Glass - An Overview

Glass is not one material but a term describing a large group of generic types of material displaying the properties which lead to them being described as glasses.

Glass is an amorphous (non-crystalline) solid usually formed by the solidification of a melt without crystallisation.  Compared with crystals, the structure of glass is devoid of a regular arrangement of atoms in a periodic lattice.

Glasses are not stoichiometric compounds but are variable compounds of substances, which can form glasses in isolation, with those that can not, thus leading to the great variety of possible compositions available to the technologist. Many glass properties are additive functions of their component materials e.g. thermal expansion, modulus of elasticity.

Glass Transformation Temperature Range

Besides an amorphous nature, inorganic and organic glasses exhibit the transformation or glass transition temperature range.

Glasses do not exhibit solidification and crystallisation at the melting point.  A glass forming liquid does not undergo any abrupt change in viscosity or a distinct melting point, changes occur over a range of temperature.

Glass Production

Glass is produced by many methods from small batch operations producing a few kilograms to large continuous processes producing tonnes per day.  The peculiar properties of glassy materials, which can be tailored to meet many demanding applications lead to its use in many varied applications. e.g. flat and container glass, glass fibres, glass tubing, bulbs, TV screens, tableware, insulators, electrical components, pharmaceuticals.

The different applications and specific properties demanded of them are too diverse to describe fully here, however the following is a summary of some of the different glass compositional groups which may be encountered.

Types of Glass

Vitreous Silica – ‘Quartz’ Glass

Ground quartz materials are melted under vacuum, to remove gas bubbles, at 2000^oC with approximately 10^-2 % impurities.

The glass possesses a low thermal expansion coefficient (6.7x10^-7/K) and is thermally stable to around 1000^oC. Devitrification to cristobalite begins between 1150^oC and 1200^oC. The vitreous melts are highly viscous.

Some vitreous silicas are produced at lower temperatures or without the use of vacuum. These are characteristically opaque due to many small bubbles in the mass of the glass.

Modifications may be made to produce highly reactive forms of silica.

Sodium Silicate – ‘Water Glass’

Sodium silicate is melted from quartz and soda at 1400°C. Dissolution of the glass granulate is carried out at elevated temperature in pressure autoclaves.

The application of sodium silicate as a binder solution in many ceramic systems makes this material attractive as an adhesive.

The content of SiO₂ in the initial glass ranges from 66-76 wt.% and the content of sodium silicate varies in the liquid form depending upon the grade.

Potassium silicate water glass is also manufactured for special purposes e.g. acid resistant cements.

Sheet and Container Glass (Soda-Lime-Silica)

The basic formulation of soda lime glasses varies little between flat and container (holloware) applications e.g. 72% SiO₂, 14% Na₂O(K₂O), 9% CaO, 2-4% MgO, 1-2 % Al₂O₃.

Due to the high alkali content the glasses have relatively high thermal expansion coefficients 8.0-9.0 x10^-6 /K and low viscosity at temperature due to the low Al₂O₃ content.

‘Crystal’ Glasses (K₂O-CaO-SiO₂, K₂O-PbO- SiO₂)

Lead and potassium oxides are characteristic components of glasses called ‘crystal’.  K₂O and PbO are also common components of optical, sealing, and other technical glasses.

The term crystal denotes a high-grade clear colourless glass with high gloss and optical transmission.  Conventionally only glass containing more than 24% PbO and having a refractive index exceeding 1.545 is termed crystal.

K₂O and PbO promote the brilliant appearance of the glass.  Glasses are formed with up to 65% K₂O in the K₂O-SiO₂ system and 80 wt.% PbO in the PbO-SiO₂ systems.

In most instances, industrially produced crystal glasses are more complex than the basic ternary system.  Other components include Na₂O, BaO, ZnO, B₂O₃ and MgO.

Lead crystal glasses contain 24-32 wt.% PbO. Typical compositions for K₂O and PbO types are given in table 1.

Table 1. Typical compositions of lead and crystal glasses.

Refining agents such as sodium sulphate NaSO_4, arsenic As₂O_3, and antimony Sb₂O₃ are often employed.

Lead glasses are easily shaped, cut and polished and have thermal expansion coefficients range from 7.5-9x10^-6/K.

Borosilicate Glasses

These are glasses which can exhibit improved thermal shock resistance due to relatively low thermal expansion coefficients <5.0x10^-7/K.  Originally developed for laboratory use, they now find wide application in industrial and domestic situations.

PYREX glasses are of this type.

These glasses tend to have low alkali contents and high SiO₂ contents e.g.>80 wt.%.

