SSRsi Note: The spelling is British. I'll get around to Americanizing it one of these days....
Making Glass| Product | Specific Qualities | Method of Manufacture | Typical Formula | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Glass Containers Bottles & Jars | Relatively cheap when mass produced. Resistant to mechanical shock, capable of being filled at very fast rates. Some bottling plants fill in excess of 1000 bottles per minute. Can be re-used and recycled. Can be sterilized at high temperatures. inert, do not impart taste or toxic substances. |
Automatically blown at high speeds. |
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| Flat Glass |
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Float process. Cast and rolled. |
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| Domestic Glassware for everyday use in home and catering |
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Mouth blown, pressed or fully automatic mass produced. | Soda-lime silica Approximate composition:
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| Radiation Shielding | High density to absorb radiation. | Extrusion and casting can be ground and polished to optical precision. |
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| Thermometer Tubing | Thermal stability over a wide temperature range, retaining transparency. | Automatic or hand drawing. | Soda-lime silica Borosilicate Lead glass Depending on temperature range required |
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| Laboratory Glassware | High chemical durability. Low thermal expansion. |
Lampworking (made from tubing by
heating and skilful manipulation). Mouth and automatic blowing. Sintering. |
Mainly Borosilicate or fused silica for extra low expansion coefficient | ||||||||||||||||||
| Full Lead Crystal Domestic Glassware |
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Hand made by skilled craftsmen. |
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| Heat Resistant Oven to Table Ware |
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Automatically pressed or blown. | Borosilicate glass
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| Optical Glass |
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Extrusion and pressing, then ground and polishing. | While range of compositions Depends on application |
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| Electrical Components: cathode-ray tubes, capacitors and resistors, computer components, printed circuits | Good dielectric properties. Low electrical losses over a wide range of temperatures. High operating temperatures. |
Blowing. Drawing - in rod form and in sheets. Sintering and Pressing - glass is ground to fine grains and then is subsequently pressed into required shape and then fired. |
Wide range of compositions | ||||||||||||||||||
| Glass Building Blocks | Resistant to normal temperature
changes. Resistant to
atmospheric conditions. Mechanical strength. Attractive. Translucent. |
Automatic pressing - pressed in halves and then fused together |
Soda-lime silica glass Similar to flat glass |
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| Ballotini: minute glass spheres (1-60 microns) which reflect light | High reflective properties: mixed with paint for road signs and cinema screens. | flame drawing - velocity of flames draws particles of glass up tower and as the softened glass falls on the outside, spheres are formed by surface tension effects. |
Soda-lime silica glass Similar to flat glass |
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| Glass Fibre | High strength-to-weight ratio. Resistant to attack by corrosive substances. Resistant to high temperature. Flame resistant. High electrical resistance. |
Filament drawing. Continuous filament. White wool. Crown process. Can be woven into textiles or incorporated with plastics to form insulating materials, boat hulls, car bodies, etc. |
Soda lime silica and where
resistance to weathering is necessary, a borosilicate glass is used,
e.g. Soda-lime silica glass
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| Lighting Glassware 1. Electric Light Bulbs |
Economical to produce. Easy to manufacture by mass production methods. Resistant to shock. Impermeable and inert to gas, vapor and liquid. Durable. Transparent or translucent. |
Ribbon Machine - produces bulb at the rate of over 1,000 per minute. Blanks used in the manufacture of vacuum flasks are also produced by this machine |
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| 2. Special Glasses (a) High pressure mercury vapor lamps |
Low electrical conductivity. Resistance to intense chemical activity of mercury vapor. |
Special glasses can be formed by using manufacturing | Wide range of compositions | ||||||||||||||||||
| (b) Aircraft fire-warning sensors | Low melting point. | processes, or, in some cases, laminated onto ordinary glasses | |||||||||||||||||||
| (c) Glass for encapsulating electric components | High electricity conductivity. | i.e. sodium discharge lamps. | |||||||||||||||||||
| 3. Tubing for fluorescent lighting | Low electrical conductivity. Resistance to intense chemical activity. Electrical discharge generates UV light which then causes fluorescent powder to emit visible light. High efficiency. Long life: 3,000 - 5,000 hours, i.e. about one year of continuous use. |
Automatic Drawing |
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| 4. Domestic and industrial shades and bulkhead lights: inc. lenses for traffic lights, car lights and railway signal lights | Resistant to high temperature. Resistant to thermal shock. Resistant to weathering. Accurate and non fading color: subject to strict BS specifications. |
Mouth blowing. Hand and automatic pressing - depending on quantities required. |
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WHAT IS 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 crystallization, i.e., the molecules cannot arrange
themselves into a crystalline pattern. The fast rate of cooling to
prevent crystallization 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.
