Types

Silicate

Silicon dioxide (SiO₂) is a common fundamental constituent of glass. Fused quartz is a glass made from chemically-pure silica. It has very low thermal expansion and excellent resistance to thermal shock, being able to survive immersion in water while red hot, resists high temperatures (1000–1500 °C) and chemical weathering, and is very hard. It is also transparent to a wider spectral range than ordinary glass, extending from the visible further into both the UV and IR ranges, and is sometimes used where transparency to these wavelengths is necessary. Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc. However, its high melting temperature (1723 °C) and viscosity make it difficult to work with. Therefore, normally, other substances (fluxes) are added to lower the melting temperature and simplify glass processing.

Soda-lime

Sodium carbonate (Na₂CO₃, "soda") is a common additive and acts to lowers the glass-transition temperature. However, Sodium silicate is water-soluble, so lime (CaO, calcium oxide, generally obtained from limestone), some magnesium oxide (MgO) and aluminium oxide (Al₂O₃) are other common components added to improve chemical durability. Soda-lime glasses (Na₂O) + lime (CaO) + magnesia (MgO) + alumina (Al₂O₃) account for over 75% of manufactured glass, containing about 70 to 74% silica by weight. Soda-lime-silicate glass is transparent, easily formed, and most suitable for window glass and tableware. However, it has a high thermal expansion and poor resistance to heat. Soda-lime glass is typically used for windows, bottles, light bulbs, and jars.

Borosilicate

Borosilicate glasses (e.g. Pyrex, Duran) typically contain 5–13% boron trioxide (B₂O₃). Borosilicate glasses 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). They are, therefore, less subject to stress caused by thermal expansion and thus less vulnerable to cracking from thermal shock. They are commonly used for e.g. labware, household cookware, and sealed beam car head lamps.

Lead

The addition of lead(II) oxide into silicate glass lowers melting point and viscosity of the melt. The high density of Lead glass (silica + lead oxide (PbO) + potassium oxide (K₂O) + soda (Na₂O) + zinc oxide (ZnO) + alumina) results in a high electron density, and hence high refractive index, making the look of glassware more brilliant and causing noticeably more specular reflection and increased optical dispersion. Lead glass has a high elasticity, making the glassware more workable and giving rise to a clear "ring" sound when struck. However, lead glass cannot withstand high temperatures well. Lead oxide also facilitates solubility of other metal oxides and is used in colored glass. The viscosity decrease of lead glass melt is very significant (roughly 100 times in comparison with soda glass); this allows easier removal of bubbles and working at lower temperatures, hence its frequent use as an additive in vitreous enamels and glass solders. The high ionic radius of the Pb2+ ion renders it highly immobile and hinders the movement of other ions; lead glasses therefore have high electrical resistance, about two orders of magnitude higher than soda-lime glass (108.5 vs 106.5 Ω⋅cm, DC at 250 °C).

Aluminosilicate

Aluminosilicate glass typically contains 5-10% alumina (Al₂O₃). Aluminosilicate glass tends to be more difficult to melt and shape compared to borosilicate compositions, but has excellent thermal resistance and durability. Aluminosilicate glass is extensively used for fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.), top-of-stove cookware, and halogen bulb glass.

Other oxide additives

The addition of barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eyeglasses. Iron can be incorporated into glass to absorb infrared radiation, for example in heat-absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs ultraviolet wavelengths. Fluorine lowers the dielectric constant of glass. Fluorine is highly electronegative and lowers the polarizability of the material. Fluoride silicate glasses are used in manufacture of integrated circuits as an insulator.

Glass-ceramics

Glass-ceramic materials contain both non-crystalline glass and crystalline ceramic phases. They are formed by controlled nucleation and partial crystallisation of a base glass by heat treatment. Crystalline grains are often embedded within a non-crystalline intergranular phase of grain boundaries. Glass-ceramics exhibit advantageous thermal, chemical, biological, and dielectric properties as compared to metals or organic polymers.

The most commercially important property of glass-ceramics is their imperviousness to thermal shock. Thus, glass-ceramics have become extremely useful for countertop cooking and industrial processes. The negative thermal expansion coefficient (CTE) of the crystalline ceramic phase can be balanced with the positive CTE of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net CTE near zero. This type of glass-ceramic exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C.

