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 & ITS USES

1. SODA 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 colorless, 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 colors 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 colors are not mixed and that the cullet is free from impurities, especially metals and ceramics.

Uses

It is the cheapest & most common glass. It is prepared by fusing soda ash, sand, and limestone. It is also called soft glass. it fuses at comparatively low temperatures. The major disadvantage of using this glass is that it is brittle & breaks easily. It cracks when subjected to sudden changes of temperature. Soda glass is used for the manufacture of window glass, mirrors, common glassware etc. it is easily attacked by chemicals.

Disadvantage

The disadvantages of soda-lime glass are 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.

2. HARD GLASS:

it is obtained by fusing potassium carbonate & limestone. It is used for making hard glass apparatus. It is more resistant to the action of acids.

3. LEAD CRYSTAL 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.

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

Uses

it is used for making expensive glass ware. The surface of lead glass objects is often cut into decorative patterns to reflect light. Cut glass show extraordinary sparkle.

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

Chemical Resistance

Borosilicate glass is inert to almost all materials with the exception of hydrofluoric acid, hot phosphoric acid and hot alkalies. Of these, hydrofluoric acid has the most serious effect and, even when a solution contains a few parts per million, attack will occur.

Phosphoric acid and caustic solutions cause no problems when cold but at elevated temperatures corrosion occurs. Caustic solutions up to 30% concentration can be handled safely at ambient temperatures.

Physical Properties :

Composition

Low-expansion borosilicate glass has the following approximate chemical composition:

SiO2		81%
Na2O		4.0%
K2O		0.5
B2O3		13.0%
Al2O3		2.0%

Linear Coefficient of Expansion:

Between 32°F and 572°F [0°C and 300°C], per ASTM Method E 228)
18.1 x 10-7 in/in/7°F
32.5x10-7 cm/cm/°C

Annealing:

All fittings and all straight lengths are annealed to reduce internal stress. This also makes the pipe easier to field fabricate.

Thermal Conductivity:

0.73 Btu/hr-ft2-°F/ft
0.0035 cal/sec-cm2-°C/cm

Specific Heat:

0.20 Btu/lb-°F
0.20 cal/gm-°C

Dialectric Constant:

at 23°C and 1M Hz per ASTM Method D 150: 4.6 ±-0.2

Density:

Approximately 139 lb/ft3 (2.23 gm/cm3)

Young's Modulus:

per ASTM Method C215: in the range of 9 x 106 to 10 x 106 psi.

Mechanical Strength :

The mechanical properties of glass differ from those of metals. The lack of ductility of glass prevents the equalization of stresses at local irregularities or flaws and the breaking strength varies considerably about a mean value. This latter is commonly found to occur at a tensile strength of about 70 kg/cm2 (1000 psi). The glass should be adequately supported and appropriate allowance should be made for special conditions such as high temperatures, dense liquids, etc. Subject to the above, maximum working pressures are as specified in the following table.

Working Temperatures

Borosilicate glass retains its mechanical strength and will deform only at temperatures which approach its strain point. The practical upper limit for operating temperatures is much lower and is controlled by the temperature ditterentials in the glass, which depend on the relative temperatures of the contents of the equipment and the external surroundings.

Provided borosilicate glass is not subjected to rapid change in temperature, creating undue thermal shock, it can be operated safely at temperatures up to 450°F (232°C). The normal limiting factor is actually the gasket material. The degree of thermal shock (usually defined as sudden chilling) which it can withstand depends on many factors, for example: stresses due to operating conditions; stresses imposed in supporting the equipment; the wall thickness of the glass, etc. It is therefore undesirable to give an overall figure but, as a general guide, sudden temperature changes of up to about 216°F (120°C) can be accommodated at sub-zero temperatures, the tensile strength of borosilicate glass tends to increase and equipment can be used with safety at cryogenic temperatures.

Uses

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 fibers for textile and plastic reinforcement.

5. OPTICAL GLASS:

Optical glasses will be found in scientific instruments, microscopes, and 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 colors of the rainbow. Glass makers use the variations in these characteristics to develop optical glasses.

Uses

it is specially made so as to be free of strains & defects. It is used for making lenses for spectacles, microscopes, cameras, telescopes & other optical instruments.

6. PHOTOCHROMIC GLASS:

This is actually a complex process so buckle up, there's some serious science ahead!

Photo chromic glass is a form of photosensitive glass, which is the name for a family of glasses whose optical properties (namely transparency) changes with incident light.

Photochromitism starts with a glass containing silver and some halide. Small silver halide crystals ( ~10 nm) form in the glass during initial processing by heat treatment at temperatures near the glass softening point. When this glass is exposed to high energy light (UV light) those tiny crystals facilitae the formation of non-linear silver specks. This process causes a darkening of the glass through colloidal particle interference and reverses when the glass is no longer exposed to the UV source.

Some photo chromic glasses utilize copper cadmium halides for a green color rather than the grey or brown associated with silver.

7. SAFETY GLASS:

It is prepared by placing a layer of transparent plastic between the two layers of glass by means of suitable adhesive. The 3 layers are joined together by the action of heat & pressure. The glass does not break easily under ordinary impact, & that is why is known as safety glass. It is used in making wind screens of aero planes, automobiles, bulletproof glass etc.

8. GLASS FIBRES:

Glass fiber 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 fiber 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 fibers which can guide light and thus transmit images round corners. These fibers 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.

it is form of fibers, has varied applications in most of the industries . Glass wool is a bundle of loose glass fibers, which is an excellent heat insulator.

Uses

it is used as insulating material in refrigerators, ovens etc.another important class of glass fiber is optical fibers which are extensively used in telecommunication, surgical operation etc. great progress has been made in making optical fibers, which can guide light, & thus transmit images round corners. The fibers are used in endoscopy to examine the internal human organs. They are also made in telecommunications & are much more efficient than copper cables.

9. COLOURED GLASS:

These are obtained by adding certain coloring material, such as metallic oxides, to the molten mass. Different additions may produce different colored glasses.

10. OTHER GLASS:

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 750°C.

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 450-550°C, 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 colors 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.

References:

www.indianofficer.com
www.lenntech.com
www.britglass.org.uk
www.pegasus-glass.com
answers.yahoo.com
www.wrap.org.uk
www.crystalartusa.com
www.oharacorp.com

Editorial Team, Mindfiesta