Glass
Glass istransparent, relatively strong, hard-wearing, essentially inert,biologically inactive material which can be formedvery smoothimpervious surfaces. These desirable properties lead tovery many usesglass.Glassesuniform amorphous solid materials, usually produced whensuitably viscous molten material cools very rapidly, thereby not giving enough time forregular crystal latticeform.
Common glassmostly amorphous silicon dioxide (SiO2), which issame chemical compound as quartz, orits polycrystalline form, sand. Pure silica hasmelting pointabout 2000 Celsius, so two other substancesalways added tosand inglass-making process. Onesoda (sodium carbonate Na2CO3), or potash,equivalent potassium compound, which lowersmelting pointabout 1000 Celsius. However,soda makesglass soluble, whichobviously unhelpful, so lime (calcium oxide, CaO) isthird component, addedrestore insolubility.
One ofmost obvious characteristicsglassthat ittransparentvisible light. This transparencydue tofact that there are no atomic line transition states withenergyvisible light inmaterial that makes up glass, andfact that glasshomogeneous on all lengthscales greater than aboutwavelengthlight (inhomogeneities cause lightbe scattered, breaking up any coherent image transmission).
Extremely pure glass can be made so transparent that hundredskilometersglass can be 'seen through' at infrared wavelengthsfibre optic cables.
Most common glass has other ingredients addedchange its properties. Leaded glassmore brilliant, becauseits increased refractive index, while boron may be addedchangethermalelectrical properties, asPyrex. Adding barium will also increaserefractive index,ceriumusedglass that absorbs infrared energy. Other metal oxidesaddedchangecolor. Additional Soda or potashsometimes addedfurther lowermelting point,manganese can be addedremove unwanted colors.
Glasssometimes created naturally from volcanic flows informobsidian.
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2 Fine glassware 3 Architectural Glass 4 Does glass flow? 5 References: 6 External links: |
Historyglass
Naturally occurring glass, such as obsidian, has been used sincestone age. The first documented glass makingin Egypt around 2000 BC glass was first used asglazepotteryother items. Infirst century BCtechniqueblowing glass was developedwhat had once been an extremely rarevaluable item became much more common. DuringRoman Empire many formsglass were created mostlyusevasesbottles.
Until12th century stained glass (whichglasssome metals addedcolor) was not widely used.
The centreglass making from14th century was Venice which developed many new techniquesbecamecenter oflucrative export tradedinner ware, mirrors,other luxury items. Eventually some ofVenetian glass workers movedother areasnorthern Europeglass making spreadthem.
Used up tomid-1800s wasCrown glass process,whichglassblower would spin around 9 lbs.molten glass atend ofrod untilflattened intodisk approximately 5 feetdiameter. The disk would then be cut into panes. Venetian glass was highly prized between10th14th centuries askeptprocess secret. Around 1688,processcasting glass was developed, which ledit beingmuch more commonly used material. The invention ofglass pressing machine1827 allowedmass productioninexpensive glass articles.
The word glass, Latin glacis (ice) German Glas, M.E. glas, A.S. glaes was also used byAesti-Old Prussians. They usedword glaesdescribe amber, recorded by Roman historians as glaesum. Angle-Saxons usedword glaeramber. Another German wordamber, Bernstein (English translation : burning stone), came into use becauseits transparency as glass,shine (glare)its abilitymelt.
Fine glassware
Even withavailabilitycommon glassware, there remains placehand blown glassware. Some artistsglass include Sidney Waugh, René Lalique, Dale Chihuly,Louis Comfort Tiffany, who were responsibleextraordinary glass objects. The term "crystal glass", derived from rock crystal, has comedenote high-grade colorless glass, often containing lead,is sometimes appliedany fine hand-blown glass.
Architectural Glass
Float (annealed) glass
90% ofworld's flat glassproduced byfloat process developed by Pilkington Glass,which molten glasspoured onto molten tin. The glass floats ontin, spreading outgivingsmooth faceeither side. The glass slowly solidifies astravels overmolten tin, leavingtin bath incontinuous ribbonis then fire-polished, leavingproductnear-perfect parallel surfaces.
Toughened glass
Toughened glassmade from ordinary annealed float glass viathermal tempering process. The float glasscut torequired sizeany required processing (such as polishingedges or drilling holes inglass)carried out beforetoughening process starts. The glassplaced ontoroller table, takingthroughfurnace which heats itabove its annealing point600 Celsius. The glassthen rapidly cooledforced draughtsair. This rapid cooling induces compressive stresses insurface ofglass balanced by tensile stresses inbody ofglass. The patterncooling can be revealed by observingglasspolarised light. Toughened glasstypically assumedbe six timesstrengthannealed glass.
However, this strength comes withpenalty. Due tobalanced stresses inglass, any damage toglass edges will result inglass shattering into thumbnail sized pieces. Thiswhyglass must be cutsize before tougheningcannot be re-worked once toughened. Also,toughened glass surfacenot as tough as annealed glassis more susceptiblescratching.
Toughened glasstypically usedunframed assemblies such as frameless doorsin structurally loaded applications.
Toughened glassconsideredsafety glass dueits increased strengthits tendencyshattersmall, rounded pieces.
Laminated glass
Laminated glassproduced by bonding two or more layersordinary annealed glass together withplastic interlayer, usually polyvinyl butyral (PVB). The PVBsandwiched byglass whichthen heatedaround 70 Celsiuspassed through rollersexpel any air pocketsforminitial bond.
The PVB interlayer also givesglassmuch higher sound insulation rating, due todamping effect,also blocks 99%transmitted UV light.
Laminated glassnormally used when there ispossibilityhuman impact or whereglass could fall if shattered. Shopfront glazingtypicallylaminated glass.
Laminated glassconsideredsafety glass dueits abilityhold togetherremain in-situ even when shattered.
Does glass flow?
Itsometimes claimed that glass may show some ofpropertiesliquids that flow at room temperature, albeit very slowly. This has ledcontroversial statements such asclaim that "glass issupercooled liquid". Itsometimes claimed that old windowsoften thicker atbottom than attop,that this might be dueflow. It isbit unclear where this belief came from, or if there was ever any evidencesupport it.
One possible sourcethis beliefthat when panesglass were commonly made by glassblowers,technique that was used wasspin molten glass so ascreateround, mostly flateven plate (the Crown glass process, described above). This plate was then cutfitwindow. The pieces were not, however, absolutely flat;edges ofdisk would be thicker duecentrifugal forces. When actually installed inwindow frame,glass would be placed thicker side down forsakestability. Thereanecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness attimeinstallation.
The "glass flows" issue has been discussed at great length inalt.folklore.urban newsgroup, andconsensus there (supported by citations from glass experts)that glass does not flow at room temperature. Note, however, that glass candoes 'creep' , just like crystalline solids do,response toload. Furthermore,some applications (such as some laboratory thermometers), glass gets heated abovetransition temperature at whichactually does becomesupercooled liquid. This can causecalibrationthermometerschange slightly overcoursemany yearsuse.
Arguments against glass flow:
- if medieval glass has flowed perceptibly, then ancient RomanEgyptian objects should have flowed proportionately more - but thisnot observed.
- if glass flows atrate that allows changesbe seen withnaked eye after centuries, then changesoptical telescope mirrors should be observable (by interferometry) inmatterdays - but this alsonot observed.
References:
- "Do Cathedral Glasses Flow?", Am. J. Phys. v66, pp 392-396, May, 1998
- Noel C. Stokes; The GlassGlazing Handbook; Standards Australia; SAA HB125-1998