White Opaque – Opal Glass

Opal glasses show a milky opalescence.  Opacity may be due to the presence of a dispersed crystalline, vitreous or gaseous phase.  In practice it is usually developed by the introduction of fluorides to the batch. An example formula is given in table 2.

Table 2. A typical composition for an opal glass.

Special Glasses

Ceramic Glazes

Ceramic glazes cover the full spectrum of glass compositions including alkaline earth alumino-silicates, alkali/alkaline earth alumino-silicates, borosilicates, and lead silicates and crystallising compositions.

Commonly encountered glazes are designed to possess higher viscosity at their maturity than a glass would in its working range, as excessive flow is undesirable.

Glazes may be formulated with thermal expansion coefficients to suit the substrate, typically ranging from 0.25-0.5 linear % at 500^oC.  Glass ceramic glazes are often employed to produce super-low thermal expansion coatings.

Conventional glazes contain higher alumina levels, around 8-12 wt.%_, than industrial glasses and are often more complex in composition.

Sealing and Solder Glasses

The different sealing glasses for producing glass to metal seals are grouped according to the thermal expansion ranges of the metals to be joined. For this reason glasses with an expansion coefficient somewhat lower than the chosen metal are desirable (table 3).

Table 3. Thermal expansion co-efficients of metals and their respective glass seal materials

Sealing glasses may be for applications in low or high temperature environments.

Glassy solders do not crystallise noticeably during soldering.  They are generally lead-borate (60-90% PbO) with SiO₂ and Al₂O₃ to improve chemical resistance. Solder glasses used for joining glass-glass and glass to metal components generally operate (flow) at low temperatures 450°C-550^oC.

Crystallising Solders

These maintain glassy characteristics until the soldering temperature is reached. At this point they transform into a glassy-crystalline structure producing an irreversible joint.

For temperatures >550°C zinc-borate and silicon borate glasses can be used, e.g.:

50-65 % ZnO, 0-15% SiO₂, 20-35% B₂O₃

Phosphate Glasses

A major phase in bone china is calcium phosphate glass.  Phosphate glasses may be produced using Zn, Mg, Ca, Ba or Na.  Thermal expansions range from 7.0-13.0x10^-6/K (Ca-P =8.0 x10^-6/K).

Combining with Al improves resistance to water vapours and lowers the thermal expansion.

Alkali phosphate glasses show solubility in water.

A calcium phosphate glass may be produced by melting at 1300°C a mixture of 49.6 P₂O₅, 49.6 CaO and 0.8 SiO₂ mol%.

Chalcogenide Glasses

This group of glasses includes those formed from chalcogenous elements S, Se, Te. These glasses also contain Ge, Si, P, As, Sb and Bi and possibly Ti Cd Pb.

The glasses are usually simple containing 2-4 elements.  They find application in special optical situations.

Glasses of the As-Se, As-S and As-Se(S) –Te systems form around 200°C. Other chalcogenide glasses form above 500°C.

Key Properties

There are no fixed properties for glasses as their composition and use is so varied.

The application of glass may be purely for aesthetic reasons, functional purposes or both.

The following properties are considered of importance depending on the application and product/process types.

Crystallisation

The tendency of glasses to crystallize, particularly when undergoing heating is known as devitrification.  The rate will depend on the composition in question and the thermal treatment.

Many glasses are extremely chemically durable being resistant to concentrated acidic and alkaline solutions.  This is therefore an important property in the choice of a glass in certain applications.  Other glasses are soluble and this property can be exploited.

This is one of the most important properties for glass production and working.  Viscosity is affected by composition and temperature. 

Density varies according to composition and can therefore be used as a production control measure.

Mechanical Properties

The mechanical strength of a glass depends on the condition of the glass surface.  The compressive strength of glass is approximately 10 times the tensile strength.  Surface treatment such as toughening can increase the strength of glass.

The colour, refractive index and dispersion of the glass will affect its potential uses in for example flat glass or for optical components.

Deliberately coloured glasses are produced by the addition of oxides such as chromium for green.

Perhaps the most important of the thermal properties for glasses.  The coefficient of thermal expansion varies strongly with composition e.g. quartz glass 3.2 x 10^-6/K heavy lead flint 8.0 x 10^-6/K.

Glass can also have sound insulating properties and is used in double and triple glazing for this purpose.

Electrical Properties.

Usually considered an electrical insulator at high temperature, glasses are good conductors of electricity.  At room temperature, the resistivity is around 10¹² ohms compared to 5-50 ohms at 1400°C for soda lime glass.