The very first glass making occurred naturally in the form of obsidian formed during volcanic eruption.
Man made glass has been around since at least 5,000 BC when it was discovered by accident when the heat from fires melted the basic ingredients of glass together. It was not until about 3,500 BC that it was used, firstly for glazing clay pots and then for making objects such as vases. Various objects using different production methods were made throughout the centuries until the beginning of the 20th Century when modern, mass production techniques were developed.
This page gives an introduction to modern glass making techniques. The process may have changed in recent years but the principals have not changed since glass making began 5,000 years ago.
There are a few main ingredients used in glass making plus many additives for particular properties.
1. Sand or Silica
The main ingredient of glass making is Silica, which as a very high melting point of over 2,000 deg C. It is possible to make highly specialised products from just silica such as halogen lamps or one of the materials in fibre optic cables. This is because it has such a high melting point and is stronger than normal glass.
2. Sodium Carbonate
Sodium Carbonate lowers the melting point of Silica to about 1,000 deg C and is therefore added to make the process more efficient. The Sodium Carbonate will, however, cause the finished glass to be water soluble which is not desirable in glass making. Sodium Carbonate was originally found in the ash of certain plants – soda ash – but is now commonly produced from table salt.
3. Lime or Calcium Oxide
Calcium Oxide is extracted from limestone and counters the effects of the Sodium Carbonate making the glass non-soluble in water. Magnesium Oxide and Aluminium Oxide can also be used to enhance the properties of the glass.
These ingredients, along with other ingredients, are carefully weighed and mixed together to form ‘batch’. The batch is placed in a furnace with cullet (recycled glass) where it melts and can be used for the production of glass products.
4. Other Additives
A number of different ingredients can be added to the glass to change the properties of the finished product, for example:
Lead – to make leaded or crystal glasses. The lead has better reflective properties and therefore the glass seems to ‘sparkle’. This kind of glass also lends itself to be cut to form decorative patterns on the glass.
Boron – this changes the thermal and electrical properties of the glass and is used to make Pyrex glassware which can withstand extremes of heat and cold.
Lanthanum Oxide – this has excellent light reflective properties and is used to make high quality lenses in glasses. Thorium Oxide was formerly used but due to its radioactivity this is no longer the case.
Iron – used to absorb infrared energy in, for example, heat absorbing filters in movie projectors.
5. Colour Additives A range of additives can be used to make glass into different colours. Although glass appears to be clear it is actually green. It is extremely difficult to make completely clear glass. Metals and oxides can be used to colour glass.
Additive(s) Colour Comments
Iron Oxide Blue-green Used for beer bottles
Iron Oxide & Chromium Richer green Used for wine bottles
Sulphur, carbon & iron salts amber, yellow Can be almost black
Manganese Removes green Can give purple colour
Selenium Red or Pink Copper and gold both produce similar colours
Cobalt Blue Very small amounts required
Tin oxide, antimony & arsenic White opaque Used to mimic porcelain
Copper Oxide Turquoise Nickel Blue, violet or black Chromium Dark green Titanium Yellow / brown Uranium fluorescent yellow or green Not radioactive in glass but Can be if ground down.
Silver Orange or red The way the glass is cooled affects the colour
In the 1950’s, Alastair Pilkington developed a new, patented system of making large, good quality sheets of flat glass for windows and glass sheets. Flat glass making had only been achieved with limited success up until then and was expensive. The new Pilkington method allowed glass to be made cheaply in bulk and to a good standard.
Molten glass is drawn on to molten metal and floats along the metal until cooled and cut – thus its name, float glass. This is achieved in a continuous process using machinery which can be up to 500m long. The machines are never switched off throughout their life span of about 10 – 15 years and produce up to 6,000 of glass per week – that’s about 6,000 km per year. It is such a specialised operation that there are only about 260 plant worldwide.
