About Glass

History of Glass

A Brief History of glass

From our earliest origins, man has been making use of glass. Historians have discovered that obsidian - natural glass made within the mouth of a volcano when the intense heat of an eruption melts sand - was first used by man as tips for spears.

The oldest examples of glass were in the form of Egyptian beads, dating from 12,000 BC. It was not until 1500 BC that the first hollow glass container was made by covering a sand core with a layer of molten glass.

Glass blowing became the most common way to make glass containers from the First Century BC. However, the glass made during this time was highly coloured due to the impurities of the raw material. It was not until the First Century AD when colourless glass was produced and then coloured by the addition of colouring materials.

The secret of glass making came to Britain with the Romans. However, the skills and technology required to make glass were closely guarded by the Romans and it was not until the Roman Empire disintegrated that skills for glass making spread throughout Europe and the Middle East.

The Venetians, in particular, gained a reputation for technical skill and artistic ability in the making of glass bottles and a fair number of the city's craftsmen left Italy to set up glassworks throughout Europe.

In Britain, there is evidence of a glass industry round Jarrow and Wearmouth dating back to 680AD, while from the 13th Century, there is evidence of there having been a glass industry in the Weald and the afforested area of Surrey and Sussex around Chiddingford.

A major milestone in the history of glass occurred with the invention of lead crystal glass by George Ravenscroft. He attempted to counter the effect of clouding that sometimes occurred in blown glass by introducing lead to the raw materials used in the process.

The new glass he created was softer and easier to decorate and had a higher refractive index, adding to its brilliance and beauty, and it proved invaluable to the optical industry. It's thanks to Ravenscroft's invention that optical lenses, astronomical telescopes, microscopes and the like became possible.

The modern glass industry only really started to develop in Britain after the repeal of the Excise Act in 1845 relieved the heavy taxation that had been enforced. Before that time, excise duties were placed on the amount of glass melted in a glasshouse and levied continuously from 1745 to 1845.

Joseph Paxton's Crystal Palace at the Great Exhibition of 1851 marked the beginning of the discovery of glass as a building material. The revolutionary new building encouraged the use of glass in public, domestic and horticultural architecture. Glass manufacturing techniques also improved with the advancement of science and better technology.

By 1887 glass making developed from traditional mouth blowing to a semi-automatic process when Ashley introduced a machine capable of producing 200 bottles per hour in Castleford, Yorkshire - more than three times quicker than the previous production methods.

Twenty years later, in 1907, the first fully automated machine was developed in America by Michael Owens from major glass manufacturers Owens of Illinois, and used at its factory in Manchester, Illinois making 2,500 bottles per hour.

Other developments followed rapidly, but it was not until the First World War, when Britain became cut off from essential glass suppliers that glass became part of the scientific sector. Up until then glass was seen as a craft rather than a precise science.

Today, glass making is a modern, hi-tech industry operating in a fiercely competitive global market where quality, design and service levels are critical to maintaining market share.

Modern glass plants are capable of making millions of glass containers a day in many different colours, but green, brown and clear remain the most popular.

Few of us can imagine modern life without glass. It features in almost every aspect of our lives - in our homes, our cars and whenever we sit down to eat or drink. Glass packaging is used for many products, wines, spirits and beers all come in glass as do medicines and cosmetics not to mention numerous foodstuffs.

With increasing consumer concern for the environment, glass has again come into its own proving to be an ideal material for recycling. Glass recycling is good news for the environment. It saves used glass containers being sent to landfill and less energy is needed to melt recycled glass than to melt down raw materials, thus saving energy. Recycling also reduces the need for raw materials to be quarried thus saving precious resources.

 

Types of Glass

There are many different types of glass with different chemical and physical properties. Each can be made by a suitable adjustment to chemical compositions, but the main types of glass are:

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. Types of special glass include:

 

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 1700oC. 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 800oC. 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.

Most commercial glasses have roughly similar chemical compositions of:

70% - 74% SiO2 (silica)
12% - 16% Na2O (sodium oxide)
5% - 11% CaO (calcium oxide)
1% - 3% MgO (magnesium oxide)
1% - 3% Al2O3 (aluminium oxide)

Flat glass is similar in composition to container glass except that it contains a higher proportion of magnesium oxide.

Within these limits the composition is varied to suit a particular product and production method. The raw materials are carefully weighed and thoroughly mixed, as consistency of composition is of utmost importance in making glass.

Nowadays recycled glass from bottle banks or kerbside collections, known as cullet, is used to make new glass. Using cullet has many environmental benefits, it preserves the countryside by reducing quarrying, and because cullet melts more easily, it saves energy and reduces emissions.

Almost any proportion of cullet can be added to the mix (known as batch), provided it is in the right condition, and green glass made from batch containing 85% to 90% of cullet is now common.

