Glasses

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
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Discussion

Glass is not a material like steel or concrete. It's the name given to a phase of matter equal in importance to that of the solid, liquid, gas, or plasma phases. Glasses combine the properties of both liquids and solids but are also distinctly different from each. In one sense, glasses are supercooled liquids, but one should not take this description too literally. While glasses share an equal level of disorder with liquids they flow about as much as solids do; which is to say, glasses don't flow at all. In another sense, glasses are just disordered solids, but again one should not take this description too literally. Glasses are no more disordered than are liquids; which is to say, glasses are semi-ordered (or semi-disordered for you pessimists).

The atoms of a glass are held firmly in place like they are in a solid but are arranged randomly like those in a liquid. This gives glasses some mechanical strength, but not as much as a true solid. When glasses fail, they fracture along curved surfaces that reveal the flow patterns of a liquid frozen in time. Glasses are usually formed by melting crystalline materials at very high temperatures and then cooling them "quickly" before the atoms get a chance to settle into a nice, well-ordered arrangement.

Glasses share properties with both solids and liquids.
solid glass liquid
long range atomic order short range atomic order short range atomic order
when solids break, they fracture along planar surfaces when glasses break, they fracture along curved surfaces liquids can't withstand shear forces
very little atomic mobility very little atomic mobility limited atomic mobility
retain their shape for long periods of time and return to their original shape after stresses have been removed retain their shape for long periods of time and return to their original shape after stresses have been removed take the shape of their container up to the limit of their volume, which is more or less fixed

Indefinite melting point. No plateau on heating/cooling curve.

Glasses are nonequilibrium materials, so their physical properties depend on the process used to make them.

Commercial glasses …

The first glass containers were not molded or blown, but were sculpted from glass blocks 4000 years ago in the eastern Mediterranean (Mesopotamia, Egypt, and surrounding territories).

These glass object were formed well over 3000 years ago in Egypt, yet they have not changed shape in any appreciable way. That's because glasses are not liquids. They do not flow at temperatures far below their melting point.

Until the mid Nineteenth Century there were only two kinds of commercial glass: soda-lime, crown glass and lead-containing, flint glass. Then in 1884, Otto Schott …

Most of this was adapted from the Corning Museum of Glass
http://www.cmog.org/page.cfm?page=279
A smaller portion came from British Glass
http://www.britglass.co.uk/aboutglass/types.htm

Types or categories based on chemical composition.

  1. Soda-lime glass [crown glass] is the most common (90% of glass made), and least expensive form of glass. It usually contains 60-75% silica [silicon dioxide SiO2], 12-18% soda [sodium oxide Na2O], 5-12% lime [calcium oxide CaO]. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair. [Transmits visible light, but absorbs ultraviolet. Used in windows and bottles.]

    {Crown glass}, a variety of glass, used for making the finest plate or window glass, and consisting essentially of silicate of soda or potash and lime, with no admixture of lead; the convex half of an achromatic lens is composed of crown glass; -- so called from a crown-like shape given it in the process of blowing.
  2. Lead glass [flint glass] has a high percentage of lead oxide [PbO] (at least 20% of the batch). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. It is somewhat more expensive than soda-lime glass and is favored for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature. [Denser with higher refractive index than soda-lime glass. Lead oxide instead of lime? Used in CRT-TV screens to block out x-rays.]

    Flint glass \Flint" glass`\ (Chem.) A soft, heavy, brilliant glass, consisting essentially of a silicate of lead and potassium. It is used for tableware, and for optical instruments, as prisms, its density giving a high degree of dispersive power; -- so called, because formerly the silica was obtained from pulverized flints. Called also {crystal glass}. Cf. {Glass}. Note: The concave or diverging half on an achromatic lens is usually made of flint glass.
  3. Borosilicate glass [Pyrex] is any silicate glass having at least 5% [typically 10-25%] of boric oxide [B2O3] in its composition [and a small amount of Al2O3]. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass, and not as low in cost as lime, borosilicate's cost is moderate when measured against its usefulness. Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware are examples of borosilicate products.

    Boron. Nonmetallic element, fifth in atomic table, distinguished by the symbol B, atomic weight 10.8. Boron does not occur free in nature but always in combination with oxygen and other elements, notably sodium and/or calcium. Borax. Natural or refined sodium tetraborate with ten molecules of water, i.e., Na2B4O7.10H2O. Borax also has pentahydrate (five molecules of water) and anhydrous forms. Borate. Generic term for boron compounds containing oxygen. Boric acid. An acid formed form reaction of borates with H2SO4. Formerly called boric acid, its formula is H3BO3. Borosilicate. General term referring to a range of glasses containing silica (typically over 60%) and boric oxide (5-20%). Two specific glasses cover most applications: low expansion Pyrex-type glass; and highly durable 'neutral' glass. Glass fibers[?], ceramic glazes, and vitreous enamels are also borosilicates.
  4. Aluminosilicate glass [?] has aluminum oxide in its composition. It is similar to borosilicate glass but it has greater chemical durability and can withstand higher operating temperatures. Compared to borosilicate, aluminosilicates are more difficult to fabricate. When coated with an electrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry

    According to British Glass: "For textiles, an alumino-borosilicate glass with very low sodium oxide content is preferred because of its good chemical durability and high softening point. This is also the type of glass fibre used in the reinforced plastics to make protective helmets, boats, piping, car chassis, ropes, car exhausts and many other items."
    "A small, but important type of glass, aluminosilicate, contains 20% aluminum oxide (alumina-Al2O3) often including calcium oxide, magnesium oxide and boric oxide in relatively small amounts, but with only very small amounts of soda or potash. It is able to withstand high temperatures and thermal shock and is typically used in combustion tubes, gauge glasses for high-pressure steam boilers, and in halogen-tungsten lamps capable of operating at temperature as high as 750 oC."
  5. Ninety-six percent silica glass [?] is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements from the piece. By reheating to 1200 °C the resulting pores are consolidated. This glass is resistant to heat shock up to 900 °C.
  6. Fused silica [optic fibers] glass is pure silicon dioxide in the non-crystalline state. It is very difficult to fabricate, so it is the most expensive of all glasses. It can sustain operating temperatures up to 1200 °C for short periods. [Transparent to a wide range of visible wavelengths.]
  7. Alkali-barium Silicate Glass [CRT glass]
    "Without this type of glass watching TV would be very dangerous. A television produces X-rays that need to be absorbed, otherwise they could in the long run cause health problems. The X-rays are absorbed by glass with minimum amounts of heavy oxides (lead, barium or strontium). Lead glass is commonly used for the funnel and neck of the TV tube, while glass containing barium is usually used for the screen." From British Glass.

Composition

  1. Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a former. (With enough heat, 100% of the earth's crust could be made into glass.) Most commercial glass is made with sand that contains the most common former, Silica. Other formers include …
    • Silica (sand, quartz pebbles) silicon dioxide SiO2
    • Anhydrous Boric Acid
    • Anhydrous Phosphoric Acid
  2. Fluxes help formers to melt at lower, more practical to achieve temperatures. But melting sand by itself is too expensive because of the high temperatures required (about 1850 °C, or 3360 °F). Therefore fluxes are added, which let the former melt more readily and at lower temperatures (1300 °C, or 2370 °F). These include …
    • Soda (soda ash, natron, marine plant ashes) (Na2CO3): sodium oxide Na2O
    • Potash (ashes of inland plants) potassium oxide K2O
    • Lithium Carbonate
  3. Stabilizers combine with formers and fluxes to keep the finished glass from dissolving, crumbling, or falling apart. But fluxes also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. Therefore stabilizers are added to make the glass uniform and keep its special structure intact. These include …
    • Lime (chalk, limestone (CaCO3)) calcium oxide CaO
    • Litharge
    • Alumina
    • Magnesia
    • Barium Carbonate
    • Strontium Carbonate
    • Zinc Oxide
    • Zirconia
  4. Additives?
    • Lead (oxidized lead metal) lead oxide PbO
    • Boron (modern mineral) boric oxide B2O3
    • Magnesium (impurity) -- MgO
    • Aluminum (impurity) -- Al2O3
    • Iron (impurity) level must be reduced to less than 1 ppb in optic fibers
    • Copper -- CuO copper oxide makes glass green
    • Iron -- Fe2O3 iron oxide makes glass green (brown?)
      • brown bottle glass with from the addition of a sulfur-iron colloidal solution
      • brown beer bottle glass is made from brown sand containing iron impurities
    • Cobalt oxide CoO -- deep blue cobalt glass
    • Chromium -- green bottle glass contains small amounts of chromium

Natural glasses …

Candy glasses …

Candies
crystalline
solid-like		 noncrystalline
glass-like
rock candy with large visible crystals, cream candies (fondant, fudge) with very small homogenous, but imperceptible crystals hard candy (lemon drop, clear mint, butterscotch, lollipop), caramel, taffy, peanut brittle, marshmallow, divinity, nougat
sucrose glucose & fructose (invert sugar)

Water has a glass phase. Glassy water can be produced by …

Just like ordinary window glass, water glass requires annealing to relax into a metastable state. Depending on pressure and temperature conditions, one of three glassy states are currently known: low-density amorphous ice (LDA), high-density amorphous ice (HDA), and very-high-density amorphous ice (VHDA). These glassy states differ microscopically in structure and macroscopically in properties like density. The commonly accepted temperature for water's glass phase transition 136 K at one atmosphere.

Summary

Problems

practice

  1. Write something.
    • Answer it.
  2. Write something else.
    • Answer it.
  3. Write something different.
    • Answer it.
  4. Write something completely different.
    • Answer it.

numerical

  1. problems

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