The process is achieved in 6 stages:
1. Melting & Refining
Batch and cullet (Recycled Glass)are heated in the furnace to about 1,500 deg C. There are different areas of the furnace through which the glass flows. Impurities and bubbles are removed and the molten glass is cooled to 1,100 deg C by the time it is ready to move on. This process is key to producing high quality glass and can last up to 50 hours. It is also where the glass is mixed with different ingredients to produce different finished products.
2. The Float Bath
The molten glass from the furnace flows on to mirror like molten tin and forms a ribbon about 6.8mm thick. It flows over the tin and is cooled to 600 deg C when it leaves. Thicker glass up to 25mm is produced by not allowing it to settle at 6.8mm. The ribbon is stretched to produce thinner glass.
Many coatings can be applied to the glass to make it suitable for a range of products during this stage of the process. These are normally applied via chemical vapour and more than one coating can be added. These coatings can change the optical and absorption qualities of the glass.
The cooling process is carefully controlled in a part called a lehr – the glass is under stress when it cools and care is taken to prevent it from breaking.
The glass is automatically checked by lasers throughout the process at every stage. Action can be taken during processing to correct a blemish or cutters can avoid the area.
The final stage of glass making is the cutting. Diamond wheel cutters are used to cut the glass made to the required size. Customer requirements are fed into the computer system which works out the best way to cust the glass to avoid waste.
Jars and bottles are made from one of two similar processes known as Blow & Blow or Press & Blow. The process used to make them is known as the IS process.
PRESS & BLOW METHOD
All containers are made in a 2-step process. Firstly a ‘parison’ or pre-form is made. This is then put into a mould and blown so that it is pushed against the surface of the mould to form the finished shape.
In the press and blow method, the parison is formed by pressing molten glass called gobs into a mould. This is then cooled so that it keeps its shape whilst being transferred to another mould. Heat and compressed air or a vacuum causes the pre-form to push against the sides of the new mould, this time in the shape of the finished bottle.
The bottles are reheated again before the glass is cooled in a lehr. This final re-heating removes the stress caused by the blowing process and prevents the bottle from breaking.
BLOW & BLOW METHOD
This time, the pre-form is made in 2 steps. The molten gobs of glass are firstly poured into a mould and the neck formed. The rest of the parison is formed by blowing the liquid to the edges of the mould. This is then removed and put into another mould to be blown into the finished shape as with the Press & Blow method.
Modern machinery makes it possible to produce multiple containers simultaneously to so that more bottles can be made – up to 600 bottles per minute is possible.
Glasses and light bulbs are made in a Westlake machine. The machine was originally designed to make light bulbs but can be adapted for drinking glass making. In some respects, the method is similar to making containers and automates the traditional glass-blowing techniques.
Molten glass is gathered by vacuum and fed into moulds to make pre-forms or parisons. The parison is formed when air is blown into the mould and pushes the molten glass to the edge. This is transferred to another mould where it is blown into the final shape. This mould I kept wet with water which forms a cushion of steam so that the glass does not actually touch the sides of the mould. The neck is finished by stretching it to the required size and shape. Excess glass formed at the gathering stage is burnt off. The glass is finally cooled in a part of the machine called a lehr which is an important part of glass making since the glass can easily break if this is not controlled.
The modern machines effect the glass making rate achievable, but this can be up to 75,000 light bulbs and 55,000 drinking glasses can be produced per day.
Drinking glasses and other objects such as glass bowls and vases were originally made by hand blowing the glass into a mould. This glass making method is still in use today and is a highly skilled craft producing high quality and more unique products.
A hollow blowing-iron or pipe is dipped into the molten glass which is rotated and ‘gathers’ the glass. This is then rolled on an iron slab to form the parison. It is further manipulated by reheating and blowing so that it forms the rough shape of the mould. This is then transferred to the final mould which is wet with water. The water forms a steam cushion so that the glass does not actually tough the mould. The craftsman blows air into the pipe to blow the parison into shape whilst continuously rotating the pipe to prevent blemishes from forming.
These are the main methods of modern production of glass.