TYPES OF GLASSES
A large variety of glass with different chemical and physical properties
can be made by a suitable adjustment to chemical compositions. Further
sections of this booklet deal with various glasses, including crystal
and optical glasses of high refractive index and high lead content.
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 1700oC. 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 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. 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 (aluminum oxide) |
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 harmonized to make this possible.
It is, however, important that glass colors 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.
Other types of glasses
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.
Soda-lime glasses
These are the most common commercial glasses and have been described in
the previous chapter. The chemical and physical properties of soda-lime
glasses make them suitable for a visible light and hence applications.
The nominally colorless 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 aluminum
oxide. They are characterized 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 fibers 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.
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
specialized 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 1500oC 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 600oC 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% aluminum 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. 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 750oC.
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 (450o - 550oC) well below that at which the
glass to be sealed will deform.
Some solder glasses do not crystallize 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
characterized 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 crystallization 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 fibers.
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 color 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 color 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.
COLOURS
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 colorants 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 decolorize
the glass by adding minute amounts of other colorants which produce
complementary colors to green so that the finished articles appear
colorless. Thus selenium (which gives a pink color) 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 decolorizing of
lead crystal glass.
Different additions may produce different colored
glasses, the range of possible colors being almost infinite. Some of
the most frequently used colorants and the colors they produce are
listed below. The color depends on the state of oxidation of the
colorant, the type of glass in which it is used, and thermal treatment.
| COLORANT | GLASS COLOUR/S |
|---|---|
| Iron | Green, brown, blue |
| Manganese | Purple |
| Chromium | Green, yellow, pink |
| Vanadium | Green, blue, grey |
| Copper | Blue, green, red |
| Cobalt | Blue, green, pink |
| Nickel | Yellow, purple |
| Uranium | Yellow, brown, green |
| Titanium | Purple, brown |
| Neodymium | Purple |
| Praseodymium | Green |
| Cerium | Yellow |
| Carbon & Sulphur | Amber, brown |
| Cadmium Sulphide | Yellow |
| Antimony Sulphide | Red |
| Selenium | Pink, red |
| Gold | Red |
The use of large amounts of several different
colorants 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 devitirification.
GLASS MELTING FURNACES
There are two types of glass melting furnaces.
1. Pot Furnaces are structures built of
refractory materials in which there is no contact between the furnace
and the glass. Glass is melted in several pots made of refractory
materials which are resistant to glass attack at high temperatures. The
pots are charged with a batch, which is melted over a number of hours
and worked on a 24 or 18 hour cycle. An average pot can hold 600-700 Kg
of glass. Pot furnaces are used where the glass is formed by hand and
mouth blowing. One of the main advantages of this system is that several
types of glasses can be melted at the same time. A pot can be used for
about 30 melting cycles and thus produce between 18 and 21 tons of
glass.
Fuel economy is normally achieved by recuperation,
i.e., the pre-heating of combustion air by waste heat from the furnace
exhaust gases. In this system the pre-heating of the combustion air is
done by passing the air through metal tubes on the outside of which the
exhaust gases flow towards the chimney. Thus the heat exchange is
continuous. Electricity can also be used for melting.
2. Tank Furnaces are used where continuous
flow of glass is needed to feed automatic glass forming machines. They
are more economical in their use of fuel and are used mainly for the
large scale production of containers, flat glass, electric bulbs, tubing
and domestic machine made tableware. A large float glass furnace can
have a capacity of 2,000 tons.
