Glass

Glass is a material (see below) and a drinking vessel made of this material. See also glasses.


Glass is a transparent, relatively strong, hard-wearing, essentially inert, and biologically inactive material which can be formed with very smooth and impervious surfaces. These desirable properties lead to the very many uses of glass.

Glasses are uniform amorphous solid materials, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form.

Common glass is mostly amorphous silicon dioxide (SiO2), which is the same chemical compound as quartz, or in its polycrystalline form, sand. Pure silica has a melting point of about 2000 Celsius, so two other substances are always added to the sand in the glass-making process. One is soda (sodium carbonate Na2CO3), or potash, the equivalent potassium compound, which lowers the melting point to about 1000 Celsius. However, the soda makes the glass soluble, which is obviously unhelpful, so lime (calcium oxide, CaO) is the third component, added to restore insolubility.

One of the most obvious characteristics of glass is that it is transparent to visible light. This transparency is due to the fact that there are no atomic line transition states with the energy of visible light in the material that makes up glass, and the fact that glass is homogeneous on all lengthscales greater than about a wavelength of light (inhomogeneities cause light to be scattered, breaking up any coherent image transmission).

Extremely pure glass can be made so transparent that hundreds of kilometers of glass can be 'seen through' at infrared wavelengths in fibre optic cables.

Most common glass has other ingredients added to change its properties. Leaded glass is more brilliant, because of its increased refractive index, while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium will also increase the refractive index, and cerium is used in glass that absorbs infrared energy. Other metal oxides are added to change the color. Additional Soda or potash is sometimes added to further lower the melting point, and manganese can be added to remove unwanted colors.

Glass is sometimes created naturally from volcanic flows in the form of obsidian.

Table of contents
1 History of glass
2 Fine glassware
3 Architectural Glass
4 Does glass flow?
5 References:
6 External links:

History of glass

Naturally occurring glass, such as obsidian, has been used since the stone age. The first documented glass making is in Egypt around 2000 BC glass was first used as a glaze for pottery and other items. In the first century BC the technique of blowing glass was developed and what had once been an extremely rare and valuable item became much more common. During the Roman Empire many forms of glass were created mostly for use in vases and bottles.

Until the 12th century stained glass (which is glass with some metals added for color) was not widely used.

The centre for glass making from the 14th century was Venice which developed many new techniques and became the center of a lucrative export trade in dinner ware, mirrors, and other luxury items. Eventually some of the Venetian glass workers moved to other areas of northern Europe and glass making spread with them.

Used up to the mid-1800s was the Crown glass process, in which a glassblower would spin around 9 lbs. of molten glass at the end of a rod until it flattened into a disk approximately 5 feet in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the 10th and 14th centuries as they kept the process secret. Around 1688, the process for casting glass was developed, which led to it being a much more commonly used material. The invention of the glass pressing machine in 1827 allowed the mass production of inexpensive glass articles.

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 describe 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 transparency as glass, to shine (glare) and its ability to melt.

Fine glassware

Even with the availablity of common glassware, there remains place for hand blown glassware. Some artists in glass include Sidney Waugh, René Lalique, Dale Chihuly, and Louis Comfort 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.

Architectural Glass

Float (annealed) glass

90% of the world's flat glass is produced by the float process developed by Pilkington Glass, in which molten glass is poured onto molten tin. The glass floats on the tin, spreading out and giving a smooth face to either side. The glass slowly solidifies as it travels over the molten tin, leaving the tin bath in a continuous ribbon and is then fire-polished, leaving a product with near-perfect parallel surfaces.

Toughened glass

Toughened glass is made from ordinary annealed float glass via a thermal tempering process. The float glass is cut to the required size and any required processing (such as polishing the edges or drilling holes in the glass) is carried out before the toughening process starts. The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point of 600 Celsius. The glass is then rapidly cooled with forced draughts of air. This rapid cooling induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. The pattern of cooling can be revealed by observing the glass with polarised light. Toughened glass is typically assumed to be six times the strength of annealed glass.

However, this strength comes with a penalty. Due to the balanced stresses in the glass, any damage to the glass edges will result in the glass shattering into thumbnail sized pieces. This is why the glass must be cut to size before toughening and cannot be re-worked once toughened. Also, the toughened glass surface is not as tough as annealed glass and is more susceptible to scratching.

Toughened glass is typically used in unframed assemblies such as frameless doors and in structurally loaded applications.

Toughened glass is considered a safety glass due to its increased strength and its tendency to shatter in small, rounded pieces.

Laminated glass

Laminated glass is produced by bonding two or more layers of ordinary annealed glass together with a plastic interlayer, usually polyvinyl butyral (PVB). The PVB is sandwiched by the glass which is then heated to around 70 Celsius and passed through rollers to expel any air pockets and form the initial bond.

The PVB interlayer also gives the glass a much higher sound insulation rating, due to the damping effect, and also blocks 99% of transmitted UV light.

Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Shopfront glazing is typically a laminated glass.

Laminated glass is considered a safety glass due to its ability to hold together and remain in-situ even when shattered.

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. This has led to controversial statements such as the claim that "glass is a supercooled liquid". 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 is not observed.

Tar pitch, on the other hand, is a highly viscous liquid which appears solid, and, unlike glass, does flow at room temperature, very very slowly. See
pitch drop experiment for more details.

References:

  • "Do Cathedral Glasses Flow?", Am. J. Phys. v66, pp 392-396, May, 1998
  • Noel C. Stokes; The Glass and Glazing Handbook; Standards Australia; SAA HB125-1998

External links:




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