The Mystery of Softening, Annealing and Strain Points

The strain point is the temperature corresponding to a viscosity of 1014.5 Poise. At this viscosity glass will hold its shape because it is a ... ChatterGlass AWordPress.comsitewithchatteraboutstainedglasscrafts. Skiptocontent ←Whatnext? UseYourGlassGrinder Waste!→ TheMysteryofSoftening,AnnealingandStrain Points Postedon23March,2014bychatterglass Thequestion“Why?”ratherthan“What?”motivatesthisposting.LetmeexplainbecauseIwanttobemorespecific… WhenIboughtanelectrickilnitcamewithsomepre-programmedschedules.TheyworkedbutIwantedtoknowwhy.IthenreadwidelyontheInternetandfoundamillionmoreschedulesthatdidthesameandaddedafewmoreofmyowncreation.Buttheyareallwildlydifferent,eventhoughtheydothesamething.Butwhy? SoIstoppedaskingthequestionslike“WhatschedulecanIusetodoabottleslump?”andstartedtoaskquestionslike“Whydotheseschedulesdoabottleslump?”.Ihadtochangefrom“What?”to“Why?”becauseIcouldnotseeanyconsistencyinwhatIwasreading.ItseemedtomethatIwaslivinginaneraof“superstitionandblindfaith”ratherthan“theageofenlightenment”. Becausethemanyphysicalpropertiesofglassarewelldefinedandsimplephysicalprocessesaredeterministic,anddespiteglassbeingrather“weird”asanamorphouscrystallinesubstance,itstruckmethatthedesignofkilnschedulesseemedtobemoreabouttrialanderror,and“ruleofthumb”,allofwhichcanbetakentobeaeuphemismforguesswork. SoIhavetoaskwhyarewebehavinglikealchemistswhenweshouldbebehavinglikescientists?Whyareweasking“How?”whenweshouldbeasking“Why?” So,thispostingisonesmallsteptowardsansweringabigquestion:“WhyarethekilnschedulesI’musingthewaytheyare?”andIstartbylookingintothe“importanttemperatures”relatingtoglasswork. Itturnsoutthattherealstartingpointistolookattheviscosityofglassatdifferenttemperatures. ViscosityPoints Thereareseveralnotableviscositiesforglassthatarewidelyrecognised.Foreachofthemthereisanassociatedtemperatureandstandardisedteststhatcanbeusedtodeterminethem.Butyouwillprobablyrecognisethenamesasbeingrelatedtotemperaturesratherthanviscosities. ThesearethewidelyrecognisedviscositiesforglassthatIfound: Workingpoint         104Poise Flowpoint               105Poise Softeningpoint       107.6Poise(Bullseye&Wiki)or107.65Poise(Britannica) Annealingpoint      1013Poise Glasstransition       1012Poise Strainpoint             1014.5Poise Exactlywhat“Poise”meansasameasureofviscositydoesn’treallymattersomuchasrecognisingthatsmallernumbersmeanlessviscouswhichmeans“runny”.Thisinturnmeanshighertemperatures. Ifyou’re“mathematicallychallenged”thentermslike“104”mayleaveyoubaffled.Don’tworry.Itisan“exponential”orshorthandnotationthatmeans“10multipliedby10fourtimes,or1000”.So,thesuperscriptnumbertellsyouhowmanyzeroesareinvolvedandsavestimeandconfusionwhenwritingverybignumbers. So,thereasonforchoosingthesespecificviscositiesasstandardsisbecausewecancomparedifferenttypesofglass.Andthereasonwehavetobeabit“arbitrary”aboutchoosingthesestandardviscositiesisbecauseglassdoesnothaveasharplydefinedmeltingpoint. TemperatureEquivalents