANDRUNIFNOTCONTAINED.Itisnotfuntochipthisoffofshelves,orfinditinglazedwork.
2. Glazematuritytemperaturecones.Glazeconepackshouldcontain6,8,9,10.Whilethe#6coneisnot
necessary,itgivesaguidepartwaythroughthefiringtotemperatureandkilnevennesstoptobottom
andisahelpfulindicator.Pleasemakesuretofillinakilnchartandlabelthenumbersoftheconesinthe
conepackssothefirersknowwhatthey'relookingat.
Placementofconepacks
Conepacksareplacedinfrontofthepeepholes,usuallywiththereductionconestothefront(theywillmeltout
ofthewaybeforetheglazeconesareneeded)facinginonedirection,andtheglazeconesinbackfacingthe
oppositeway.Checkpositionforvisibilitywiththekilndoorclosedusingaflashlightorapapertorchinthekiln.
Glazing
Mosthighfireglazedbenefitfromevenglazeapplication.Thisismosteasilyaccomplishedbydipping.See
informationonglazingandhighfireshopglazes.
Potbottomsshouldbewaxed(dryfooted,i.e.unglazed)toresistglazeapplication,aswellaslidseatsandlid
edgesifyouplantofirewiththelidinplacetoreducewarping.Highfireglazestendtomove(run)duringthe
firing,and1/8to1/4"shouldbeleftbetweenthepotbottomandthebeginningoftheglazedareatopreventruns
ontothekilnshelf.Manypottersconsiderthiswhenmakingwork,andmakeadeepbevelonthefootthatcreates
Cone10reductionconepacksafterfiring.
Backconepack,ltor,cones011,010,09,4,
front,ltor,6,8,9,10.Notethatpacksare
facedinoppositedirectionstomake
sightingconesmoredefinite.Inspiteofa
boat,theconesran.Helpstoputonakiln
shelfshardforeasiercleanup.
Bisqueconepacks,cone05.Left, 06,right
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ashadowsothepiecevisuallysitsgracefullyandalsoliftsthepottheareaneededabovethekilnshelfw/o
showingaringofbareclayatthefoot.
Ifglazesticksthepiecetothekilnshelf,itmaybreakthepieceasitcontractsincooling,andtheworksownerwill
beaskedtochiseltheglazeoffthekilnshelf.Thickerglazeapplicationstendtorunmorethanthinones.
Combinationsofglaze(e.g.doubledips)mayformaeutecticberunnierthaneitheronealone. Ifyoufeelyouare
usingathickglazeapplicationand/ordoingexperimentalcombinationsofglaze,pleasemakesureyouworkis
loadedintothekilnwithaclaywasterofapieceofbrokenkilnshelfunderittocatchanyrunnyglazedrips.
Loading
Beforeyouloadcheckthatthekilnisfreeofdebrisaroundtheportsandbehindthebagwall.Thetargetbrick
behindthebagwallistheretodeflecttheflameandshouldremaininplace,butcheckforshardsofwork,etc.
ChecktoseethatallthegasvalvesareclosedandtheblowersystemareOFF.Ifyouhaveanyreasontosuspecta
problemwiththekiln,testlightitbeforeloadingtomakesureitisworkingproperly.
Usetheblacksiliconcarbideshelvesforthegaskilns.THESESHELVESAREHEAVY.THESESHELVESAREEXPENSIVE.
Pleasebecarefulanddonotpropthemagainstmovableitems.Findhelpifyouhavetroubleliftingandplacing
highershelvesinthekiln.Checkeachshelfforproperkilnwash.Thisprotectstheshelffromglazedripsandmakes
iteasiertochiseloffanyglazeruns.Kilnwasshouldbemixedupinacontainerbythesinkinthekilnroom.Dry
mixisinthecupboardunderneath.Applyw/abrushorpaintroller.Kilnwashshouldbetheconsistencyofcream.
Severalthinner
layers
stick
better
than
one
thick
one.