A tank furnace consists of a bath, built of a very
special high refractory material, which can resist chemical attack of
molten glass at temperatures in excess of 1500oC and a
superstructure where combustion takes place. The quality of refractory
materials, used for building the bath, has improved to such an extent
that whereas some 30 years ago, the life of a furnace was well below 2
years, it is now over 9 years.
In order to achieve high melting temperatures and
fuel economy, a regenerative or recuperative system is used. Both these
systems utilize the waste heat of combustion for pre-heating the
incoming combustion air.
While in the recuperative system the heat exchange
between the combustion air and waste gases is continuous, in the
regenerative system the waste gases are passed through a large chamber
packed with refractory bricks arranged in a pattern which permits free
flow of the gases. The brickwork is heated by the waste gases and after
having been heated for some 20 minutes, the direction of firing is
reversed. Combustion air is passed through the chamber and the heat thus
collected in the brickwork is used for pre-heating the combustion air.
The firing is thus from right to left, normally for 20 minutes, during
which time the right hand generator is heated and so there is a reversal
of firing every 20 minutes. The cycle time can be changed for best heat
exchange results and modern furnaces have computer managed control
systems, which adjust the time of firing in each direction to achieve
the best heat exchange conditions.
Heavy fuel oil or natural gas is normally used for
firing tank furnaces. Glass, being an electrical conductor at high
temperature, can also be melted by electricity. However, electricity is
far too expensive in the UK and is normally used to boost the output
from a gas or oil fired furnace. Nevertheless, technological progress in
electric melting has enabled the use of all electric glass melting
furnaces even at the high cost of electricity.
GLASS FORMING PROCESSES
Like treacle and pitch, glass is fluid at high
temperature and its fluidity decreases at the temperature is reduced. In
other words its viscosity decreases as the temperature increases. Unlike
water, which turns from liquid to a solid at a specific temperature,
glass has no specific melting or freezing point but is gradually changed
from a stiff solid to a liquid mass as the temperature is increased. It
is this property of variable viscosity which is utilized in forming a
mass of glass into articles of beauty or utility.
Glass Blowing
For nearly 2,000 years mouth blowing 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 mechanization.
For mouth blowing, a hollow blowing-iron or
pipe is dipped into a pot containing molten glass and the glass is
gathered at the end of the pipe by rotating it, similarly to gathering
treacle onto a spoon. The collected glass, known as the gather, cools to
about 1000oC and is marvered (rolled on an iron slab) to form
a parison. The parison is then manipulated by allowing it to elongate,
re-heating it and blowing air into it to bring it into a shape which
resembles the final article to be formed. It is then placed in an iron
or wooden mould, which is kept wet by water, and the glass is blown to
the final shape of the interior of the mould. There is no contact
between the glass and the mould, due to the water being present, a
cushion of steam forms a barrier preventing this. During the blowing the
pipe is rotated continuously, thus preventing mould joints or other
mould imperfections appearing in the glass.
Semi Automatic Bottle Making
Until the second half of the 19th century bottles
were made by hand gathering, mouth blowing and finishing the neck, which
was to receive a closure, by manual manipulation with simple tools. The
mouth of the bottle, being made last, was known as the "finish".
No way was discovered of imitating this process
by semi-mechanical or mechanical means until it was realized that the
only way was to make the "finish" first.
The glass is thus gathered on a solid rod and is
allowed to flow into a parison mould until sufficient glass is judged to
have entered the mould. It is then cut off by means of hand shears. At
the bottom of the parison mould is the "finish" with a plunger, which
forms the opening into which compressed air is blown. A puff of
compressed air blows the glass upwards against the sides of the parison
mould and a plate at its top. Thus a parison, which is a thick walled
bottle vaguely resembling the final product, is formed. The parison is
removed from the mould and by absence of contact with the iron, the heat
from the outer surface is no longer conducted away and the parison
surface is re-heated. It is then transferred to the final mould and
blown again and the parison surface is re-heated. It is then transferred
to the final mould and blown again by compressed air to its final shape.
The mould is opened, the bottle is removed and placed in a re-heating
tunnel, called a lehr, for annealing.