Temperaturesassociatedwiththeviscositiesdependsonseveralfactors,suchastheformulationoftheglass,includingadditivessuchasmetaloxidesthatproducecolours.Thereforethetemperatureatwhichoneparticularpieceofglasswillhaveaparticularviscositywilldifferfromanotherpieceofglass.Consequentlytherewillbearangeoftemperaturesforeachoftheviscositiesevenfora“compatible”rangeofglasssuchastheBullseyeandUroborosFX90productsofferings. TogiveaflavourandsubstancetothesignificanttemperaturesrelatingtoBulleye“COE90”glassandcompatibleUroboros“FX90”glassIwillworkthrougheachofthe“special”viscositiesinturn.IwillalsoshowyouthatyouhavetobecarefulwithinformationyoufindontheInternetandtrytoreachsomeconclusionsofmyownthatwillhelpmeunderstandwhykilnschedulesarethewaytheyare.Ihopeyoufinditusefulforyourownpurposestoo. Butawordofwarning.Ihaven’tvisitedeverywebsiteandeveryforumontheInternetwhichmeansIhaven’tgatheredtogethereverypieceofinformationthatisavailable.ButIdohopeIfoundenoughinformationwithwhichtoreachcomerationalconclusions. Workingpoint Theworkingpointisthetemperaturecorrespondingtotheviscosityof104Poise.Atthisviscosityglassissufficientlysoftforshapingactivitiessuchasblowingorpressinginaglassformingprocess. Aforumposterwithanun-identifiedBullseyeGlassRodchartsays“WorkingTemp1750°F”(954°C)andagraphfromBullseye(comparingSpectrumandBullseyeworkingranges)wasusedtoestimatetheworkingpointasabout934°C(1713°F)withinamarginofcoupleofdegreeseitherway.ButthisisallIcouldfind. Atentativeconclusionisthattheworkingpointtemperatureistowardstheuppertemperaturelimitforatypicalglasskiln,sosomewhereintherange1713°F(934°C)to1750°F(954°C). Flowpoint Theflowpointisthetemperaturecorrespondingtotheviscosityof105Poise.Atthisviscosityglassbeginstoflowfreelyifunrestrained.Inpracticaltermsthisrepresentstheuppertemperatureofthefusingrange. NousefulinformationwasfoundaboutflowpointtemperaturesotherthanageneraldescriptioninBullseyeTechnote4thatsaysglassflowsundertheforceofgravityattemperaturesabove1500°F(816°C). Atentativeconclusionisthattemperaturesintheapproximaterange800°C(1472°F)to850°C(1562°F)maybeappropriate. Softeningpoint Thesofteningpointisthetemperaturecorrespondingtoaviscosityof107.6Poise(Bullseye&Wiki)ormaybe107.65poise(Britannica).Ididn’tfigureoutwhyIfoundtwodefinitionsforthesofteningpoint.Atthisviscosityglasssoftenssufficientlytobeworked.Itrepresentsarelativelylowtemperaturewithinthefusingrangeinwhichglasswillsaginakilnorcanbebentintheflameofalamp-blowingburner. SomeoneonaForumwithanun-identifiedBullseyeGlassRodchartsays“SofteningPoint1250F”andanothersays“Thesofteningpointisfrom1180-1270forBullseyeglasses” Threepeopleforumquote,withoutprovidingareference,toBullseyepublishedinformationthatIcouldnotfind,suggestingsofteningpointsasbeing1250°F(677°C)fortransparents,1270°F(688°C)forOpalsand1180°F(638°C)forGoldPinktransparents. InTechnote5Bullseyesaythatasoakinthe1150°F(621°C)to1250°F(677°C)rangeisoftenusedtosqueezeorminimizetrappedairfrombetweenlayers.Thisimpliesthatat621°Cglasshassoftenedenoughtoslowlysag,implyingthesofteningpointhasbeenreached. AgraphfromBullseye(comparingSpectrumandBullseyeworkingranges)wasusedtoestimatethesofteningpointasbeingabout1265°F(685°C)withinacoupleofdegreeseitherway. FromagraphinBullseyeTechnote4asofteningtemperatureof1090°F(588°C)wasestimated. Basedonasuggestiontodobubblesqueezebyaslowrampofabout200°Fperhourfrom1000°Fupto1200°F,andacommentsayingglasswon’tfirepolishuntil1300°F, impliesthatthesofteningpointtemperatureisnohigherthan1300°F. SeveralpublishedBullseyeschedules(includingfromWarmGlassUK)havesegment1oftheirkilnscheduleswithatargettemperatureof1250°F(677°C). Asanaside,Spectrum’sownbasicscheduleshavesegment1risingtotargettemperatureof1050°F(566°C)and1250°F(677°C)whenheating.ThiscombinesnicelywithasuggestionatGlassCampusthat“IffiringtoaspecifictemperatureproducesadesiredeffectinSpectrumglass,youwillhavetofireBullseyetoahighertemperature(about25ºF)toproducetheidenticaleffectinBullseyeglass.”ThisleadstoanequivalentforBullseyeglasstobeapproximately1030°Fand1275°F,whichseemstomirrorexpectations. SeveralsetsofnumberscanbeusedforanalysisofwhatI’vejustfound.ThereisreasonableconfidencethatBullseyehavedefined1250°Fforopals,1270°Ffortransparentsandexceptionally1180°FforGoldPinktransparent.SeveralforumsourceshintatthissamerangeandclaimBullseyeasthesourcebutIcouldnotfindtheprimarysourceatBullseye.GraphsfromBullseyeimplyapproximately1090°Fand1265°Fassofteningtemperatures.Bubblesoakssuggestsofteningtemperaturessomewherebetween1150°Fand1250°Forsomewherebetween1000°Fand1200°Fwithanupperlimitof1300Fasusedforfirepolishing. Aconclusionisthatthesofteningpointtemperaturemightbetakentobeanaverageofaround1260°F(682°C),anarrowrangeas1250°F(677°C)to1270°F(688°C)andanall-encompassingrangeasabout1090°F(588°C)to1270°F(688°C). Annealingpoint Theannealingpointisthetemperaturecorrespondingtoaviscosityof1013Poise.Atthisviscosityglasswillholditsshapequitewellbecauseitis“notquitesolid”.Theannealingpointrepresentsagoodtemperatureatwhichtoannealglassquicklysuchthatinternalstressesarerelievedinminutes,asopposedtohoursnearthestrainpoint. Theannealingpointisdefinedbyaspecificviscositysotherecanonlybeoneannealingtemperatureforaparticularpieceofglass.Thismeansthat“upper”and“lower”annealingpointsdonotexist,despitewhatsomepeoplearesuggesting.Moreaboutsuch“confusionalhelp”towardstheendofthisposting! AforumcontributorwithanunknownBullseyeGlassRodchartsaysthe“AnnealingTemp940°F”.Anothersays“Bullseyesaystheirglassannealsat960°F,(thoughJamesKervinlistsitasaround980°F.)”withoutidentifyingeithersource.ThreepeoplequoteinformationfromBulleye,withoutprovidingaverifiablereference,thatsuggestaverageannealingpointsfortransparentsas990°F(532°C),opalescentsas935°F(501°C)andGold-Pinktransparentsas882°F(472°C). BullseyeTechnote5saysglassisrigidwithnovisibleshapechangesunder1000°F(538°C)andexpandssubjecttoCOE.ThesameTechnotealsosuggestsanannealsoaktemperatureof 900°F(482°C)andthatglasswillbesubjecttothermalshockbelow900°F(482°C).AgraphinTechnote4suggesttheannealsoaktempisat900°F(482°C)butanotheratWarm-GlassclaimingtobefromBullseyesuggestsatemperatureofabout970°F(521°C).Theoldertemperatureisapre-2009recommendationfromBullseye…“AsofJune2009,Bullseyehaschangeditschartforannealingthickslabs.Specifically,therecommendedannealsoaktemperaturehasbeenloweredfrom960°F/516°Cto900°F/482°C.” InformationatUroborosforFX90glass(compatiblewithBullseye)is“Usethesemeasuredpointsas‘typical’oranaverageovertheFX90colorsmadebyUroboros”:TYPICALANNEALINGPOINTS:Transparents:977°F(525°C)andOpals:955°F(512°C) MyanalysisofwhatIfoundisasfollows.Lessreliablesourcesforannealpointtemperaturessuggest940°For960°For980°F.SeveralsourcespointtoanunidentifiedpublicationfromBullseyesuggestingannealpointtemperaturesintherange935-990°F,exceptionallyto882°FforGoldPinktransparents. BullseyeTechnoteinformationsuggestsglassisrigidat1000°F/538°Cyetsimultaneouslysuggestsanannealsoaktemperatureof900°F/482°Cwhichisalsoquotedasbeingatemperaturebeneathwhichglassissubjecttothermalshock.AlsoconsiderBullseye’schangefrom960°Fto900°Fforannealsoaksandthereasoningbehindit.ButtherearereliableaveragesforUroborosinthe955°Fto977°Frange. Myconclusionitthatwithnoclearlystatedaveragetemperaturetouseastheannealpoint,960°F(516°C)isreasonablemidpointcapturingbothBullseyeandUroborosFX90glasstypesandmatchesthepre-2009Bullseyeannealsoaktemperature.Butforanarrowrange935°F(502°C)to990°F(532°C)isreasonable,withanall-encompassingrange882°F(472°C)to 990°F(532°C). Glasstransition Theglasstransitiontemperature(orsimply“glasstransition”)correspondstoaviscosityof1012Poise.However,inreality,theglasstransitionhappensoveratemperaturerangebecauseitrelatestoreversiblechangesinwhichthereissmoothbutsuddenchangeinphysicalpropertiesofglasssuchasthermalexpansioncoefficient,specificheatandviscosity.Inthissensetheglasstransitioncanbeseenasachangefromahardandrelativelybrittlestateintoaviscousmoltenorrubberyplasticstate(orthereverse). Unlikeother“special”temperaturesforglasswork,theglasstransitiontemperatureistheonlyonewhereaveryclearandscientificallyobservablechangeistakingplaceinglasssofarasIcouldtell.Alltheother“special”temperaturesseemtobearbitrarilydefinedbycommonconsent.Itisarathercurioussituationtonotethatglasstransitiontemperatureisabouttheonlytemperatureforartglassworkersthatdoesnoteverseemtobementioned! Ihavereadthat“bygeneralagreementitisconsideredthataliquidonbeingcooledbecomespracticallyaglasswhentheviscosityequals1013poiseorwheretherelaxationtimeis102s.”Thisisaninterestingsentencebecausetheviscositysuggeststheannealingpointwhereasdescribinga“liquidonbeingcooledbecomespracticallyaglass”suggeststheglasstransitionpoint. ThisconundrumseemstoberesolvedbyacommentinEncyclopaediaBritannicawhichsays“theannealingpointandthestrainpointlieintheglasstransformationrange;often,theglasstransitiontemperatureandtheannealingpointareusedsynonymously,andthestrainpointmarksthelow-temperatureendoftherange.” So,perhaps,theglasstransitiontemperatureisasinglemeasurablepointonatemperature-viscositygrapharoundwhichisaless-welldefinedglasstransitiontemperaturerangethatisboundedattheupperendbyanannealingpointandatthelowerendbyastrainpoint.Butaretheannealpointandstrainpointscoincidentwiththeboundsoftheglasstransitiontemperaturerange? Toaddtomyconfusion,BullseyeTechnote4saythatinthe538°Cto677°Ctemperaturerangeglassholdsitsshapebutisbeginningtosoftenandcallsthisthetransformationrange.So,ontheonehandthewords“transformationrange”hintattheglasstransitionrangebutthedescriptionandtemperaturerangebothhintatathetemperaturerangebeingboundedbytheannealpointandthesofteningpoint. Myanalysisofthisconfusionisthatbecausethe538°Cto677°Crangeisequivalenttothe1000°Fto1250°Frangethenthe“transformationrange”cannotbethe“glasstransition”rangebecause538°Ciswithinwhatappearstobetherangeofexpectedannealpointtemperaturesand677°Ciswithinthesofteningpointtemperaturerange.Withnootherinformationthanviscosities,theglasstransformationtemperatureissomewhereinthemiddleofaglasstransformationtemperaturerangewhichappearstobeboundedbyanannealpointandastrainpoint. MyconclusionisthatI’mstillnotsurewhat’shappeninghere! Strainpoint