WASH
ONLY
THE
TOP
OF
THE
SHELF.
Wash
on
the
sides
or
underneathmaypopoffandfallintotheworkbelow.Kilnwashforlowfireandregularreductionis50silica+50
EPKbyweight.
WARNING:ifyouareusingthethin,blackbondedsiliconcarbideshelvesDONOTEVER
firethemwetordamp.Iftheyneedtobekilnwashed,theymustbecompletelydried
beforeusingthem.Iftheseshelvesareheatedwet,theymayEXPLODE.
Postthefirstshelfseveralinchesoffthekilnbottomforadequateheat/aircirculation.
Generallyshelvesaretriposted,sharingendposts.Postsarealwaysputaboveeachother
whenloadingsubsequentlevelssothattheposts,nottheshelves,beartheweightofthe
load.
Wareis
grouped
by
height,
and
similar
heights
are
loaded
on
the
same
shelf
to
use
the
space
efficiently.
Very
tall
piecesareoftenputonthetopshelfsothatmultiplepostsdonthavetobeusedtoachievetallheights.Theware
selectedforthebottomshelfshouldbeatleast6"high.Averylowfirstshelf(say,platesortiles)sometimesdoes
notletheatandgassescirculatewell,andmayresultinunderfiredorunderreducedworksonthebottom.Check
thebottomsofallwareasyouloadtomakesurethereisnoglaze.AlsochecktomakesuretheclaydoesNOTlook
liketerracotta.Iflowfireclayisaccidentallyfiredtocone10,itwillmeltintoapancake,sticktothekilnshelf,and
hasthepotentialtoruinadjacentworks. Ifanythinglookslikeitmayruntoomuch,putitonapieceofbrokenkiln
shelforabricktoprotectthekilnshelf.Glazedwaremaybeplacedtotheedgeoftheshelf,closetogether,butnot
touching.Makesureyounotethepositionofthepeepsandleaveroomfortheconepacks.
Firing
Whenyouaredoneloading,checkthekilnchartandmakesuretheconesfortheconepacksarenoted.Withthe
damperopen,
the
air
off,
light
the
pilots.
We
generally
candle
the
gas
kiln
on
very
low
heat
overnight
to
get
them
firedoffatareasonablehourthenextday.Itssafetohavethepilotson.Turningtheburnersonlowsettingisa
judgmentcall.Ifthisisdoneearlyandthenextpersonwatchingthekilncomesinlate,youcouldheatthekiln
enoughthatyoumisstherighttimetodoreduction. Generally,weturntheburnersonverylowovernightonlyif
thekilnisturnedonlateintheevening,andsomeoneiscominginearlytocheckonit.
Glazedwareholdslessphysicalwaterthangreenware,andthechemicalwaterhasalreadybeenfiredoff,soitis
lesslikelytoexplodefromwaterturningtosteamat212Fthanbisque.However,recentlyglazedpieces,
especiallythickonesthatarestillcoldtothetouch,mayholdwater.Temperatureshouldbeincreasedgradually
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untilthekilnispastredheat(about1000F),whichisalsoquartzinversion,wheresilicachangesshapeandgets
2%bigger.
OXIDATIONiscompletecombustionoffuelinthepresenceofadequateoxygen.Theflamewillappearblue,short
andbushy,thekilnatmospherewilllookclear,andthekilnmakesaroaringsound.Nobackpressure(flames)from
damperorpeeps.Thegascombustsattheburnertips.Thisisthemostefficientcombustionandwillcause
temperaturetorisethefastest.Oxidizeuntilreduction(about010).Itishelpfulforsmoother,brighterglaze
surfacestooxidizefor1530minutesattheendofthefiring(beginaroundcone9softforacone10softfiring).
REDUCTIONisincompletecombustionoffuel,causedbyashortageofoxygen,whichproducescarbonmonoxide.
