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1、Materialsforlecture16-18Readingpassage1Questions1-13arebasedonthefollowingpassage.Sheetglassmanufacture:thefloatprocessGlass,whichhasbeenmadesincethetimeoftheMesopotamiansandEgyptians,islittlemorethanamixtureofsand,sodaashandlime.Whenheatedtoabout1500degreesCelsius(°C)thisbecom
2、esamoltenmassthathardenswhenslowlycooled.Thefirstsuccessfulmethodformakingclear,flatglassinvolvedspinning.Thismethodwasveryeffectiveastheglasshadnottouchedanysurfacesbetweenbeingsoftandbecominghard,soitstayedperfectlyunblemished,witha‘firefinish’.However,theprocesstookalongtime
3、andwaslabourintensive.Nevertheless,demandforflatglasswasveryhighandglassmakersacrosstheworldwerelookingforamethodofmakingitcontinuously.Thefirstcontinuousribbonprocessinvolvedsqueezingmoltenglassthroughtwohotrollers,similartoanoldmangle.Thisallowedglassofvirtuallyanythicknessto
4、bemadenon-stop,buttherollerswouldleavebothsidesoftheglassmarked,andthesewouldthenneedtobegroundandpolished.Thispartoftheprocessrubbedawayaround20percentoftheglass,andthemachineswereveryexpensive.ThefloatprocessformakingflatglasswasinventedbyAlistairPilkington.Thisprocessallowst
5、hemanufactureofclear,tintedandcoatedglassforbuildings,andclearandtintedglassforvehicles.Pilkingtonhadbeenexperimentingwithimprovingthemeltingprocess,andin1952hehadtheideaofusingabedofmoltenmetaltoformtheflatglass,eliminatingaltogethertheneedforrollerswithinthefloatbath.Themetal
6、hadtomeltatatemperaturelessthanthehardeningpointofglass(about600°C),butcouldnotboilatatemperaturebelowthetemperatureofthemoltenglass(about1500°C).Thebestmetalforthejobwastin.Therestoftheconceptreliedongravity,whichguaranteedthatthesurfaceofthemoltenmetalwasperfectlyflatandhoriz
7、ontal.Consequently,whenpouringmoltenglassontothemoltentin,theundersideoftheglasswouldalsobeperfectlyflat.Iftheglasswerekepthotenough,itwouldflowoverthemoltentinuntilthetopsurfacewasalsoflat,horizontalandperfectlyparalleltothebottomsurface.Oncetheglasscooledto604°Corlessitwastoo
8、hardtomarkandcouldbetransportedoutofthecoolingzonebyro