Semi-automatic bottle making has practically
disappeared in the developed countries and has been replaced by fully
automatic production.
Making Glass Container by Semi-Automatic Process
[Picture Missing]
The gob is gathered by hand
on an iron, and the correct amount of glass is dropped into a
preliminary mould. Compressed air is introduced to form the neck of the
article. The embryo shape (parison) is then transferred to the finishing
mould in which the final shape is blown.
Automatic Container Production
The principle of automatic production is exactly
the same as that previously described, except that instead of gathering
the glass on an iron rod and allowing it to flow into the parison mould,
gobs of glass of pre-determined shape and weight are formed above the
parison mould and are allowed to drop into it.
Making Glass Containers by Automatic Process
The Press and Blow Process
[Picture Missing]
The Blow and Blow
Process
The machine which has almost replaced all others
is the IS machine. It is not certain whether "IS" denotes its inventors,
Ingle and Smith, or its main characteristic which is independent
synchronized units with a synchronized gob distribution system each
section.
It can consist of several sections and 10 section
machines are by no means uncommon. The machine can operate on blow and
blow or press and blow principle and double gob production, i.e.
delivery of two gobs of glass at the same time is quite common. Triple
gob machines are also in existence. The machine is capable of producing
more than 200 containers per minute.
Flat Glass
The main flat glass products are float glass for
high quality glazing in homes, offices, hotels, shops, transport and
public buildings glass for horticulture: wired glasses for fire
resistance; patterned glass for privacy and decoration; and a wide range
of glasses for environmental control and energy conservation.
Other flat glass products include toughened glass
doors, suspended window assemblies, cladding glasses for the exterior of
buildings, mirrors and diffuse reflection glass for reducing reflection
on glazed pictures and instrument dials.
The two manufacturing processes for producing flat
glass in the UK are the float glass process and the rolled process.
The Float Glass Process
The float glass process, invented by Pilkington
Brothers PLC and introduced in 1959, is now the principal method of
producing flat glass throughout the world.
The glass is held in a chemically controlled
atmosphere at a high enough temperature (1000 ºC) for a long enough time
for irregularities to melt out and for the surfaces to become flat and
parallel. Because the surface of the molten tin is flat the glass
becomes flat and the thickness of the ribbon, in the range 2.5 to 25mm,
is controlled at this stage. The ribbon is cooled down while still
advancing along the molten tin until the surfaces are hard enough (600
ºC) for it to be lifted onto the conveyor rollers without marking the
bottom surface. The ribbon passes through the annealing lehr to the
automatic warehouse where computers govern the cutting of the ribbon to
match customer's orders. A large modern float glass plant will produce
5000 tons of glass per week. It operates continuously 24 hours a day,
365 days a year for several years. The glass produced has a uniform
thickness and bright fire-polished surfaces without the need for
grinding and polishing.
The Rolled Glass Process
The rolling process is used for the manufacture of
patterned flat glass and wired glass. A continuous stream of molten
glass is poured between water cooled rollers.
Patterned glass is made in a single pass
process in which glass flows to the rollers at a temperature of about
1050 ºC. The bottom cast iron or stainless steel roller is engraved with
the negative of the pattern; the top roller is smooth. Thickness is
controlled by adjustment of the gap between the rollers. The ribbon
leaves the rollers at about 850 ºC and is supported over a series of
water cooled steel rollers to the annealing lehr. After annealing the
glass is cut to size.
Wired glass is made in a double pass
process. The process uses two independently driven pairs of water cooled
forming rollers each fed with a separate flow of molten glass from a
common melting furnace. The first pair of rollers produces a continuous
ribbon of glass, half the thickness of the end product. This is overlaid
with a wire mesh. A second feed of glass, to give a ribbon the same
thickness as the first, is then added and, with the wire mesh
"sandwiched", the ribbon passes through the second pair of rollers which
form the final ribbon of wired glass. After annealing, the ribbon is cut
by special cutting and snapping arrangements.
Glass Fibers
Glass in the form of fibers has found wide and
varied applications in all kinds of industry. Its composition depends on
the intended use.
