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1、1.Initialheatingofaglassformingbatchusuallyresultsinthereleaseofsomemoisture,whichmayhavebeenabsorbedontheparticles,orcombinedaswaterofhydrationorashydroxyl.Manyofthecomponentsofcommonglassformingbatchesaresomewhathygroscopic,readilyabsorbingsomewaterfromthesurroundings.Boricoxidema
2、ypartiallyconverttoboricacid(H3BO3),CaOmayformCa(OH)2,etc.Othercomponentsalreadycontainwater,e.g.,NaOH,clays,hydratedalumina,NaB4O7.l0H2O,whichwillbereleasedatmoderatetemperatures.Thetemperatureatwhichthiswaterisreleasedwilldependuponthenatureofitsbondingtothematerials,i.e.,physical
3、orchemical,andthestrengthofthesebonds.Removalofthiswatercarriesheatfromthebatchandincreasesthecostofprocessing.Farmoregasisreleasedduringthedecompositionofcarbonates,sulfates,andnitrates.Thegasesreleasedexpandtovolumesmuchgreaterthanthatofthestartingbatch,resultinginconsiderablemixi
4、ngandstirringaction,whichaidsinhomogenizationofthemelt.Thecreationofsomuchgas,however,alsoleadstotheformationofanextremelylargenumberofbubbles,whichmustberemovedfromthemeltbeforeprocessingiscompleted.Therapidformationofaliquidcanentrapaportionoftheairwhichinitiallyoccupiesthespacebe
5、tweenparticles,andresultinbubbleformation.Rapidheatingofsuchameltcanleadtoexpansionofthesebubblesandfoamingofthemelt.Itispossibleforthefoamingduetotrappedairtocausethemelttoriseabovetheupperrimofacrucible,ifthecrucibleisinitiallyfilledtothetopwithbatch.Sinceglassformingmeltsareusual
6、lyveryefficientatdissolvingthebottomoffurnaces,careshouldalwaysbetakentoensurethatthisdoesnotoccur.2.Glassformingabilityisdefinedintermsofresistancetocrystallizationofameltduringcooling;glassstabilityisdefinedintermsofresistancetocrystallizationofaglassduringheating.Glassformingabil
7、ityismostimportantduringprocessesrequiringproductionofaninitialglass,whileglassstabilityismostimportantduringprocessesinvolvingre-formingofanexistingglass.Glassstabilityisfrequentlycharacterizedbythedifferenceintemperaturebetweentheonsetoftheglasstransformationregion(Tg)andtheoccurr
8、enceofcrystallizati