Reducingflameswillappearsoft,licking,long,orange/yellow.Thekilnatmospherewillbemurky.Backpressure
(incompletelyburnedgasses)willbeseen(visibleorangeoryellowflamesatthedamperandpeepholes).Black
smokeindicatestoomuchgas(waste),andcancauseblackcoringofclaybodiesanddullglazes.Itisnotnecessary
toreducethismuch.Duringreduction,thelackofsufficientoxygenforcombustioninthekilncausesthewhole
kilntobecomeacombustionarea.Thisisinefficientcombustion,slowsthetemperatureclimb,butwillhelpeven
outthekilntemperature.Youshoulddoreductionaboutcone010,andreducefor2030mintues.Ifyoubegin
reducinglaterthen06,youmayhavemissedreductionandwillnotgetany.
NEUTRALorslightreductionatmospherewillproducegreenishflamesandconditionsbetweenoxidationand
reduction.Afterreduction,itiscommontoleavethekilninaneutralatmospheretilcone9,thenoxidize.
KilnControl
ControlsonthekilnconsistoftheGASvalve(generallyameterisattached,althoonsomekilnsyoumayregister
justthepositionofthehandletoindicateamountofgas),thePRIMARYAIR,whichisthebloweronforcedair
kilns,andSECONDARYAIR,ortheairsuckedinaroundtheburnerportsandcontrolledbytheDAMPERonthe
flue.
Thesecontrolsworktogethertoproducethedesiredeffect.Toproduceanoxidizingflame,turnofthegastothe
desiredlevel.Youmayopenthedamperand/orincreasetheprimaryorforcedairorreducetheamountofgasto
produceablue,bushyflame.
Toputthekilnintoreduction,closethedamperpartiallyand/ordecreasetheprimaryair,and/orincreasethegas
pressuretoproducereducingconditions.Makesurethatyougetbackpressure,oratleastcanseeswirlinggasses
inthe
kiln
at
the
bottom
peep
for
good
reduction.
Thefiringisoxidizedfromthebeginninguptoaboutcone010.Thekilnisreduced,thenputintoaneutral
atmosphereuntilcone9isdown.Ascone10beginstomove,thekilnshouldbeputintocompleteoxidationto
finishthefiring(usuallythelast20minutesorso).Thiswillallowglazematerialstosettledown. Oncethekiln
reachestheglazematuritypoint(forourshopglazessomeplacebetweencone10beginningtobendandcone10
over),turnoffthekiln.Ifyouturntheelectricalswitchesoff,everythingwillturnoff.Closethegasvalves.Note
thetimeonthekilnchart.Closethedampertokeepcoldairfromblowingthroughthekiln.Mostglazedbenefit
fromslowcooling,andallneedtimeafterfiringinamoltenstatetoallowbubblestosmoothout.Colddraftscan
causeunevencoolingduringquartzinversionorcristobaliteinversionmaycausedunting,orcrackingthoughthe
bodyduringcooling,fromstresses.
SafetyPleasetielongand/orfluffyhairbackbeforecheckingthekiln.Removeandreplacepeepplugsfromthesideto
avoidanybackpressure.Ifyouhavetroubleviewingtheconesw/backpressurecomingoutthepeeps,reducethe
backpressurebyturningdownthegasand\oropeningthedamperabit.Whenviewingkilnsaboveorangeheat,
useprotectivedarkglassesintendedforeyeprotection.
TROUBLESHOOTING
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Youneedtoreviewthekilnconditionstomakeappropriatejudgmentsforadjustments.Downdraftkilnsreact
somewhatdifferentlythanupdrafts,andeachkilnhadit'sownpersonality.Differencesinloadingandweather
mayalsoaffectfiring.Generalizationsbelowarefortheupdrafts.
IFyouhavethedamperwideopen,youmaybelosingsomuchheatoutthefluethatthebottomwillbecoldin
spiteofalow,busy,oxidizingflame.Ifthebottomseemscold(oftenthecaseearlyinthefiringasthekilnheats),
closethedamperabit.