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 (E glass) is preferred because
of its good chemical durability and high softening point. This is also
the type of composition employed for the fibers used in the
reinforcement of plastics, familiar for their application in protective
helmets, boats, piping, car chassis and many other articles.
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 applicable to endoscopes for examination
of internal human organs, changeable traffic message signs now in common
use on motorways for speed restriction warnings and communications
technology for transmitting telephone conversations much more
efficiently than copper cable.
There are two broad groups of glass fiber
products: continuous glass fiber which is used for the reinforcement of
plastics, rubber and cement; and glass wool, which is used for thermal
insulation and which is produced by the Crown process.
Glass Fiber Manufacture
Continuous glass fiber is a continuous strand,
made up of a large number of individual filaments of glass.
Molten glass is fed from the furnace or "tank"
through a channel or "forehearth" to a series of bushings which contain
over one thousand six hundred accurately dimensioned holes or "forming
tips" in its base.
A constant head of glass is maintained in the tank
and forehearth and the temperature of the glass in the bushings is
controlled to very fine limits. Fine filaments of glass are drawn
mechanically downwards from the bushing tips at a speed of several
thousand meters per minute, giving a filament diameter which may be as
small as nine microns, or one tenth the diameter of a human hair. From
the bushing the filaments run to a common collecting point where size is
applied and they are subsequently brought together as bundles, or
"strands", on a high speed winder.
Glass fiber is produced in a range of filament
diameters and strand dimensions to tight tolerances for different end
uses. It is used to strengthen and stiffen thermosetting plastics,
thermoplastics, nylon and polypropylene as well as inorganic matrices,
such as gypsum.
Glass Wool Manufacture
Glass wool is made in the Crown process. From the
forehearth of the "tank" a thick stream of glass flows by gravity from
the bushing into a rapidly rotating alloy steel dish "Crown" which has
several hundred fine holes round its periphery.
The molten glass is thrown out through the holes
by centrifugal force to form filaments which are further extended into
fine fibers by a high velocity blast of hot gas. After being sprayed
with a suitable bonding agent, the fibres are drawn by suction onto a
horizontally moving conveyor positioned below the rotating dish.
The mat of tangled fibers formed on the conveyor
is carried through an oven which cures the bonding agent, then to
trimmers and guillotines which cut the product to size. The mat may be
further processed into rigid sections for pipe insulation. The mats are
made into many products for heat and sound insulation in buildings,
transport vehicles and domestic appliances.
Optical Fiber Manufacture
Communications are increasingly based on eletro-optic
systems in which telephones, television and computers are linked by
fiber optic cables which carry information by laser light.
Making glass optical fibers is a highly specialized aspect of glass manufacture. Optical
fibers consist of two
distinct glasses, core of highly refracting glass surrounded by a sheath
of glass with lower refractive index between the two glasses, it is
guided by total reflection at the core-sheath interface to the other end
of the fiber. In theory, a wide range of glasses can be used as long as
the difference in refractive index is appropriate but the higher the
refractive index of the core relative to that of the sheath glass, the
greater the carrying capacity of the fiber. A typical system available
commercially comprises a germanium doped silica core and a borosilicate
cladding.
The aim in manufacture is to produce a fiber of
glass which is so pure and free form defects that light inserted at one
end will emerge at the other end a distance of 1 kilometer or more away.
There are many manufacturing processes being used to produce cored fiber;
two of these will illustrate the principles. All the processes require
ultra-pure starting materials.
Chemical vapor deposition - high silica
glass fibers are prepared by chemical vapor deposition in which layers
of SiO, are deposited to make a preform, either on the outside of a
mould or on the inside of a fused silica tube. The layers are doped
during the deposition to control the refractive index. The preform is
then drawn to a rod and subsequently to a fiber of 100-125mm diameter.
The surface is protected from damage by a plastic coating.
[Picture Missing]
The double
crucible method - The double crucible uses purified glasses in
separate crucibles in a controlled atmosphere furnace. Fiber drawn from
the tip consists of a uniform core drawn from the central crucible and a
cladding drawn from the outer crucible.
