Types of glass

Glasses may be devised to meet almost any imaginable requirement. For many specialised applications in chemistry, pharmacy, the electrical and electronics industries, optics, the construction and lighting industries, glass, or the comparatively new family of materials known as glass ceramics, may be the only practical material for the engineer to use.

Each can be made by a suitable adjustment to chemical compositions, but the main types of glass are:

Borosilicate Glass

Most of us are more familiar with this type of glass in the form of ovenware and other heat-resisting ware. Borosilicate glass, the third major group, is made mainly of silica (70-80%) and boric oxide (7-13%) with smaller amounts of the alkalis (sodium and potassium oxides) and aluminium oxide.

This type of glass has a relatively low alkali content and consequently has good chemical durability and thermal shock resistance (it doesn't break when changing temperature quickly.)

As a result it is widely used in the chemical industry, for laboratory apparatus, for ampoules and other pharmaceutical containers, for various high intensity lighting applications and as glass fibres for textile and plastic reinforcement.

Commercial Glass

Most of the glass we see around us in our everyday lives in the form of bottles and jars, flat glass for windows or for drinking glasses is known as commercial glass or soda-lime glass, as soda ash is used in its manufacture.

The main constituent of practically all commercial glass is sand. Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700°C. Adding other minerals and chemicals to sand can considerably reduce the melting temperature.

The addition of sodium carbonate (Na2CO3), known as soda ash, to produce a mixture of 75% silica (SiO2) and 25% of sodium oxide (Na2O), will reduce the temperature of fusion to about 800°C. However, a glass of this composition is water-soluble and is known as water glass. In order to give the glass stability, other chemicals like calcium oxide (CaO) and magnesium oxide (MgO) are needed. These are obtained by adding limestone which results in a pure inert glass.

Commercial glass is normally colourless, allowing it to freely transmit light, which is what makes glass ideal for windows and many other uses. Additional chemicals have to be added to produce different colours of glass such as green, blue or brown glass.

Glass Fibre

Glass fibre has many uses from roof insulation to medical equipment and its composition varies depending on its application.For building insulation and glass wool the type of glass used is normally soda lime. 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.In recent years, great progress has been made in making optical fibres which can guide light and thus transmit images round corners.

These fibres are used in endoscopes for examination of internal human organs, changeable traffic message signs now on motorways for speed restriction warnings and communications technology without which telephones and the internet would not be possible.

Lead Glass

Commonly known as lead crystal, lead glass is used to make a wide variety of decorative glass objects.It is made by using lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide. The traditional English full lead crystal contains at least 30% lead oxide (PbO) but any glass containing at least 24% PbO can be described as lead crystal. Glass containing less than 24% PbO, is known simply as crystal glass.

The lead is locked into the chemical structure of the glass so there is no risk to human health.Lead glass has a high refractive index making it sparkle brightly and a relatively soft surface so that it is easy to decorate by grinding, cutting and engraving which highlights the crystal's brilliance making it popular for glasses, decanters and other decorative objects.

Glass with even higher lead oxide contents (typically 65%) may be used as radiation shielding because of the well-known ability of lead to absorb gamma rays and other forms of harmful radiation.