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History of Glass
Modern Glass
Types of Glass
Strengthening Glass
Reflective, Low Emissivity and Insulating Glass
Specialty Glass
History of Glass
The origin and use of glass goes back almost 5,000 years. Archaeologists
have discovered evidence of glass objects dating as early as 3,000 B.C.
The ancient Greeks used glass in their buildings. Historical records
from the period refer to baths and rooms lined with glass. Window glass
dates from the end of the third century.
Experts believe that the ancient Syrians discovered glassmaking, probably
by accident, about 3,000 B.C. A manufactured green glass rod found in
ancient Babylonia (modern Iraq), dates to 2,600 B.C. Syrian glass was
a simple melted mixture of soda ash, lime and sand. Glassmakers formed
it into final shapes by sculpting it while still hot.
When Egypt conquered Syria in 1,400 B.C., the captured Syrian glass
workers were sent back to Egypt. They refined glass making into a higher
art. Glassworkers produced vessels, vases and eating utensils by pouring
several thin layers of molten glass into sand molds in the shape of
the object. The glassware was decorated by adding molten colored glass
drips to the final layer.
The Egyptians discovered that they could blow a gob of molten glass
from the end of a hollow metal tube into the mold. This technique evolved
into what we now know as glass blowing. Glass blowing remains a useful
technique for creating many types of glassware.
Window glass originated in Rome, but it was very thick and translucent.
That is, it let light in but people couldn't see out. In 1291, on the
Italian island of Murano, workers developed a clear, almost transparent
glass called "cristallo." This is where the word "crystal" comes from.
Murano glassware became popular throughout Europe, and Italy built up
a thriving export trade.
In the Middle Ages, glass making was still a hand-made process. Window
glass was made by blowing the molten glass into a flat disc which was
then spun so that centrifugal force caused the glass to thin out and
flatten. These discs were cut into small panes of glass, usually limited
to 18 square inches. Glass workers searched for improvements.
Cylinder glass was one such improvement. The molten glass was blown
into a cylinder which was cut apart, then reheated and flattened. In
the 16th and 17th Centuries, the English discovered that using coal
instead of wood in their furnaces produced a much clearer glass. Although
the panes were wavy and full of bubbles, and sometimes light amethyst
or amber in color, people could actually see through their windows.
Louis Lucas de Nehou, a Frenchman, developed a manual process for making
plate glass in 1688. The method was cumbersome-it took 16 days from
start to finish and produced glass so expensive that only the very rich
could afford it. For the next two hundred years, improvements were made
in this process, primarily in the power sources needed to melt the raw
materials into glass, and in methods to increase the amount of glass
that could be produced. But the French plate glass method remained the
basic technique. Finally, in the 1900's, technological improvements
were developed which made possible large scale glass manufacturing as
we know it today.
Modern Glass
Today, the glass making industry is very sophisticated. Glaziers use
a wide variety of glass, depending upon the functions that the glass
must perform. What Is Glass? Basically, glass is sand-a very high quality
silica sand, to which other materials are added. The resulting mixture
is called a batch. Some of the other materials included in the batch
are salt cake, limestone, dolomite, feldspar, soda ash and powdered
cullet. Cullet is broken glass. It can be left over from a previous
batch or from the edges that remain after a batch of glass has been
formed and cut to size. Adding cullet helps the batch melt more easily.
Glass is made by melting and cooling the batch. As the batch cools,
it becomes solid without forming crystals. Crystals are three-dimensional
building blocks that make a substance internally rigid. The lack of
crystals makes glass technically a liquid, not a solid. It also makes
glass transparent.
Types of Glass
Glass comes in many shapes and forms. At one time most of the glass
manufactured in the United States was plate glass. Plate glass was made
by a process of grinding and polishing. No longer made in this country,
plate glass has been replaced by float glass.
Float glass is a term that refers to a process of making glass that
was perfected in 1959 by Pilkington Brothers, Ltd. of England. Float
glass is made by pouring the molten glass from a furnace into a chamber
that contains a bed of molten tin. The atmosphere inside the chamber
is carefully controlled. The glass floats on the tin and forms itself
in the shape of the container. It spreads 90 to 140 inches wide at a
thickness determined at the time of manufacture. The length of the glass
from the furnace to the cutter is about a mile. The upper surface of
the glass is called the air side or score side. It is polished with
fire. The lower surface is called the tin side. It is not fire-polished.