[Picture Missing]
Tubing
Manufacture of Tubing Danner Process
The Danner Process was developed for the
continuous production of glass tubing and rod. Subject to equipment
design the process can make tubing of 1.6mm to 66.5mm diameter and rods
of 2.0mm to 20mm diameter at drawing rates of up to 400m a minute for
the smaller sizes.
Glass flows from a furnace forehearth in the form
of a ribbon which falls on to the upper end of an inclined refractory
sleeve carried on a rotating hollow shaft or blowpipe. The ribbon is
wrapped around the sleeve to form a smooth layer of glass which flows
down the sleeve and over the tip of the shaft. Tubing is formed by
blowing air through a blowpipe with a hollow tip and rods are made by
using a solid tip on the shaft.
The tubing is then drawn over a line of support
rollers by a drawing machine situated up to 120m away. The dimensions of
the tubing are determined as the glass cools through its setting point
at the catenary or unsupported section between the blowpipe and the
first line roller. A given range of size is based on the diameter of the
refractory sleeve, and variations within the range are obtained by
adjusting the temperature of the glass, the rate of flow, the pressure
of the blowing air and the speed of the drawing machine.
Manufacture of Tubing Vello Process
The Vello process was a later development with a
production capacity greater than that of the Danner process but based on
a different principle.
Glass flows from a furnace forehearth into a bowl
in which a hollow vertical mandrel is mounted or a bell surrounded by an
orifice ring. The glass flows through the annular space between the bell
and the ring and travels over a line of rollers to a drawing machine up
to 120m away. Tubing is made by blowing air through a bell with a hollow
tip and rod is produced by using a bell with a solid tip. The dimensions
of the tubing are controlled by the glass temperature, the rate of draw,
the pressure of the blowing air and the relative dimensions of the bell
and ring.
Automatic Domestic Glassware Production
The Westlake machine was developed for blowing
bulbs for domestic lamps and radio valves at production rates of up to
75,000 a day (gross). It has since been adapted for making drinking
glasses, including stemmed ware, at up to 55,000 a day (gross).
The machine copies the action of a handblower in
gathering glass from the furnace, forming a parison and blowing the
article in a cast iron mould. Twelve pairs of spindles or blowpipes,
together with their blowing air valves and past moulds, travel around a
central column. The gathering equipment is carried on top of the column
and sets of cams are fitted around the column to control the sequence of
operations.
Glass is gathered by vacuum into a pair of blank
moulds and the pairs of blanks are transferred in turn to each pair of
spindles. The spindles are rotated and swung down, and air is introduced
to form each blank into a parison, controlling the profile and
distribution of the glass before blowing the required shape in the
wetted mould.
The mould opens and the spindle jaws release the
article which is then transferred to the stemming machine. Here the neck
formed in the mould is reheated and stretched to the required length.
The article then passes to the burn-off machine where oxygen-gas flames
remove the "moil" or waste glass which was originally formed at the
gathering position, and the finished piece is conveyed to the lehr for
annealing.
Electric Light Bulb Envelope Production
The ribbon machine was developed for the high
speed manufacture of bulbs for domestic lamps, auto lamps, vacuum
flasks, etc. Its main feature is that glass travels through it in a
straight line rather than on a rotary path as with the Westlake
machines. Production rates in excess of 1000 a minute can be achieved.
From the furnace forehearth molten glass flows
down between two rotating water cooled rollers and on to the Ribbon
machine. On leaving the rollers the ribbon of glass is carried through
the machine on a series of orifice plates, forming a continuous belt
pierced with holes.
As the ribbon moves forward, a continuous chain of
blowheads does the glassblower's job for him. It blows the glass through
the hole and the "blister" forms into a bulb inside a rotating mould
which meets and closes around it from below. Still moving forward on the
ribbon, the shaped bulb is released form its mould, cooled by air jets
and then tapped off the ribbon to fall onto the scoops of a rotary
turntable which tips it on to a conveyor belt. This carries it through
an annealing lehr and air cooling to inspection and packing. The unused
part of the ribbon passes direct to a cullet system for re-melting.