IFyou
have
very
high
primary
air
you
may
cause
agreat
deal
of
turbulence
in
the
kiln
and
such
astrong
draft
and
intakeofsecondaryairthatthebottomstayscold.Reducetheprimaryair(blower)andadjustthegasanddamper
forproperflame.
IFyouhavethedamperclosedtoomuch,youmayfinditdifficulttogetaclean,oxidizingatmospherew/oback
pressure.Openthedamperabitand/orreducethegas.
IFyoudonothaveenoughturbulenceinthekiln,youwillhavedifficultygettinggoodreductionofthebottom.
Increasetheprimaryairand/oropenthedamperabit.Turbulencehelpstheheatandkilngassescirculatetoall
partsofthekiln.
IFthekilnseemstostalltowardtheendofthefiring,checkthegas.Sometimesveryhighgasandenoughairfora
neutralatmosphere
causes
too
much
draft
and
heat
is
lost.
Turning
down
the
gas
abit
for
better
combustion
with
thedesiredamountofairmayactuallyincreasetherateofclimb.
REMEMBERthatanychangeyoumakemaytake1530minutestoshowinoveralleffect.Constantlychangingthe
kilnsettingsmayslowfiring.Noteallchangesandresultsonthekilnchart.
Whenyoureachthedesiredtemperature,turnoffthegasvalves,flipofftheelectricsystem,turnofftheblowers,
andshutthepilotvalves.Closethedampersothatcoolairdoesnotcontinuetoflowthroughthekiln.Notethe
timeonthekilnchart.
Cooling/Unloading
Althoughitispossibletospeedcoolakilnbyleavingthedamperopenfromtheendoffiringthroughredheat,this
ishardonthekilnstructureandmaynotproducethebestglazesurfaces,especiallyincrystalsurfaceglazes(the
crystalsform
during
cooling,
and
slow
cooling
promotes
more
crystals).
Around
red
heat
quartz
(1000
deg
F)
goes
throughQUARTZINVERSION,andthepiecebecomesabout2%smaller.Thisshouldhappenslowlytoavoid
dunting(coolingcracks).Freesilicaintheclaycanbeconvertedtocristobalitewhenfiredaboveabout1940degF.
Thiswillinvertandgetabout3%smalleroncoolingat439degF.Thiscanalsocauseduntingifithappensquickly.
Aclassictestforwhetherakilniscoolenoughtopullthepeepsandcrackthedoor/lidistoputpaperintoaspy
hole.Whenapapernolongerignites(paperburnsat 451degreesF),youmaypullthepeepsandopenthe
damperacrack.Unloadwhencool.
Personsunloadingshouldnoteanyunusualresultsfromreductionorlackofwhilethekilnisstillloadedandnote
positionsofanyunreducedorunderfiredwork(notethisonthebackofthekilnchartforreference).Unloadthe
workontoacart.Removeshelves.Chipanyglazerunsoffwithachiselandhammerand/orruboffw/apieceof
blacksiliconcarbideshelf.Toavoidcrackingtheshelfwhenchippingglazeoffshelves,itishelpfultoputtheshelf
onaresilient
surface
(e.g.
wood),
or
lean
against
an
immovable
object
at
an
angle
and/or
chip
at
an
angle,
not
straightdown.Reapplykilnwashandstack.PLEASEWEARSAFETYGLASSESTOPROTECTYOUREYESFROM
FLYINGCHIPSOFWASH.Replacetheshelvesintheholder,stackingtheshelvesfacetofaceandbacktoback(to
preventwashchipsonthebacksofshelvesthatcouldfallintotheworkinstackingthekiln).Restackkilnposts.
Sweepoutthekilnifneeded.Sweepuptheareaaroundthekiln.
Bibliography
Hamer,FrankandJanet,ThePottersDictionaryofMaterialsandTechniques4th
ed,Univ.OfPAPress,A&CBlack,
1997.Thoroughdiscussionofcristobalite,reduction,andmanyothertopics