From the chamber, the glass enters an oven, called a lehr. There it
is slowly cooled at a specific rate. This process, called annealing,
relieves the glass of internal stresses. The rate of cooling is crucial
to the success of the final product. The glass emerges from the lehr
at room temperature as a continuous ribbon. It is flat, fire-finished
on the top, and has smooth, parallel surfaces. Automatic cutters trim
the edges and cut the glass to length.
Because the process is so highly automated, individual lites of glass
are not labeled. Shipments of large custom-cut lites are generally shipped
in cases that list size, quantity and quality. Each case weighs from
3,000 to 4,000 pounds. Glass can also be shipped in a loose pack, called
a stoce. The stoce is bound together by banding material. Stoce glass
weighs from 4,000 to 10,000 pounds.
There are two types of glass made by the float process:
· Clear glass
· Tinted or heat absorbing glass
Most of the flat glass made by the float process is clear glass. As
its name implies, clear glass is transparent and colorless. Depending
upon its thickness, clear glass allows about 75 to 92 percent of the
visible light to pass through. This characteristic of glass is called
its light transmittance.
The specifications written by the American Society for Testing Materials
(ASTM) separates tinted and heat absorbing glass into two categories.
This is misleading, however, because all tinted glass absorbs heat.
Tinted, or heat absorbing glass, is made by adding coloring agents to
the batch mix. These agents include bronze, gray, green and blue. What
tinting does:
· Tinting glass: Reduces the amount of light that passes through the
glass.
· Causes the glass to absorb more of the sun's rays.
As the glass gets thicker, the density of the color also increases.
This causes the glass to transmit less visible light. The light transmittance
of tinted glass varies from 14 to 83 percent depending upon its color
and thickness.
Edge conditions are crucial to the effectiveness of tinted glass because
a flaw at the edge can cause the glass to fail as it absorbs heat. Two
types of failures are:
· Heat breaks
· Pressure breaks
A heat break occurs at an angle of 90 degrees to the surface of the
glass. Heat breaks resemble smooth curves. A pressure break occurs along
the surface, usually starting at a corner.
Rolled glass is manufactured by pouring glass from the furnace into
a series of rollers. It is then shaped to the desired thickness, annealed
and cut to size. The two basic types of rolled glass are:
· Patterned Glass
· Wired Glass
Patterned glass is also called figured glass, obscure glass, and decorative
glass. It is available in thicknesses from 1/8" to 3/8". Patterned glass
is made by passing it through rollers that have patterns on them. The
pattern is transferred to one or both sides of the glass. Each manufacturer
of patterned glass has unique patterns. Patterning glass has several
purposes:
· It controls light.
· It diffuses the details of objects.
· It is decorative.
Patterned glass is available in colors, but the choice is limited. Some
of the patterns, because of their depth, make tempering the glass impossible.
Wired glass is made by feeding a welded wire net of a particular design
into the molten glass just before it enters the rollers. The wire does
not add to the strength of the glass but it does hold the lite in the
sash if it shatters. Although manufacturers have unique wire patterns,
there are some common ones. A diamond shaped pattern is called misco.
A baroque pattern is square. Wired glass can be patterned on one or
both sides. If the glass is patterned on both sides, it is usually called
rough glass.
Wired glass is used in fire-rated windows and doors because it meets
most fire codes. For these applications, all the wires must be embedded
in the glass. There are limitations on the square footage allowed in
openings. In other applications, the edges of the glass must be sealed
to prevent the wires from rusting. However, even though it meets fire
codes, wired glass is not a safety glass. In fact, it has only one half
the strength of annealed glass of the same thickness. In addition, wired
glass cannot be tempered.
Strengthening Glass
The rate of cooling directly affects the strength of glass. The regular
process of cooling - or annealing - float glass results in a slow rate.
