Glass Basics
1. What is Glass?
Following ASTM (US Standard) and DIN (German Standard) definition, glass is an inorganic product of fusion which has been cooled to a rigid condition without crystallizing.
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.
Chart 1: Structure of Glass
2. Types of Glass
A large variety of glass with different chemical and physical properties can be made by a suitable adjustment to chemical compositions. Glasses vary widely in their chemical make-up; indeed, there are very few element in the periodic table that have not been incorporated in a glass of some kind. However, most of the glasses produced commercially on a large scale may be classified into three main groups: soda-lime, lead and borosilicate, of which the first is by far the most common.
2.1 Commercial Glasses
The main constituent of practically all commercial glasses 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 chemicals to sand can considerably reduce the temperature of fusion. The addition of sodium carbonate (Na2CO3), known as soda ash, in a quantity to produce a fused 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. The raw materials used for introducing CaO and MgO are their carbonates CaCO3 (limestone) and MgCO3 (dolomite), which when subjected to high temperatures give off carbon dioxide leaving the oxides in the glass.
Most commercial glasses whether for containers, i.e. bottles and jars, flat glass for windows or for drinking glasses, have somewhat 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)
Chart 2: Composition of Soda-Lime Glass
Within these very wide limits the composition is varied to suit a particular products and production method. The raw materials are carefully weighed and thoroughly mixed, as consistency of composition is of utmost importance. To the mixture of chemicals a further raw materials added – broken glass, called cullet. Cullet can come from factory rejects, it can be collected by the public in Bottle Banks or from the bottling industry. 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 95% of cullet is by no means uncommon. Although the glass collected by Bottle Banks may come from several manufacturer, it can be used by one of them, as container glass compositions have been harmonised to make this possible. It is, however, important that glass colours are not mixed and that the cullet is free from impurities, especially metals and ceramics.
Flat glass is similar in composition to container glass except that it contains a higher proportion of magnesium oxide.
Soda-lime glasses
These are the most common commercial glasses and have been described. The chemical and physical properties of soda-lime glasses make them suitable for a visible light and hence applications. The nominally colourless types transmit a very high percentage of visible light and hence have been used for windows since at least the time of the Romans. 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.
One of the main disadvantages of soda-lime is their relatively high thermal expansion. Silica does not expand very greatly when heated but the addition of soda has a dramatic effect in increasing the expansion rate and, in general, the higher the soda content of a glass, the poorer will be its resistance to sudden changes of temperature (thermal shock). Thus, care is needed when soda-lime containers are filled with hot liquids to prevent breakages due to rapid thermal expansion.
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 relatively soft surface so that they are easy to decorate by grinding, cutting, engraving. The overall effect of cut crystal is the brilliance of the two.
Glasses with even higher lead oxide contents (typically 65%) may be used as radiation shielding glasses because f the well-known ability of lead to absorb gamma rays and other forms of harmful radiation.
Borosilicate glasses
As the name implies, borosilicate glasses, the third major group, are composed mainly of silica (70-80%) and boric oxide (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 permanently 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, possibly better known under the trade name of the first glass of this type to be placed on the consumer market- Pyrex.
2.2 Other types of glasses
Special glasses
Glasses with specific properties may be devised to meet almost any imaginable requirement, the main restriction normally being the commercial considerations, i.e., whether the potential market is large enough to justify the development and manufacturing costs. 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.
Vitreous silica
As mentioned previously, silica glass or vitreous silica is of considerable technical importance. However, the fact that temperature above 1500°C are necessary in the melting makes the transparent variety (often known as fused quartz or quartz glass) expensive and difficult to produce. The less expensive alternative for many applications is fused silica, which is melted at somewhat lower temperatures; in this case small gas bubbles remain in the final product which is therefore not transparent.
Another substitute for vitreous silica can be produced by melting a suitable borosilicate glass and then heating it at around 600°C until it separates into two phases. The alkali-borate phase may be leached out with acids, leaving a 96% silica phase with open pores of controllable size which can be converted into clear glass. Porous glasses of this kind, commonly known as Vycor, from the first commercial version produced by Corning Glass Works Ltd, may be used as membranes for filtration purposes and for certain biological applications.
Aluminosilicate glasses
A small, but important group of glasses is that known as aluminosilicate, containing some 20% aluminium oxide (Al2O3) often including calcium oxide, magnesium oxide and boric oxide in relatively small amounts, but with only very small amounts of soda or potash. They tend to require higher melting temperatures than borosilicate glasses and are difficult to work, but have the merit of being able to withstand high temperatures and having good resistance to thermal shock. Typical applications include 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 glasses
In normal operation, a television produces X-rays which need to be absorbed by the various glass components. This protection is afforded by glasses with minimum amounts of heavy oxides (lead, barium or strontium). Lead glasses are commonly used for the funnel and neck of the tube, while glasses containing barium are usually employed for the face or panel.
