Glas
Glas ist ein gleichförmiges amorphes Material. Glas entsteht normalerweise, wenn geschmolzene Materialien sehr schnell abkühlen, so dass nicht genügend Zeit bleibt, damit sich ein regelmäßiges Kristallgitter ausbilden kann.
Gewöhnliches Glas besteht hauptsächlich aus amorphem Siliziumdioxid (SiO2), dem gleichen Material, aus dem auch Quarz besteht und das in seiner polykristallinen Form Sand ergibt. Die offensichtlichste Eigenschaft von Glas ist, dass es durchsichtig ist. Extrem reines Glas kann so transparent sein, dass infrarotes Licht durch Hunderte von Kilometern Glas in Form von Glasfaserkabeln geschickt werden kann.
Die meistesten Glassorten enthalten noch weitere Zusatzstoffe, um ihre Eigenschaften zu verändern. Bleiglass funkelt stärker, weil es einen höheren Brechungsindex hat, während Bor als Zusatz die thermischen und elektrischen Eigenschaften verändert. Barium erhöht ebenfalls der Brechungsindex und Cer wird für Glas verwendet, das Infrarotstrahlung absorbiert. Andere Metalloxide als Zusätze ändern die Farbe. Soda und Pottasche werden manchmal zugegeben, um den Schmelzpunkt zu erniedrigen, und Mangan kann unerwünschte Farben entfernen.
Glas kann manchmal auch auf natürliche Weise entstehen, zum Beispiel als Obsididan in Vulkanen. Bei Blitzeinschlägen in Sandboden oder Meteoriteneinschlägen entsteht ebenfalls Glas.
Geschichte des Glases
Natürlich vorkommmendes Glas wie Obsidian wurde seit der Steinzeit benutzt. Dokumentiert wurde die Glasherstellung zum ersten mal in Ägypten um 2000 v.Chr, wo Glasscheiben hergestellt wurden, indem Sand auf geschmolzenem Blei geschmolzen wurde. Die Römer erfanden viele Arten von Glas, aber lange Zeit war Glas undurchsichtig, bis im 14. Jahrhundert italienienische Glasmacher das "Cristallo" erfanden, ein farbloses und durchsichtiges Glas. Anfangs wurde Glas geschleudert. Dazu wurde einige Kilo geschmolzenes Glas am Ende einer Stange schnell gedreht, so dass sich eine Scheibe mit etwa 1,50 Meter Durchmesser bildete. Daraus wurden dann Glasscheiben geschnitten. Um 1688 wurde die Methode des Glasgießens erfunden, wodurch Glass stärkere Verbreitung fand. Ab 1827 konnte Glas durch neue Maschinen als Massenware für billige Artikel produziert werden.
Glas ist durchsichtig, hart, chemisch weitgehend träge, biologisch inaktiv und kann mit sehr glatten und undurchlässigen Oberflächen hergestellt werden. Deshalb gibt es soviele Verwendungsmöglichkeiten für Glas.
The word glass, Latin glacis (ice) German Glas, M.E. glas, A.S. glaes was also used by the Aesti-Old Prussians. They used the word glaes to described amber, recorded by Roman historians as glaesum. Angle-Saxons used the word glaer for amber.
Another German word for amber Bernstein (english translation : burning stone) came into use because of its tranparency as glass, to shine (glare) and its ability to melt.
Fine glassware
Even with the availablity of comman glassware, there remains place for hand blown glassware. Some artists in glass include Sidney Waugh, René Lalique, Dale Chihuly, and Louis Tiffany, who were responsible for extraordinary glass objects. The term "crystal glass", derived from rock crystal, has come to denote high-grade colorless glass, often containing lead, and is sometimes applied to any fine hand-blown glass.
Toughened glass
Glass is a tremendously strong material, but very brittle. Glass articles intended to survive rough handling, or rapid temperature changes, are sometimes toughend by rapid and localised cooling of their surfaces during the manufacturing process (called "tempering"). This prestresses the material and reduces its tendency to crack at the surface when stressed. When tempered glass does shatter, it tends to break into rounded granules that are not as dangerous as ordinary glass shards. The pattern of cooling is revealed by observing the glass with polarised light.
Laminated glass
Large sheets of glass can cause a serious hazard when broken, as they tend to form shards with very sharp edges. This risk is unacceptable in applications such as automobile windscreens, or large shop windows. This hazard can be reduced by laminating the glass with layers of plastic material. Laminated glass tends to hold together when shattered, the shards bound to the flexible plastic layer where they are less likely to cause injury. Often, large sheets of glass will be both toughened and laminated. The laminate can be either within the bulk of the material, or applied to the surface.
Does glass flow?
It is sometimes claimed that glass may show some of the properties of liquids that flow at room temperature, albeit very slowly. As an example, it is sometimes claimed that old windows are often thicker at the bottom than at the top, and that this might be due to flow. It is a bit unclear where this belief came from, or if there was ever any evidence to support it.
One possible source of this belief is that when panes of glass were commonly made by glassblowers, the technique that was used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker due to centrifugal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability. There is anecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation.
The "glass flows" issue has been discussed at great length in the alt.folklore.urban newsgroup, and the consensus there (supported by citations from glass experts) is that glass does not flow at room temperature. Note, however, that glass can and does 'creep' , just like crystalline solids do, in response to a load. Furthermore, in some applications (such as some laboratory thermometers), glass gets heated above the transition temperature at which it actually does become a supercooled liquid. This can cause the calibration of thermometers to change slightly over the course of many years of use.
Arguments against glass flow:
- if medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more - but this is not observed.
- if glass flows at a rate that allows changes to be seen with the naked eye after centuries, then changes in optical telescope mirrors should be observable (by interferometry) in a matter of days - but this also not observed.
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