Stronger glass can be produced by changing the rate of cooling. Two
types of stronger glass are:
· Heat-Strengthened Glass
· Tempered Glass
Heat-strengthened glass is cooled at a rate faster than regular annealed
glass. Tempered glass, in turn, is cooled at a faster rate than heat
strengthened glass. Another way to strengthen glass is to use more than
one lite of glass in the application. Laminated glass consists of two
or more lites of glass, joined by a layer of plastic.
In many modern buildings, the glass must be as strong as possible. Three
basic reasons to strengthen glass are to:
· Increase Wind Load
· Increase Impact Resistance
· Combat Thermal Stress
Architects and designers must consider the force of wind on a building
or installation when choosing glass. Wind causes glass to deflect. This
deflection strains not only the glass itself but the entire glazing
system: the framework, gaskets and sealants.
Impact resistance is closely related to wind load because the wind carries
such things as hailstones, dust, small stones and other debris. During
tornadoes and hurricanes, the wind carries many larger objects.
As glass heats, it expands. The center portion of a lite gets hotter
and expands at a greater rate than the edges. The stresses on the edges
are usually greater at the center of each edge and decrease toward the
corners. The imbalance strains the edges. This is called thermal stress.
The edge strength of the lite, therefore, greatly determines its ability
to resist breaking. Clean-cut edges offer the greatest edge strength.
This is particularly crucial with heat-absorbing glass. A well-designed
glazing system also reduces stresses on the glass.
Heat-strengthened glass is made by heating annealed glass uniformly,
then cooling it at a slower rate than tempered glass.
Characteristics include:
· Is about twice as strong as regular annealed glass of the same size
and thickness.
· Is more resistant to wind loading and impacts than regular annealed
glass though less resistant than tempered glass.
· Fractures into large, jagged pieces, similar to annealed glass.
Heat-strengthened glass is generally used in high-rise buildings to
help the glass resist thermal stress. It is also used in the making
of spandrel glass. Spandrel glass is obscure glass that is used in non-vision
areas. Because heat-strengthened glass fractures into large jagged pieces,
it does not qualify as a safety glazing material. All building codes
require safety glazing for shower doors, commercial doors and store
fronts for safety purposes.
Glass gains considerable strength from tempering. A lite of tempered
glass is about four times stronger than a lite of annealed glass of
the same size and thickness. Characteristics include:
· The only characteristic of the annealed glass affected by tempering
is its bending or tensile strength:
· Tempering increases the tensile strength of glass.
· This makes tempered glass better able to resist the forces caused
by heat, wind and impact.
· Tempering does not change:
· The color, chemical composition, or light transmission characteristics
of the annealed glass.
· Its compression strength (the ability of the glass to resist crushing
forces)
· The rate at which the glass conducts and transmits heat.
· The rate at which the glass expands when heated.
· The stiffness of the glass.
The main reasons to use tempered glass are:
· Tempered glass, when broken, is designed to shatter into cube-shaped
particles. It therefore qualifies as a safety glazing material.
· Tempered glass offers greater strength against deflection, and thus,
better resistance to the force of wind, than heat-strengthened glass.
It is more effective if placed within a well-designed, overall glazing
system.
· Tempering increases the ability of glass to survive the impact of
objects that may strike the building. When tempered glass does break,
it shatters into small cubes, reducing the likelihood of serious injury
on impact.
· Tempering increases a lite's edge strength. Thus tempered glass is
specified when designers anticipate high thermal stresses.
Tempered glass is made by heating annealed glass uniformly. The glass
can be from 1/8" to 3/4" thick. The annealed glass is then cooled rapidly
by blowing air uniformly onto both surfaces at the same time. This is
known as air quenching. Rapid cooling increases the compression forces
on the surface and the tension forces inside the glass. Two processes
are used to temper glass:
· Vertical tempering
· Horizontal tempering
In vertical tempering tongs are used to suspend the glass from its top
edge. It moves vertically through the furnace in this manner. In horizontal
tempering the glass moves through the furnace on stainless steel or
ceramic rollers. Of the two processes, horizontal tempering is the more
common. Tempered glass is identified by a permanent label, called the
bug, which is placed into the corner of each tempered lite. Tempered
glass cannot be cut, drilled or edged. These processes must be performed
on the glass before tempering.
