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Glass is a non-crystalline amorphous solid that is often transparent, though it can also be colored/opaque if impurities are present. Glass is commonly used for many everyday items, like drinking glasses and windows, or in labs, as the main construction material for glassware.

Composition and effects

The composition of glass consists of the main oxide, which gives the name of the glass (silicon dioxide: silica glass, germanium dioxide: germania glass, etc.) To lower the melting point of silica or germania, an alkali oxide is added, but since this decreases the water resistance of said glass, an alkaline-earth oxide is added to increase the chemical stability. Other oxides, like boron oxide, lead oxide, or phosphorus oxide can be used, though the addition of said oxides change the properties of glass. If transition metal oxides are added, like iron oxides, cobalt oxide or manganese oxide, the glass becomes colored.

Types of glass

  • Fused-silica glass: more commonly called fused quartz, consists of very pure silica (SiO2) in vitreous (or glass) form (i.e., its molecules are disordered and random, without crystalline structure). This type of glass has very low thermal expansion, is very hard, and resists high temperatures (1000–1500 °C). Since it consists of pure silicon dioxide, fused quartz is also the most resistant against weathering (caused in other glasses by alkali ions leaching out of the glass, while staining it). Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles. It is also transparent to UV light, property which allows it to be used for UV lamps.
  • Soda-lime-silica glass: the most common form of glass, consists of silica + sodium oxide (Na2O) + lime (CaO) + magnesia (MgO) + alumina (Al2O3). A clear solid, it shows high thermal expansion and poor resistance to heat (500–600 °C), as well as poor resistance to both thermal and mechanical shock. It is used for windows, tableware, glass marbles, some low-temperature incandescent light bulbs, etc. Container glass is a soda-lime glass that is a slight variation on flat glass, which uses more alumina and calcium, and less sodium and magnesium, which are more water-soluble. This makes it less susceptible to water erosion.
  • Sodium borosilicate glass: is is one of the most commonly used glass in the manufacturing of lab items as well as heat resistant glass. Its composition is silica + boron trioxide (B2O3) + soda (Na2O) + alumina (Al2O3). Borosilicate glass tolerates thermal expansion much better than soda-lime glass. Other applications include cooking glass, car head lamps, etc. Borosilicate glasses (e.g. Pyrex, Duran) have as main constituents silica and boron oxides. They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10−6/°C as compared to about 9×10−6/°C for a typical soda-lime glass), making them more dimensionally stable. The lower coefficient of thermal expansion (CTE) also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.
  • Lead-oxide glass: commonly referred to as crystal glass or lead glass, it consists of silica + lead oxide (PbO) + potassium oxide (K2O) + soda (Na2O) + zinc oxide (ZnO) + alumina. Because of its high density (resulting in a high electron density), it has a high refractive index, making the look of glassware more brilliant (nicknamed "crystal", though of course it is a glass and not a crystal). Crystal glass also has high elasticity, making glassware "ring" if struck. It is also more workable in the factory, but cannot stand thermal effects very well. This kind of glass is also more fragile than other glasses and is easier to cut.
  • Aluminosilicate glass: having the composition silica + alumina + lime + magnesia + barium oxide (BaO) + boric oxide (B2O3), and displaying good mechanical properties, this type of glass is used in the manufacturing of fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.) and for halogen bulb glass. Aluminosilicate glasses are also resistant to weathering and water erosion, due to low alkali content.
  • Germanium-oxide glass: an extremely clear glass, is is used in the manufacturing of fiber optics. Unlike the other types of glass, this one lacks silica from its composition, and instead contains germania (GeO2), having the composition alumina + germanium dioxide GeO2.
  • Chalcogenide glass: contains one or more chalcogens (S, Se, Te, but not oxygen), commonly used in the manufacturing of CDs.

Types of glass materials

Common materials made of glass include:

  • Glass planes: used in window-making, shelves, external decorations
  • Tempered glass: commonly used for car windshields and other places where mechanical stress is expected. Some types are laminated.
  • Fiber optics: used in fiber optics communication, are flexible tubes of glass with good optic properties.
  • Fiberglass: used in construction, for making composite materials, cables
  • Glass wool: made of fiberglass or other fibrous glass material, they are commonly used as insulating materials, suitable for high temperatures
  • Glass-ceramic: used in the construction of cooktops, have an amorphous phase and one or more crystalline phases

General properties

In general, glass is a hard solid, more often than not transparent, electrical insulator and a poor thermal conductor. Except for fused quartz and borosilicate glass, most glass types do not handle thermal shock and will crack.

Glass is chemically inert, with only a few reagents capable of attacking it, like fluorine, hydrofluoric acid, molten alkali (NaOH e.g.), hot phosphoric acid, certain fluorine interhalogens (ClF3, ClF5). Despite this, in contact with water or aqueous solutions, glass will slowly leak alkali in the water, although this phenomenon is extremely slow.


Glass is commonly encountered in everyday items, but identifying each type of glassware is not easy.

Soda-lime glass is commonly found in glass panels like windows and drinking glass.

Lead glass can be found in vintage crystal glassware items.

Borosilicate glass can be found in Pyrex cookware and lab glassware.

Fiberglass can be found in most composite materials, but fiber glass rolls are a better source.

Germania glass can be found in fiber optics, which you can get by buying rolls online or removing them from fiber optics lamps.

Fused quartz can be bought from hardware suppliers, taken from heating or UV lamps. Small pieces of fused quartz can be found in EPROMs or other electronic components.

Glass can also be found in nature (not as trash), in the form of fulgurite (created by lightning strikes), obsidian (from volcanic activity) or tektite (meteoric impact). These types of glass don't have much use beyond collectibles.

Make your own glass

Making glass at home is not easy peasy, but if you have a good kiln, you can make almost any type of glass. You can also make colored glass or various glass "sculptures".



Glass is inert, but pulverized glass is harmful to touch, eye contact or if inhaled. Broken glass is sharp and may lead to injuries.

See also


Relevant Sciencemadness threads