Thestrainpointisthetemperaturecorrespondingtoaviscosityof1014.5Poise.Atthisviscosityglasswillholditsshapebecauseitisasolidinpracticalterms.Atthestrainpointittakesseveralhoursforstressestorelaxwithinasopposedtominutesattheannealingpoint.Furthermore,stressesthatremaininglassattheendoftheannealingprocessbecomepermanentbelowthestrainpoint. Apracticalconsequenceisthatinitialheatinguptothestresspoint(andfinalcoolingdownfromthestresspoint)forapieceofglasswillintroducetemporarystressesthatwillresultinthermalshockandbreakageiftemperatechangesaretoodramatic. Thestrainpointisdefinedbyaspecificviscositysotherecanonlybeonestraintemperatureforaparticularpieceofglass.Thismeansthat“upper”and“lower”strainpointsdonotexist,despitewhatsomepeoplearesuggesting.Moreaboutsuch“confusionalhelp”towardstheendofthisposting! Onecontributortoaforumwithanun-identifiedBullseyeGlassRodchartsays“Strainingpoint820°F.”butanothercontributorsaysthe“strainpointis750F.”AndonanotherwebsiteIfind800ºFforeverythingbutSatakeglass. IalsofoundthreepeoplequotethesameinformationfromBullseye,withoutprovidingareferencetoinformationthatIcannotfindatBullseye,thatsuggestanaveragestrainpointfortransparentsas920°F(493°C),opalescentsas865°F(463°C)andGold-Pinktransparentsas820°F(438°C). Ialsofoundasuggestionthatbecausethestrainpointisthe“solid–notsolid”boundaryaslowrampingtoavoidthermalshockcracksisonlyneededbelow1000°F(515°C).AlthoughIlikereasonedargumentsIamalwayssuspiciousof“roundnumbers”whenphysicalmeasuresareconcerned.Igetdoublysuspiciouswhenthetemperaturequotedliesintheannealpointrangeratherthanthestrainpointrange. AtUroborosIfoundexplicitinformationforFX90glass(compatiblewithBullseye)as“Usethesemeasuredpointsas‘typical’oranaverageovertheFX90colorsmadebyUroboros”:TYPICALSTRAINPOINTS:Transparents:910°F(488°C)andOpals:875°F(468°C)”. TheonlyprimarysourceinformationIcouldfindrelatingtoBullseyeglasswasanexplicitstatementinTechnote4thatsays“Subjecttothermalshockbelowapproximately850°F(454°C)”buttherearemuchhigherstrainpointtemperaturesforsomemanykindsofBullseyeandUroborosglasssuggestingthatsomekindsofglassaresusceptibletothermalshockathighertemperatures. Myanalysisisthattheunreliablespecificationsforthestrainpointare750°F,800°F,820°Fand1000°F.Becausetheoriginalsourcecannotbefound,IamcautiouslyconfidentthattheBullseyestrainrangeliesaround865°Fto920°F,with820°FforGoldPinktransparent. Slightlyworryingly,Bullseyewarnaboutthermalshockfortemperaturesunder850°F(454°C).ButUroborosarequiteexplicitwithtypicalstrainpoints875°F(468°C)to910°F(488°C). Myconclusionisthatasingleaveragestrainpointmightbesomewherearound890°Fwithaminimalrangeofaround850°F(454°C)to920°F(493°C)andanall-encompassingrangeofabout820°F(438°C)to920°F(493°C). Andasafinalpoint,InoticedasIbrowsedtheInternetareminderthatglassdoesn’tsuddenlychangefromliquidtosolidoncooling:“Uponfurthercooling,belowthestresspoint,viscosityincreasesrapidlytowellbeyond1018poise,whereitcannolongerbemeasuredmeaningfully.”Whatthisissayingisthatoncoolingglassbecomessoviscousthatyoucan’tmeasureit! WorkingRange Athttp://www.bullseyeglass.com/the-working-range-and-what-it-means-to-fusing.htmlBullseyecompareSpectrumandBullseyeglassintermsof“workingrange”inthecontextofanold(butstillinteresting)legaldispute.Theysaytheworkingrangeis“Therangeoftemperaturesinwhichglassisformedintowareinaspecificprocess.Forcomparisonpurposes,whennospecificprocessisconsidered,theworkingrangeofglassisassumedtocorrespondtoaviscosityrangefromtheworkingpointtothesofteningpoint.