Although the recycled glass may come from manufacturers around the world, it can be used by any glassmaker, as container glass compositions are very similar. It is, however, important that glass colours are not mixed and that the cullet is free from impurities, especially metals and ceramics.

 

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.

 


Borosilicate Glass

Most of us are more familiar with this type of glass in the form of ovenware and other heat-resisting ware, better known under the trade name Pyrex.

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.

 

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.

 

Vitreous Silica

Silica glass or vitreous silica is of considerable technical importance as it has a very low thermal expansion. This difficult to make glass contains tiny holes created using acids and is used for filtration. Porous glasses of this kind are commonly known as Vycor.

 

Aluminosilicate Glass
A small, but important type of glass, aluminosilicate, contains 20% aluminium oxide (alumina-Al2O3) often including calcium oxide, magnesium oxide and boric oxide in relatively small amounts, but with only very small amounts of soda or potash. It is able to withstand high temperatures and thermal shock and is typically used in combustion tubes, gauge glasses for high-pressure steam boilers, and in halogen-tungsten lamps capable of operating at temperature as high as 750oC.

 

Alkali-barium Silicate Glass

Without this type of glass watching TV would be very dangerous. A television produces X-rays that must be absorbed, otherwise they could in the long run cause health problems. The X-rays are absorbed by glass with minimum amounts of heavy oxides (lead, barium or strontium). Lead glass is commonly used for the funnel and neck of the TV tube, while glass containing barium is used for the screen.

 

Technical Glass

Technical is the term given to a range of glasses used in the electronics industry.

Without borate glass the computer revolution would not have been possible as it's vitally important in producing electrical components. This type of glass, contains little or no silica and is used for soldering glass, metals or ceramics as it melts at the relatively low temperature of 450-550oC, well below that of normal glass, ceramics and many metals.

Glass of a slightly different composition is used for protecting silicon semi-conductor components against chemical attack and mechanical damage. Known as passivation glass it is vital in microelectronics technology and the production of the silicon chips inside computers.

Another type of glass - Phosphate Glass - which is a semi conductor, is used in the construction of secondary electron multipliers.

Chalcogenide glass - Similar semi conductor effects are also characteristic of a type of glass that can be made without the presence of oxygen. Some of them have potential use as infrared transmitting materials and as switching devices in computer memories because their conductivity changes abruptly when particular threshold voltage values are exceeded.

 

Glass Ceramics

Some of these "Glass ceramics", formed typically from lithium aluminosilicate glass, are extremely resistant to thermal shock and have found several applications where this property is important, including cooker hobs, cooking ware, windows for gas or coal fires, mirror substrates for astronomical telescopes and missile nose cones.

An essential feature of glass is that it does not contain crystals. However, by deliberately stimulating crystal growth in glass it is possible to produce a type of glass with a controlled amount of crystallisation that can combine many of the best features of ceramics and glass.

 

Optical Glass
Optical glasses will be found in scientific instruments, microscopes, fighter aircraft and most commonly in spectacles.

The most important properties are the refractive index and the dispersion. The index is a measure of how much the glass bends light. The dispersion is a measure of the way the glass splits white light into the colours of the rainbow. Glass makers use the variations in these characteristics to develop optical glasses.

 

Sealing Glass

A wide variety of glass compositions are  used to seal metals for electrical and electronic components. Here the available glasses may be grouped according to their thermal expansion which must be matched with the thermal expansions of the respective metals so that sealing is possible without excessive strain being induced by differing levels of expansion.

For sealing to tungsten, in making incandescent and discharge lamps, borosilicate alkaline earths-aluminous silicate glasses are suitable. Sodium borosilicate glasses may be used for sealing to molybdenum and the iron-nickel-cobalt (Fernico) alloys are frequently employed as a substitute, the amount of sodium oxide permissible depending on the degree of electrical resistance required. With glasses designed to seal to Kovar alloy, relatively high contents of boric oxide (approximately 20%) are needed to keep the transformation temperature low and usually the preferred alkali is potassium oxide so as to ensure high electrical insulation.

Where the requirement for electrical insulation is paramount, as in many types of vacuum tube and for the encapsulation of diodes, a variety of lead glasses (typical containing between 30% and 60% lead oxide) can be used.

 

Glass Forming

THE GLASS FORMING PROCESSES

Like treacle, glass is fluid at high temperature and its fluidity decreases as the temperature is reduced. Unlike water, glass has no specific melting or freezing point but is gradually changed from a solid to a liquid as the temperature is increased. It is this property of 'variable viscosity', which is used in forming a mass of glass into articles of beauty or utility.

 

Glass Blowing

For nearly 2,000 years glass blowing by hand was the main method of forming glass articles. The last few years of the 19th century saw the beginnings of blowing glass by compressed air and the 20th century brought in the revolution of mechanisation. Today glass blowing is still carried out by the craftsmen of Tai-Ocean.