Laminated glass, sometimes called "lami," is made by placing a layer
of polyvinyl butyral (PVB) between two or more glass lites. The PVB
can be clear or tinted and commonly varies in thickness from .015" to
.090", but it can be as thick as .120" for special applications. The
entire unit is then fused under heat and pressure in a special oven
called an autoclave. The laminating process can be performed on clear,
tinted, reflective, heat-strengthened or tempered glass. Characteristics
include:
· When laminated glass breaks, the glass particles adhere to the PVB
and do not fly or fall. Certain combinations of glass and PVB thicknesses
qualify as safety glazing materials under the health and safety standards
set by the American National Standards Institute (ANSI). For example,
laminated glass with a .030 PVB layer sandwiched between two pieces
of two-millimeter annealed glass meets the minimum requirement for safety
glazing.
Applications-In addition to safety glazing, laminated glass has many
specialty applications, including sound reduction and security.
Reflective, Low Emissivity, and Insulating
Glass
Modern glass is called upon to perform many tasks. An important one
is to control the amount of heat and light that passes through the glass.
Three types of glass designed for this purpose are:
· Reflective Glass
· Low Emissivity Glass
· Insulating Glass
Reflective glass is clear or tinted glass that has a very thin layer
of metal or metallic oxide on the surface. The reflective coating reduces
heat gain and glare from the outside while allowing visible light to
enter. Characteristics include:
· Appearance. Reflective glass gives a building a mirror-like appearance.
The coatings are available in silver, copper, gold and earthtone. They
can be combined with tinted glass to give a building a beautiful exterior.
· Energy savings. Because it reflects and absorbs the sun's rays, reflective
glass reduces the amount of solar radiation that enters the building.
This can save money in heating and air-conditioning costs.
· Comfort. Reflective glass reduces variations in the interior temperature
of a building.
Three different processes are used to deposit the coating on the glass:
· Wet Chemical Deposition
· Vacuum Deposition
· Pyrolitic Deposition
Wet Chemical Deposition-In this process the glass is submerged in a
tank containing a chemical solution. The metallic oxide is transferred
to the surface of the glass by a chemical reaction.
The coating is very fragile and must be protected immediately. The usual
method of protection is to use the glass in an insulating glass unit
or in a laminated glass product.
Vacuum Deposition-In this process the glass is placed in a vacuum chamber
containing a special atmosphere. When electrical energy is added to
the chamber, a complex magnetic reaction takes place that causes the
metal atoms to strike the surface of the glass at high speed. The atoms
coat the surface of the glass uniformly. This process is commonly called
sputter coating. Sputter coated reflective glass has a few disadvantages:
· It cannot be heat strengthened or tempered because the heat would
destroy the coating. And, since the coating increases the amount of
the sun's rays the glass absorbs, it may be necessary to heat treat
the glass before coating.
· The soft coating can be damaged easily before installation.
· The glass has limited compatibility with sealants.
Pyrolytic Deposition-The word pyrolytic is used to describe a change
brought about by heat. In pyrolytic deposition the metallic oxide is
added to the glass while the glass is hot. This can be done in an oven
or during the process of making the float glass. Generally, pyrolytic
coated glass is installed with the coated side facing outdoors. The
coating itself reflects most of the sun's rays before they reach the
glass. In many cases, this eliminates need for heat strengthening or
tempering that might be required with other types of reflective glass.
Characteristics include:
· Pyrolytic coatings are more durable than wet chemical or sputter coatings.
· Annealed glass with a pyrolitic coating can be heat treated or tempered
without affecting the coating.
Low emissivity glass, commonly called low E glass, is a type of reflective
glass that is gaining in popularity, especially in residential and office
applications. Low E coatings are very thin metallic coatings that reduce
visible light transmission by about 10 percent compared to uncoated
glass. They are applied using either the vacuum (sputter) or pyrolytic
process. Characteristics include:
· Low E glass:
· Reduces heat loss through windows.
· Re-radiates the heat absorbed from sunlight back inside the room.
· Allows sunlight into a room without letting heat escape outdoors.
· Resists ultraviolet light, which results in less damage to carpets,
draperies and other furnishings.