Electric Light Bulb Envelope Manufacture
Molten glass flows continuously between water
cooled rollers and the ribbon so formed on orifice plates. Blowheads
from above blow the glass through the holes in the plates. Moulds form
below meet and close round these "blisters". The mould fall away
revealing the formed bulbs which are cooled by air jets and tapperd off
the ribbon. They fall into scoops on the rotary turntable which tip them
onto the conveyor belt to the annealing lehr. More than 1,000 bulbs per
minute can be produced on such a machine.
Pressed Glassware
Pressing is used for objects with a simple basic
shape where the opening is wider than the base, this does not restrict
surface decoration which may be complicated. A plunger is used to form
the inner surface of the article by pushing the glass against the outer
mould. Pressing can be hand-operated or fully automatic.
SECONDARY GLASS PROCESSING
Annealing
Glass, like most other materials, contracts on
cooling. However, due to its low thermal conductivity, it does not cool
uniformly and the surfaces, which cool more rapidly, shrink more quickly
than the centre. This produces uncontrolled strain in the article. If
the internal surface of an unannealed container is scratched, the
container will disintegrate. Badly annealed glass articles cannot
withstand thermal shock and are liable to break in use. The excessive
strain can be avoided by slow cooling at a controlled rate, called
annealing. Annealing is done in an oven, called a lehr, through which
glass articles pass on a slowly moving conveyor belt.
A container, for example, would enter a lehr at
approximately 450oC. As the conveyor moves through the lehr,
which is approximately 20m long, the temperature is at first increased
to about 560oC, at which the glass just begins to flow and is
then gradually reduced to a temperature at which no further strain can
be induced, and then cooled by fan air to room temperature. The time
required for this process depends on the size of the article and the
wall thickness but is normally completed in less than an hour.
Toughening
Glass has an extremely high compressive strength
and therefore when it does so due to induced tension on the surface.
Glass can be thermally strengthened by inducing invisible thin layers in
compression on the outer surfaces. In order to break such toughened or
tempered glass, the compression has to be neutralized and additional
tension applied. Toughened or tempered glass, the compression has to be
neutralized and additional tension applied. Toughening is obtained by
re-heating the glass article uniformly to a temperature just above that
at which deformation could take place and then rapidly cooling the
surfaces by jets of air. If one can imagine a sheet of glass as
consisting of 3 layers then the process becomes easier to understand.
The air jets rapidly cool and freeze solid the outer layers while the
inner layers continues to contract. While it is contracting it exerts
compression on the outer layers while putting itself under tension. This
method can be applied to flat glass or simple shapes like curved car
windscreens or even tumblers. Glass thickness must be uniform, not too
thin, and the shape of the article must be such that all surfaces can be
uniformly cooled at the same time. Bottles do not satisfy these
conditions and cannot be toughened in this way. However, it is possible
to toughen bottles chemically by immersing hot bottles in a molten
potassium salt. Potassium ions replace sodium ions on the surface and,
being larger, create a very thin layer of compression.
Toughened glass cannot be further processed since
any damage to the surface will expose the centre layer, which is in
tension, and the glass will shatter. The shattering of a car windscreen
is a good example of this phenomenon.
Coating
The coating of glass surfaces has been practiced
for centuries. Mirrors are a good example of this art. However, this
method of giving glass new physical, chemical and optical properties has
made great strides in the last few decades. Lightweight glass containers
are coated with organic compounds to give the surfaces a degree of
lubricity and thus preventing abrasion in handling. This adds strength
to the container and has enabled glass manufacturers to make a lighter
and better product. Coating containers with tin compounds also produces
a stronger product. Coating glass containers with plastic materials for
added strength and safety is a further way of lightweighting or
increasing internal pressure resistance. Other forms of decorations are
etching with hydrofluoric acid, sandblasting and vitreous enameling. In
the latter, vitreous enamels, which are low melting point glasses held
in an aqueous medium are deposited on the glass through very fine wire
mesh screens and are then fired in an enameling furnace. The enamel
thus becomes an integral part of the glass article.
Decorating
Formed and annealed glass may be further
processed. This may be done by taking away from or adding to the surface
of the glass. It may also be heated, manipulated, and reshaped. These
methods include:
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