Glass

What is glass and how is it produced?

Glass is a hard material normally fragile and transparent common in our daily life. It is composed mainly of sand (silicates, SiO2) and an alkali.

These materials at high temperature (i.e. molten viscous state) fuse together; then they are cooled rapidly forming a rigid structure, however not having enough time to form a crystalline regular structure.

Depending on the final use and application the composition of the glass and cooling rate will vary to achieve the adequate properties for the specific application. These are the common ingredients to obtain glass:

1. Sand (SiO2 silica)
In its pure form it exists as a polymer, (SiO2)n.

2. Soda ash (sodium carbonate Na2CO3)
Normally SiO2 softens up to 2000°C, where it starts to degrade (at 1713°C most of the molecules can already move freely). Adding soda will lower the melting point to 1000°C making it more manageable.

3. Limestone (calcium carbonate or CaCo3) or dolomite (MgCO3)
Also known as lime, calcium carbonate is found naturally as limestone, marble, or chalk.
The soda makes the glass water-soluble, soft and not very durable. Therefore lime is added increasing the hardness and chemical durability and providing insolubility of the materials.

Other materials and oxides can be added to increase properties (tinting, durability, etc.), produce different effects, colors, etc.

Main properties of glass These are the main characteristics of glass: - Solid and hard material - Disordered and amorphous structure - Fragile and easily breakable into sharp pieces - Transparent to visible light - Inert and biologically inactive material. - Glass is 100% recyclable and one of the safest packaging materials due to its composition and properties Glass is used for architecture application, illumination, electrical transmission, instruments for scientific research, optical instruments, domestic tools and even textiles. Glass does not deteriorate, corrode, stain or fade and therefore is one of the safest packaging materials.  These properties can be modified and changed by adding other compounds or heat treatment.

Types of glass and market application

The main types of glass are described below:

Commercial glass or Soda-lime glass:
This is the most common commercial glass and less expensive. The composition of soda-lime glass is normally 60-75% silica, 12-18% soda, and 5-12% lime. A low percentage of other materials can be added for specific properties such as coloring.

- It has light transmission appropriate to be use in flat glass in windows;

- It has a smooth and nonporous surface that allows glass bottles and packaging glass to be easily cleaned;

- Soda-lime glass containers are virtually inert, resistant to chemical attack from aqueous solutions so they will not contaminate the contents inside or affect the taste.

Whereas pure glass SiO2 does not absorb UV light, soda-lime glass does not allow light at a wavelength of lower than 400 nm (UV light) to pass. 

The disadvantages of soda-lime glass is that is not resistant to high temperatures and sudden thermal changes. For example, everybody has experienced a glass breaking down when pouring liquid at high temperature, for example to make tea. 

Some of the use of soda-lime glass is primarily used for bottles, jars, everyday drinking glasses, and window glass.

Lead glass:

Lead glass is composed of 54-65% SiO2, 18-38% lead oxide (PbO), 13-15% soda (Na2O) or potash (K2), and various other oxides. When the content of PbO is less than 18% is known as crystal glass.

- In moderate amounts lead increases durability;

- In high amounts it lowers the melting point and decreases the hardness giving a soft surface;

- In addition it has a high refractive index giving high brilliance glass.

These two last properties make it appropriate for decorating purposes.

Glass with high lead oxide contents (i.e. 65%) may be used as radiation shielding glass because lead absorb gamma rays and other forms of harmful radiation, for example, for nuclear industry.

As with soda-lime glass, lead glass will not withstand high temperatures or sudden changes in temperature.

Borosilicate glass:
Borosilicate glass is mainly composed of silica (70-80%), boric oxide B2O3 (7-13%) and smaller amounts of the alkalis (sodium and potassium oxides) such as 4-8% of Na2O and K2O, and 2-7% aluminum oxide (Al2O3).

Boron gives greater resistance to thermal changes and chemical corrosion.
It is suitable for industrial chemical process plants, in laboratories, in the pharmaceutical industry, in bulbs for high-powered lamps, etc. Borosilicate glass is also used in the home for cooking plates and other heat-resistant products. It is used for domestic kitchens and chemistry laboratories, this is because it has greater resistance to thermal shock and allows for greater accuracy in laboratory measurements when heating and cooling experiments.

There are other special types of glass by adding different substances. For example:

Recently developed forms of glass include: - Safety glass, constructed of two pieces of plate glass join by a plastic to prevent the glass from scattering when broken. - Fiberglass made from molten glass formed into continuous filaments that is used for fabrics or electrical insulation - Foam glass made by trapping gas bubbles in glass to produce a spongy material for insulating purposes.