(104to107.6Poise)” Thegraphonthatwebpageshowsthatthere’snotsomuchofadifferencebetweenSpectrumandBullseyeglass. InBullseyeTechnote4theydescribetheworkingrangeasusuallybetween1000°F(538°C)and1700°F(927°C). BuyerBeware IhavenowpassedonwhatI’velearnedbutI’mnotdonewithyouyet! You’vealreadyseenhintsofmisleadingandinconsistentinformationthatIencounteredasIdidmyresearch.IwanttogiveyouafewmoreexamplesthatrelatetowhatIcall“confusionalhelp”becauseyougetconfusedwhenyoureadmorethanoneofthem! Myfirstexamplecomesfromhttp://www.arrowsprings.com/html/annealing.htmlandyoushouldnowbeequippedtospottheproblems: Theannealingtemperatureforanyglassisactuallyarange.Thehigherendoftherangeisatemperaturesettobesafelybelowanypossiblechanceofdistortion.Thelowerendoftherangeisatemperaturehighenoughforheatsoakingtobeeffectivewithinareasonableamountoftime.Thecommonlyusedtemperaturesforanyparticularglassisactuallyjustatemperaturechosenasacompromisebetweenthehigherandlowerendsoftherange.Inotherwords,atemperatureinaboutthemiddleoftherange.Anexacttemperatureisnotwhatisimportant.Whatisimportantisthatyoukeepthetemperaturesteadyforaperiodoftimebeforeslowlycoolingtheglasstoroomtemperature. Thecluetoonereasonwhythattextiswrongisinthemeaningof“annealingpoint”.Asecondreasonisthatwhatthequoteisdescribingisanannealingprocess,nottheannealingpoint.It’seasytocreateconfusionbyusingthewrongwords(andI’msureIdoitmyself). Mysecondexampleisdifferentbutequallyconfusional.Itisfoundathttp://www.warmglass.com/Basic_Process2.htm: Unlikemanysubstances,glassdoesnotmeltorhardenatasingletemperature.Instead,itgraduallysoftensandhardensasthetemperaturechanges.Thephaseduringwhichthistransitionfromliquidtosolidoccursiscalledthe“annealingzone.”Therearethreecriticalpointswithinthiszone. TheUpperAnnealingPoint–thisistheupperendoftheannealingzone,wheretheglassbeginstoreturntosolidform. TheAnnealingPoint–thisisthetemperaturewherethemoleculesintheglassoptimallyrealignthemselvesevenlythroughouttheglass.It’salwaysbetweentheupperannealingpointandthestrainpoint. TheStrainPoint–thisisthelowerendoftheannealingzone.It’stheplacewheretheglasssolidifies.Thestress(orstrain)remainingintheglassatthispointisunlikelytobechangedorrelievedunlesstheglassisheatedupagainandannealedagain. Theproblemisthatweknowthereisjustoneannealingpoint! MythirdexampleIfoundathttp://www.glass-fusing-made-easy.com/strain-point.html: Annealingglassoccursbetweentheupperandlowerstrainpoints,thereforefindingtheannealingpointofapieceofglassisimportant.Theannealingisaccomplishedbyaslowcoolingoftheglasstoandbeyondthelowerstainpointoftheglass. Thistimewehavetwostrainpointswhenweknowthereisjustone. MyfourthexampleIreluctantlymentionbecause,onthewhole,Ilikewhattheauthoristryingtoachieveanditisotherwiseaveryhelpfularticle.Againweseemtohavetwostrainpoints.Youwillfinditat http://glasstips.blogspot.co.uk/2010/02/strain-points-and-annealing-ranges.html