1.Initialheatingofaglassformingbatchusuallyresultsinthereleaseofsomemoisture,whichmayhavebeenabsorbedontheparticles,orcombinedaswaterofhydrationorashydroxyl.Manyofthecomponentsofcommonglassformingbatchesaresomewhathygroscopic,readilyabsorbingsomewaterfromthesurroundings.Boricoxidemaypartiallyconverttoboricacid(H3BO3),CaOmayformCa(OH)2,etc.Othercomponentsalreadycontainwater,e.g.,NaOH,clays,hydratedalumina,NaB4O7.l0H2O,whichwillbereleasedatmoderatetemperatures.Thetemperatureatwhichthiswaterisreleasedwilldependuponthenatureofitsbondingtothematerials,i.e.,physicalorchemical,andthestrengthofthesebonds.Removalofthiswatercarriesheatfromthebatchandincreasesthecostofprocessing.Farmoregasisreleasedduringthedecompositionofcarbonates,sulfates,andnitrates.Thegasesreleasedexpandtovolumesmuchgreaterthanthatofthestartingbatch,resultinginconsiderablemixingandstirringaction,whichaidsinhomogenizationofthemelt.Thecreationofsomuchgas,however,alsoleadstotheformationofanextremelylargenumberofbubbles,whichmustberemovedfromthemeltbeforeprocessingiscompleted.Therapidformationofaliquidcanentrapaportionoftheairwhichinitiallyoccupiesthespacebetweenparticles,andresultinbubbleformation.Rapidheatingofsuchameltcanleadtoexpansionofthesebubblesandfoamingofthemelt.Itispossibleforthefoamingduetotrappedairtocausethemelttoriseabovetheupperrimofacrucible,ifthecrucibleisinitiallyfilledtothetopwithbatch.Sinceglassformingmeltsareusuallyveryefficientatdissolvingthebottomoffurnaces,careshouldalwaysbetakentoensurethatthisdoesnotoccur.2.Glassformingabilityisdefinedintermsofresistancetocrystallizationofameltduringcooling;glassstabilityisdefinedintermsofresistancetocrystallizationofaglassduringheating.Glassformingabilityismostimportantduringprocessesrequiringproductionofaninitialglass,whileglassstabilityismostimportantduringprocessesinvolvingre-formingofanexistingglass.Glassstabilityisfrequentlycharacterizedbythedifferenceintemperaturebetweentheonsetoftheglasstransformationregion(Tg)andtheoccurrenceofcrystallization(Tx),forasampleheatedataspecifiedlinearrate.3.Regardlessofthesourceofthecomponentsusedtoproduceaspecificglass,thebatchmaterialscanbedividedintofivecategoriesonthebasisoftheirroleintheprocess:glassformer,flux,propertymodifier,colorant,andfiningagent.Thesamecompoundmaybeclassedintodifferentcategorieswhenusedfordifferentpurposes.Alumina,forexample,servesasaglassformerinaluminateglasses,butisconsideredapropertymodifierinmostsilicateglasses.Arsenicoxidemaybeeitheraglassformerorafiningagent,dependinguponthepurposeforwhichithasbeenaddedtothebatch.4.Allglassesfoundtodatesharetwocommoncharacteristics.First,noglasshasalongrange,periodicatomicarrangement.Andevenmoreimportantly,everyglassexhibitstime-dependentglasstransformationbehavior.Thisbehavioroccursoveratemperaturerangeknownastheglasstransformationregion.Aglasscanthusbedefinedas“anamorphoussolidcompletelylackinginlongrange,periodicatomicstructure,andexhibitingaregionofglasstransformationbehavior.”Anymaterial,inorganic,organic,ormetallic,formedbyanytechnique,whichexhibitsglasstransformationbehaviorisaglass.5.Chalcogenideglassesarearecognizedgroupofinorganicglassymaterialswhichalwayscontainoneormoreofthechalcogenelements:S,SeorTe,butnotO,inconjunctionwithmoreelectropositiveelementsmostcommonlyAsandGe,butalsoP,Sb,Bi,Si,Sn,Pb,B,Al,Ga,In,Tl,Ag,lanthanidesandNa.ChalcohalideglassesadditionallycontainhalogenelementsandincludetheTeX(Xishalogen)glasses.Chalcogenideglassesaregenerallylessrobust,moreweaklybondedmaterialsthanoxideglasses.Bothheteropolar(e.g.,Ge-S)andhomopolar(e.g.,S-S;Ge-Ge)bondscanform.Thusglassformulationscancontainanon-stoichiometricamountofchalcogenandexcessSandSeatomscanformchains.Thechemicalbondingofthematrixisusuallydirectionalandcovalentandhalogensactaschainornetworkterminators.Thechalcogenideglassesarebandgapsemiconductorsandareinfraredtransmitting.6.ChalcogenideGlass:akindofnon-oxidesystemglass,containingtheelementofsulphur(S),selenium(Se)ortellurium(Te)asglassformer;combinedwithsomeothercations,As,Sn,Ga,In,Ge,Pb,Bi,etc.Chalcogenideglasspossessexcellentinfraredtransmissionability;Chalcogenideglasseshaveexcellentgoodsemiconductivity.ChalcohalideGlass:Thiskindofglasscanbeobtainedbyintroducingsomealkalimetalhalidetothebasecompositionsofchalcogenideglasses.Chalcogenideandchalcohalideglassesarehighlyinterestingmaterialsinthephotonicapplications.e.g.,fiberamplifiers,tunablelasers,upconversionemissionandwavelengthdivisionmultiplexing.7.Themostcommonmethodforfabricatingthiskindofglassismeltquenchingtechnique.Thechalcogenideglassesarepreparedbymeltingthecorrespondingmixtureoftheelementsinquartzampoulessealedunderhighvacuumconditions.Forthesakeofhomogeneity,twomeasurescanbetaken:1)Ampoulesarerotatedintheoven;2)Theovenissubjectedtoperiodicalchangeofpositionoreventolargeperiodicaloscillations.Somedifferencebetweenoxideandchalcogenideglasspreparationprocessexists:Forpreparingchalcogenideglass:1)quartzampoulesaregenerallyused;2)Themeltingtemperatureisgenerallylowerthanoxideglasses;3)Meltedinclosedsys