Fibreglass

Fibreglass (also called glass fibre reinforced plastic, GRP) is a composite material made by reinforcing a plastic resin with glass fibres. It is made by melting glass and stretching the glass into fibres. These fibres are woven together into a cloth and left to set in a plastic resin. Fibreglass has the properties of being lightweight and corrosion resistant, and is a good insulator enabling its use as building insulation material and for electronic housing for consumer products. Fibreglass was originally used in the United Kingdom and United States during World War II to manufacture radomes. Uses of fibreglass include building and construction materials, boat hulls, car body parts, and aerospace composite materials.

Glass-fibre wool is an excellent thermal and sound insulation material, commonly used in buildings (e.g. attic and cavity wall insulation), and plumbing (e.g. pipe insulation), and soundproofing. It is produced by forcing molten glass through a fine mesh by centripetal force, and breaking the extruded glass fibres into short lengths using a stream of high-velocity air. The fibres are bonded with an adhesive spray and the resulting wool mat is cut and packed in rolls or panels.

Non-silicate

Besides common silica-based glasses many other inorganic and organic materials may also form glasses, including metals, aluminates, phosphates, borates, chalcogenides, fluorides, germanates (glasses based on GeO₂), tellurites (glasses based on TeO₂), antimonates (glasses based on Sb₂O₃), arsenates (glasses based on As₂O₃), titanates (glasses based on TiO₂), tantalates (glasses based on Ta₂O₅), nitrates, carbonates, plastics, acrylic, and many other substances. Some of these glasses (e.g. Germanium dioxide (GeO₂, Germania), in many respects a structural analogue of silica, fluoride, aluminate, phosphate, borate, and chalcogenide glasses) have physico-chemical properties useful for their application in fibre-optic waveguides in communication networks and other specialized technological applications.

Silica-free glasses may often have poor glass forming tendencies. Novel techniques, including containerless processing by aerodynamic levitation (cooling the melt whilst it floats on a gas stream) or splat quenching (pressing the melt between two metal anvils or rollers), may be used increase cooling rate, or reduce crystal nucleation triggers.

Amorphous metals

In the past, small batches of amorphous metals with high surface area configurations (ribbons, wires, films, etc.) have been produced through the implementation of extremely rapid rates of cooling. Amorphous metal wires have been produced by sputtering molten metal onto a spinning metal disk. More recently a number of alloys have been produced in layers with thickness exceeding 1 millimeter. These are known as bulk metallic glasses (BMG). Liquidmetal Technologies sell a number of zirconium-based BMGs. Batches of amorphous steel have also been produced that demonstrate mechanical properties far exceeding those found in conventional steel alloys.

Experimental evidence indicates that the system Al-Fe-Si may undergo a first-order transition to an amorphous form (dubbed "q-glass") on rapid cooling from the melt. Transmission electron microscopy (TEM) images indicate that q-glass nucleates from the melt as discrete particles with a uniform spherical growth in all directions. While x-ray diffraction reveals the isotropic nature of q-glass, a nucleation barrier exists implying an interfacial discontinuity (or internal surface) between the glass and melt phases.

Polymers

Important polymer glasses include amorphous and glassy pharmaceutical compounds. These are useful because the solubility of the compound is greatly increased when it is amorphous compared to the same crystalline composition. Many emerging pharmaceuticals are practically insoluble in their crystalline forms. Many polymer thermoplastics familiar from everyday use are glasses. For many applications, like glass bottles or eyewear, polymer glasses (acrylic glass, polycarbonate or polyethylene terephthalate) are a lighter alternative to traditional glass.

Molecular liquids and molten salts

Molecular liquids, electrolytes, molten salts, and aqueous solutions are mixtures of different molecules or ions that do not form a covalent network but interact only through weak van der Waals forces or through transient hydrogen bonds. In a mixture of three or more ionic species of dissimilar size and shape, crystallization can be so difficult that the liquid can easily be supercooled into a glass. Examples include LiCl:RH₂O (a solution of lithium chloride salt and water molecules) in the composition range 4<R<8. sugar glass, or Ca_0.4K_0.6(NO₃)_1.4. Glass electrolytes in the form of Ba-doped Li-glass and Ba-doped Na-glass have been proposed as solutions to problems identified with organic liquid electrolytes used in modern lithium-ion battery cells.