Thereisanupperandlowerstrainpoint,althoughthisisdisputedbysome.Therearemathematicaldefinitionsfortheseaswellasobservationaldefinitions.Idonotunderstandthemathematicsofeither.Inlayterms,thelowerstrainpointisthattemperaturebelowwhichnofurtherannealingcantakeplace.Itissafetoassumethisis50Cbelowtheannealingpoint(Ithinkitactuallyis43C,butI’mnotcertainofthisnumber). Soitissafesttocontrolthecoolingtoatleast5Cbelowthelowerstrainpoint.Bullseyefindthatcoolingfromtheannealingsoakto370Cisbest–thisismuchmoreconservativethanistheoreticallyrequired–146Cbelowtheannealingsoakpoint.Thisdoestakecareofanyproblemofthermalshockingoftheglassduringthecooling. Theupperstrainpointmightbemoreproperlydescribedasasofteningpoint.Thisalsohasscientificdefinitions.ThewayIthinkofitisasbeingthetemperatureabovewhichnoannealingcanoccur.Anotheristothinkofitasapointbeyondwhichthemoleculesoftheglassareinrelativelyfreemotion–whichincreaseswithtemperature.Thisagaincanbeconsidered(ontherise)as50Caboveannealing.Howeveronthewaydownitissafertoconsiderittobenotmorethan30Caboveannealing.Thisisbecausetheglasstemperaturelagsbehindtheairtemperature(whichiswhatourcontrollersmeasure). Asweknow,thereisonlyonestrainpoint.Ithereforereachaconclusionthatwhentheauthorsays“lowerstrainpoint”heactuallymeans“thestrainpoint”;andwhenhesays“upperstrainpoint”hereallymeans“thesofteningpoint”.Andthere’smoreI’dargueabouttoo,suchaswhetheritissafetoassume50°Ceithersideoftheannealpoint. So,ourtallyisnowtwoannealpointsandtwostrainpointifyou’repreparedtotakewhatyoureadontheInternetatfacevalueaboutannealingglass.So,tobalancethebooks,letmedescribetheannealingprocessbrieflyandinawaythatdoesnotneed“upperandlower”annealorstrainpoints. Tobegintheprocessofannealingglassyouhavetochooseatemperatureto“soak”theglasstodissipateinternalstresses.Ifthechosentemperatureis“sensible”itwillrelievethebulkofthestressesinallthekindsofglassyouhaveusedandwillachieveitquickly. Toensurethatstressesarenotre-introducedduringthesubsequentcoolingprocess,andtofurtherreducetheinternalstresses,youthenhavetoslowlycooldowntheglasstobeyondthestainpointensuringthattemperaturedifferenceswithintheglassobjectareminimised.Thisslowcoolingistheannealcoolingperiodinyourkilnschedule.Belowthestrainpointanyresidualstressesfromannealingarepermanentbecausetheglasshasbecomerigid. Coolingdownfromthestrainpointtoroomtemperaturecanthenhappenatanyratethatdoesnotcausethermalshockbreakage. So,nomentionof“upper”or“lower”annealorstrainpoints. Butstopforamomentandthink.Thisdoesnotmeanyoudonothavemorethanoneannealandstrainpointstoconsiderwhendesigningakilnschedule.Rememberthatwhenyouhavemorethanonekindofglassinyourcreation(egpinkopalandsomepurpletransparent)theneachofthemwithhavetheirownparticularannealandstrainpoints.Lifeisneversimple.Youneedtotaketheirdifferencesintoaccount.Youmustchooseananneal“soak”temperaturethatisabletoallowalltheglasstoannealatasensiblerate.Youalsowanttheannealcoolingstepofyourscheduletoendwhenthewholepieceisatorbelowthestrainpointforallthecomponentglassparts. SomeConclusions