Introduction

This document is somewhat of a continuation of my tutorial on glass compositions and assumes a basic understanding of glasses from a structural and compositional standpoint.

Soda Lime Silica Glasses

One of the oldest glass compositions known, soda-lime silica glasses account for roughly 90% of the glass produced today. They owe their ability to stand the test of time to their ease of production, satisfactory level of mechanical and chemical durability, and their relatively cheap raw materials.

Most soda lime silica glasses are composed of soda (Na2O), lime (CaO), and silica (SiO2) in a 16-10-74 proportion. Although this ratio was discovered empirically in ancient times, it happens to coincide with a very important region on the Na2O-SiO2 binary phaes diagram (and its ternary complement, the Na2O-CaO-SiO2 phase diagram) near the lowest eutectic temperature at 788°C. While 16-10-74 melts somewhere around 800°C, it’s worthy to note that, as with all glasses, a melt at this temperature would be impractically viscous; the gob temperature (which is a fair indication of the temperatures around which a melt can be worked with) is around 1185°C.

Although its mechanical properties (namely, strength and durability) are by no means exceptional, soda lime silica glasses are widely used for most common glass applications such as in windows, glass bottles, tableware, and light bulbs. The raw materials used to create soda lime glasses are also inexpensive; soda ash, limestone, and silica sand can be purchased by the tonnage at significantly less expense than the raw materials required for borosilicate or specialty glasses.

Borosilicate Glasses

The addition of B2O3 to silica serves many purposes because it not only contributes desirable properties to glasses, but acts as a glass former much like silica does. The addition of borates allows one to use less alkali (such as soda and potash) in the glass which is often desirable, as alkali fluxes significantly decrease mechanical strength and dielectric breakdown fields of their glasses. The addition of borates also contributes significantly to the chemical durability (as in the case of sodium vapor lamp encasements) and reduced thermal expansion (in the case of Pyrex glassware).

A common borosilicate composition is

  • 80% silica
  • 12.9% B2O3
  • 3.9% Na2O
  • 2.2% alumina
  • 0.4% K2O

This has a total alkali content of 4.3% as opposed to the 16% found in the standard soda-lime silica system. While this composition (which happens to be that of Corning 7740 or Pyrex) has superior thermal shock resistance, this low alkali also makes borosilicates harder to melt. Furthermore, raw materials that contribute borates to glasses are very expensive, but borosilicate glasses are still manufactured widely into cookware, labware, and fiberglass insulation.

Lead Glasses / Lead-alkali Silicate

Lead oxide (PbO) acts as a flux in silica glass, lowering the melting points and therefore making processing and forming steps easier. Unlike alkali fluxes, though, the addition of lead oxide to silica does not detriment the dielectric loss of glasses, and its density gives lead glasses enhanced brilliance due to lead’s high refractive index. Lead oxide (PbO) is commonly added in anywhere from 18% to 65% in addition to around 11% alkali. Lead glass is most commonly used for lead “crystal” decorations and tableware (which, unlike lead-based paints, do not leech lead in any appreciable amount) and optical glasses, often being used as IR-transmittive glass (such as those needed in heat-seeking missiles) and x-ray absorptive glasses and radiation shielding.

Aluminosilicate Glasses

The addition of alumina to a silica network imparts notable strength and temperature resistances to the glass by binding up the glassy network. Aluminosilicate glasses vary widely in composition but are typically characterized as having between 20% and 40% alumina. A typical composition of an aluminosilicate glass is 57% silica, 20.5% alumina, 12% magnesia, 5.5% lime, 4.0% B2O3, and 1% soda.

Due to this very low flux content, such aluminosilicate glasses are very hard to melt and form (moreso than borosilicates), but possess superior thermal expansion (0.5 ppm in aluminosilicates versus 3.3 ppm in borosilicates), high resistance to chemical attack due to very low alkali content, good strength, and very good refractory properties. Aluminosilicate glasses are most often found in the form of cookware, glass ceramics, and fiberglass.

High Silica Glasses

As would be expected, high silica glasses, due to their lack of fluxing agents, are very hard to melt and possess working temperatures well over 2000°C. Their properties are generally superior to most other types of glasses, with the very high processing temperatures being the limiting factor in the production and application of these high-silica glasses on a larger scale. For example, these glasses possess very low thermal expansion, good chemical durability, optical properties, mechanical properties, and very good high-temperature behavior. Their primary detractor is that they have a relatively low density due to their open structure, and (combined with impurities) this makes high silica glasses good ionic conductors.

High-silica glasses have traditionally been difficult to create due to the high temperatures required to cause melting of silica. As technology has improved, though, there have been six generations of high-silica glass production technology, each generation yielding higher purities and better quality than the last.

  • Type I pure silica glass was made by the electric melting of natural quartz crystals. Because the raw material (quartz) was natural, it contained geological impurities, and this combined with low-quality melting to produce a porous, translucent silica. These glasses are still used in bunsen burner ceramic triangles and some older heating element protectors.
  • Type II silica is produced by running natural quartz powder through a hydrogen/oxygen flame to melt it, resulting in a less porous, optically transparent glass. However, the hydrogen flame produces water which acts as an IR-absorptive material in the glass, and the geological impurities inherent in natural quartz are still present.
  • Type III silica glass is synthetically produced by the gas-phase hydrolysis of silicon tetrachloride in a hydrogen flame. While this eliminates the geological/color impurities from natural quartz sources, the hydrogen flame still results in an IR-absorptive glass.
  • Type IV silica glass is produced via the gas-phase hydrolysis of SiCl4 in plasma. This results in a water-free synthetic glass that is pure and significantly more IR-transmissive than Type III silica glass.
  • Type V high silica glass is produced via solgel synthesis from triethylorthosilicate [Si(OC2H5OH)4, or TEOS) followed by full densification. Because it is synthesized at room temperature, Type V glass maintains all the purity of Type IV glass without the thermal defects.
  • Type VI silica is nanoporous “thirsty” glass, such as Vycor, also synthesized from solgel. It can be densified into a solid pure silica glass.

Applications of these pure and high-silica glasses include vast use in the semiconductor industry since silica doesn’t contaminate silicon wafers, fiber optics, UV-transmissive lamp tubes, precision optics, refractory tubes, and as a fiber reinforcer in composites.