Borate glasses
There is a range of glasses, containing little or no silica, that can be used for soldering glasses, metals or ceramics at relatively low temperatures. When used to solder other glasses, the solder glass needs to be fluid at temperatures (450°C – 550°C) well below that at which the glass to be sealed will deform.
Some solder glasses do not crystallise or denitrify during the soldering process and thus the mating surfaces can be reset or separated; these are usually lead borate glasses containing 60-90% PbO with relatively small amounts of silica and alumina to improve the chemical durability. Another group consists of glasses that are converted partly into crystalline materials when the soldering temperature is reached, in which case the joints can be separated only by dissolving the layer of solder by chemical means. Such denitrifying solder glasses are characterised by continuing up to about 25% zinc oxide.
Glasses of a slightly different composition (zinc-silicoborate glasses) may also be used for protecting silicon semi-conductor components against chemical attack and mechanical damage. Such glasses must contain no alkalis (which can influence the semi-conducting properties of the silicon) and should be compatible with silicon in terms of thermal expansion. These materials, known as passivation glasses, have assumed considerable importance with the progress made in microelectronics technology in recent years that has made the concept of the “silicon chip” familiar to all.
Phosphate glasses
Most types of glass are good insulators at room temperature, although those with a substantial alkali content may well be good conductors in the molten state. This is because the conductivity depends mainly on the ability of the alkali ions in the glass to migrate in an electric field. However, some glasses that do not contain alkalis conduct electrons which jump from one ion to another. These are known as semi-conducing oxide glasses and are used particularly in the construction of secondary electron multipliers. Typically they consist of mixtures of vanadium pentoxide (V2O5) and phosphorous pentoxide (P2O5).
Chalcogenide glasses
Similar semi conductor effects are also characteristic of a series of glasses which can be made without the presence of oxygen (non-oxide glasses). These may be composed of one or more elements of the sulphur group in the Periodic Table (called chalcogens, from the Greek word for sulphur) combined with arsenic, antimony, germanium and/or the halide (fluorine, chlorine, bromine, iodine). Some of them have potential use as infra-red transmitting materials and as switching devices in computer memories because their conductivity changes abruptly when particular threshold voltage values are exceeded, but most have extremely low softening points and much poorer chemical durability than more conventional glasses.
Glass Ceramics
An essential feature of glass structure is that it does not contain crystals. However, by deliberately stimulating crystal growth in appropriate glasses it is possible to produce a range of materials with a controlled amount of crystallisation so that they can combine many of the best features of ceramics and glass. Some of these “glass ceramics” formed typically from lithium aluminosilicate glasses, are extremely resistant to thermal stock and have found several applications where this property if important, including cooker hobs, cooking ware, windows for gas or coal fires, mirror substrates for astronomical telescopes and missile nose cones.
Some special applications of glass
Different forms and varieties of glass are used in almost every conceivable aspect of human life. Architecture, food and drink, laboratory equipment, instrumentation, the chemical, nuclear and electrical industries, lighting, optics – the list is endless. For some areas of application, one type of glass predominates: for example, soda-lime glass is used almost universally in the building and packaging industries while borosilicate tends to be standard in the chemical processing industry. However, for some purposes a wide range of glasses is required to meet different requirements, as is the case with optical glass, glasses for sealing to metals and glass fibres.
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.
The substitution of other oxides permits considerable variations to be achieved. Thus barium crown (barium borosilicate), barium flint (barium lead silicate), borosilicate crown (sodium borosilicate) and crown flint (calcium lead-silicate) are all widely used. Phosphorous and the rare earths, especially lanthanum, may also be valuable ingredients in some optical glass compositions. The inclusion of transition elements (copper, titanium, vanadium, chromium, manganese, iron, cobalt or nickel) in glass produces strong absorption bands in the ultra violet part of the spectrum as well as broad bands in the visible and infra-red, enabling a series of colour filters and glasses with modified transmission properties in the ultra-violet and infra-red to be produced.
The use of rare earth’s has less effect on colour but it is of particular significance in the manufacture of laser glasses, most of which contain neodymium. The neodymium ions in the glass, when stimulated, emit radiation at a particular wavelength (1.06um) and this is transformed into high-intensity coherent optical data, and for various measurement functions in industry.
A characteristic of some optical glasses is that when they are exposed to ultraviolet or short-wave infra-red radiation (as with sunlight) they become dark, but when removed from such exposure they revert to their original state. These, known as photochromic glasses, include in their composition silver halide crystals produced by adding silver salts and compounds of fluoride, chlorine or bromine (the halides) to the base-glass (normally borosilicate). Controlled thermal treatment during and after melting causes extremely small phase separations to occur and these are responsible for the reversible darkening effect.
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 expansion 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.
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.
