Atom

The smallest part of an element that can take part in a chemical reaction.  A speck of dust alone would contain over a million million atoms.

Borosilicate Glasses

As the name implies, borosilicate glasses, the third major group, are composed mainly of silica (70-80%) and boric acid (7-13%) with smaller amounts of the alkalis' (sodium and potassium oxides) and aluminium oxide.  They are characterised by the relatively low alkali content and consequently have good chemical durability and thermal shock resistance.  Thus they are pre-eminently suitable for process plants 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.  In the home they are familiar in the form of ovenware and other heat-resisting ware.

Colours and Glass

Unless the raw materials are very pure, glass made by mixing and heating sand, soda ash and limestone will normally be green, the depth of the colouration depending mainly on the amount of iron oxide and other colourants present in the raw materials.  A sand containing as little as one-thousandth part of iron oxide will give normal soda-lime glass, used for windows and glass containers, a greenish tint. 

For many products, instead of using high purity (and thus expensive) raw materials, glass manufacturers may decolourise the glass by adding minute amounts of other colourants which produce complementary colours to green so that the finished articles appear colourless.  Thus selenium (which gives a pink colour) and cobalt (which gives blue) can be added to soda-lime glass to offset the effect of the green or yellow due to the iron and this is done in the manufacture of glass containers.  Nickel may be used similarly in the decolourising of lead crystal glass.  Different additions may produce different coloured glasses, the range of possible colours being almost infinite.  The colour often depends on the state of oxidation of the colourant, the type of glass in which it is used, and thermal treatment.

Glass Colour/s

Green, Brown, Blue
Purple
Green, Yellow, Pink
Green, Blue, Grey
Blue, Green, Red
Blue, Green, Pink
Yellow, Purple
Yellow, Brown, Green
Purple, Brown
Purple
Green
Yellow
Amber, Brown
Yellow
Red
Pink, Red
Red

The use of large amounts of several different colourants will tend to produce black glasses.  Opaque or opal glasses can be produced by the addition of appropriate amounts of fluoride or phosphate compounds, which produce crystal growth, known in the glass industry as devitrification.

Commercial Glasses

Compound

A substance made from two or more elements whose atoms are chemically joined together.  This means that they can only be separated chemically.

Density

A measure of how tightly packed the particles are in a substance.  It is calculated from the formula density = mass ÷ volume.

Electron

One of the particles that make up an atom, they are negatively charged and are found orbiting around the atomic nucleus.

Element

A substance made of only one kind of atom.  For example, oxygen and carbon are elements, but carbon dioxide is a compound of the two, existing as molecules.  All the elements are listed in the periodic table.

Glass

Glass is a product obtained by the fusion of several inorganic substances, of which normally silica (SiO2) in the form of sand is the main one.  The fused mass is cooled to ambient temperature at a rate fast enough to prevent crystallisation, i.e. the molecules cannot arrange themselves into a crystalline pattern.  The fast rate of cooling to prevent crystallisation applies to transparent glasses, whereas in the case of translucent or opal glasses, the rate of cooling is such as to produce a pre-determined level of crystal formation.

Graph

Chart showing how one factor changes as another factor is changed.  These are shown in different ways but line graphs are the most common.  Usually done on graph (squared) paper.  Points are marked as small crosses and are usually joined up with a 'line of best fit'.

Lead Glasses

The use of lead oxide instead of calcium oxide, and of potassium oxide instead of all or most of the sodium oxide, gives the type of glass commonly known as lead crystal.  The traditional English full lead crystal contains at least 30% lead oxide (PbO) but any glass containing at least 24% PbO can be legitimately described as lead crystal according to the relevant EEC directive.  Glasses of the same type, but containing less than 24% PbO, are known simply as crystal glasses, some or all of the lead being replaced in these compositions by varying amounts of the oxides of barium, zinc and potassium.  Lead glasses have a high refractive index and a relatively soft surface so that they are easy to decorate by grinding, cutting or engraving.  The overall effect of cut crystal is the brilliance of the surface as the light is caught and refracted by the facets.  Lead glasses of somewhat different composition are used extensively in the electrical industry; they have good insulating properties partly because of the presence of lead oxide and partly because of the partial replacement of soda by potassium oxide, the sodium ion being the more conducting of the two.  Glasses with even higher lead oxide contents (typically 65%) may be used as radiation shielding glasses because of the well known ability of lead to absorb gamma rays and other forms of harmful radiations.

Mass

The amount of matter in something.  Mass is often taken as meaning the same as weight but, in science, it is definitely not.

Molecule

Two or more atoms chemically joined together.  For example, CO2 or even O2. The chemical 'bonding' that joins them together can take various forms, with the electrons taking a key role.

Mirror Boss

Metal locking base attached to windscreen which connects to interior rear view mirror.

Mix Ratio

The proportion of components in a two part adhesive system to give optimum cure rates

Moisture Cure

An adhesive that cures by reacting with moisture in the air.

Mouldings

Metal or plastic finishers for cosmetic or masking requirements.

Optical Glasses

Glasses can be designed to meet almost any specified combination of optical properties of which the most important are the refractive index (representing the deviation of a ray of light striking the glass at an oblique angle) and the dispersion (the dependence of the refractive index on wavelength).  Glasses with high dispersion relative to refractive index are called flint glasses while those with relatively low dispersions are called crown glasses.  Typically, flint glasses are lead-alkali-silicate compositions whereas crown glasses are soda-lime glasses.

Sealing Glasses

Another application for which a large variety of glass compositions is used, is sealing to metals for electrical and electronic components.  Here the available glasses may be grouped according to their thermal expansions which must be matched with the thermal expansions of the respective metals so that sealing is possible without excessive strain being induced by the expansion differences.

Soda-lime Glasses

These are the most common commercial glasses.  The chemical and physical properties of soda-lime glasses make them suitable for a wide variety of applications.  The nominally colourless types transmit a very high percentage of visible light and hence are used for windows.  Soda-lime glass containers are virtually inert, and so cannot contaminate the contents inside or affect the taste.  Their resistance to chemical attack from aqueous solutions is good enough to withstand repeated boiling (as in the case of preserving jars) without any significant changes in the glass surface.

Weight

The measure of the pull of gravity on something.  This is why you would 'weigh' less on the moon, because the moon has less of a pull of gravity than the earth.  It is measured in newtons (N).