Reduction Fire

7/25/2019 Reduction Fire

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Arbuckle: Reduction Firing

ReductionFiring.docx Page1of5

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

Documents

Reduction Fire