Thepropertiesofglassmaybedownrightweirdandunusualbutisdoesnotmeanthatthedesignofkilnschedulesshouldbeintherealmsofguesswork,naïvesuperstitionandblindfaithinsomeoneelse’skilnschedulesuggestions.Startaskingthequestion“Why?”ratherthan“How?” Makeuseofwhatyou’vejustlearned.Compareyourscheduleswithwhatishappeningtotheglass.Bemorescientific. Andfinally,myexperienceoverthelastfewdayssuggestsyoumusthaveahealthymistrustofwhatyoureadontheInternet.Lookforcorroborationbuttakestepstoverifythatcorroborationisnotactuallycut-and-pastecopying.Lookforprimarysourcesofinformation(egfromBullseyeorUroboros)ratherthantrusttheguessworkandvaguerecollectionsofsecondarysources(eginForums). Happyfusing. Sharethis:TwitterFacebookLikethis:LikeLoading... Related Aboutchatterglass Makerofstainedglassfrippery. Viewallpostsbychatterglass→ ThisentrywaspostedinAnnealing,kilnscheduleandtaggedAnnealpoint,Kilnschedule,Poise,Softeningpoint,Strainpoint,Temperaturerange,Workingrange.Bookmarkthepermalink. ←Whatnext? UseYourGlassGrinder Waste!→ 10ResponsestoTheMysteryofSoftening,AnnealingandStrain Points onjoFilms(@onjoFilms)says: 8August,2019at4:19am Verygoodarticle.Thanksforsharingyourresearch.Itisveryhelpful! Reply chatterglasssays: 23August,2019at10:15am Thankyouforyourcomment.Mostkindofyou. Reply Jesssays: 9June,2020at10:22pm Thisissousefulformyresearch!Igotsoconfusedbyallthesetemperaturepoints Reply Jesssays: 9June,2020at10:22pm butthanksforclearingitallup! Reply chatterglasssays: 11June,2020at5:58pm HiJess.Thankyouforyourkindcomments. Jesssays: 24June,2020at2:36pm Mypleasure,thankyouforwriting! MohitKumarTyagisays: 4September,2020at5:07pm Reallygoodexplanationtotheseglasstemperaturepointsandthanks Reply chatterglasssays: 7September,2020at9:49am Mypleasure.Thankyouforyourcomment. Reply DevonWillissays: 28October,2021at10:56pm Hi!I’mworkingonatechnicalquestionwithacustomerandtheydirectedmetoyourblog,specificallythisentry.Fulldisclosure,I’vebeenaBullseyeRepforthepast11years.OneofthereasonsIloveworkinghereisbecausejustlikeyou,ourteamisconstantlyresearchingthescienceandapplicationofglasstoincreaseunderstandingofit’sbehaviorwheninflictedwithheat.We’veevenenlistedthehelpofCorningGlassovertheyearstohelpsolvespecificglassrelatedissueswhichalwaysleadstosomefundiscoveries.Ilovehowyou’vetakenthingsintoyourownhandstounderstandthescientific“what’s”and“why’s”andIjustwantedtosaythatyouarewelcometocontactmeat[snip!]@bullseyeglass.comifyoueverwanttoaskspecificquestionsrelatedtoyourresearch.Fromoneglassnerdtoanother,thanksforsharingtheloveofthisfascinatingmaterialwiththeworld!Andifyouhaven’tvisitedalready,youHAVEtomakeapilgrimagetotheCorningGlassMuseum,itwillblowyourinquiringmind.GiveyourselfatLEASTthreedaystosoakitallin. Reply chatterglasssays: 29October,2021at11:32am HellothereandthankyouforyourkindcommentsandIwillbeintouchtointroducemyself.IbelieveitwasJohnConstable,thepainter,thatsaid“Weseenothingtillwetrulyunderstandit”.InthisrespectmyfalteringattemptstounderstandandthensharewhatIhavelearnedwithmyfellowglassworkerscanonlyhelpusgetbetteratwhatwedo. Reply LeaveaReplyCancelreply Enteryourcommenthere... Pleaseloginusingoneofthesemethodstopostyourcomment: Email(required)(Addressnevermadepublic) Name(required) Website YouarecommentingusingyourWordPress.comaccount. ( Log Out /  Change ) YouarecommentingusingyourTwitteraccount. ( Log Out /  Change ) YouarecommentingusingyourFacebookaccount. ( Log Out /  Change ) Cancel Connectingto%s Notifymeofnewcommentsviaemail.Notifymeofnewpostsviaemail. Δ Searchfor: RecentPosts ReduceWaste–Make Danglers! 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