· Reduces glare.
The main reason low E glass has these advantages is that it reflects
sensible heat. The heat generated by hot water or steam radiators or
the heat from hot air ducts are examples of sensible heat. Low E glass
retains more of this heat indoors than other types of reflective glass.
In northern areas, low E coatings let in the heat from the winter sun
while retaining the heat generated from inside the building. In southern
areas, low E coatings are usually applied to bronze, green or gray tinted
glass. They reduce glare and reflect the sun's heat away from the structure.
Insulating glass units, commonly called ig units, are made from two
or more lites of glass separated by a sealed air space. The metal tube
around the perimeter of the insulated unit which seperates the two lites
of glass is called the spacer. This spacer comes in thicknesses of 3/16"
and larger. It is filled with a special moisture absorbing material
called a dessicant. The perimeter of the entire unit is sealed with
a high grade sealant. Characteristics include:
· IG Units:
· Reduce the tendency of condensation to form on the room side of the
glass.
· Reduce cold transmittance at windows and helps maintain a uniform
temperature. In the winter, ig units reduce heat loss and in the summer
they reduce heat gain.
· Reduce the level of noise from the outside.
There are two types of ig units commonly manufactured:
· Single Seal Units
· Double Seal Units
The difference between the two, as their names suggest, is the presence
of a single or double seal between the spacer and the glass.
Every ig unit is fabricated according to a set process:
1. The glass is cleaned.
2. The spacers are cut to size.
3. A corner key is inserted into one end of the spacer.
4. The spacers are filled with dessicant.
5. The spacer frames are assembled.
6. If the unit is to be double sealed, a ribbon of polyisobutylene (PIB)
tape is applied to one side of the spacer frame. This tape forms the
primary seal.
7. The frame is set on the first lite.
8. If the unit is to be double sealed, a ribbon of PIB tape is applied
to the facing side to the spacer frame.
9. The second lite is set. The lites are usually clamped together or
secured by a weight.
10. The perimeter is sealed.
Single-sealed units can use several types of sealants:
· Hot melt butyl
· Polysulfide
· Silicone
· Urethane
Double-sealed units can use PIB tape for the primary seal and hot melt
butyl (one part silicone, or two-part polysulfide) for the secondary
seal.
IG units need not use the same type of glass. Tempered and annealed
glass can be used in the same unit. Patterned glass can be used but
the pattern should face the outside. If one of the lites is reflective
or tinted glass, it must face the exterior. If reflective glass is to
face the interior, it may be necessary to temper one or both lites to
guard against thermal breaking. A sandblasted finish is not recommended
for an insulating glass unit because sandblasting reduces the strength
of the glass.
Each surface of an insulating glass unit is designated by number:
Number 1 surface - faces the exterior.
Number 2 surface - inside of the first lite.
Number 3 surface - faces the number 2 surface.
Number 4 surface - faces the interior.
When reflective glass is used in an insulating glass unit, the surface
on which the reflective coating is placed makes a great deal of difference.
For example, a bronze coating placed on the number 1 surface creates
a mirror effect. The same coating placed on the number 2 or 3 surface
creates a bronze tinted effect.
How Low E, Reflective & IG Contributes to Energy Efficiency
Low E, reflective and IG glass contribute to energy efficiency by increasing
the effectiveness of the insulating system. Energy efficiency is measured
in two ways:
· U value
· R value
The U value is a measure of the heat gain or loss through glass due
to the difference between the indoor and outdoor temperatures.
· The lower the U-value, the less heat is transmitted through the glass.
The R value measures the overall resistance to heat transfer. The R-Value
is the reciprocal of the U-Value.
· The higher the R-Value, the less heat is transmitted through the glass.
For example, a material with an R value of 19 is a much better insulator
than one with an R value of 6.
Specialty Glass
There are almost as many types of glass as there are possible uses for
them. As you gain experience, you will become familiar with many types
of specialty glass. A few of these include:
· Mirrors
· Spandrel Glass
· Laminated Glass
· Art Glass
Mirrors are made from high quality annealed float glass designated either
mirror quality or mirror select. Mirrors are made by depositing a layer
of silver on one surface of the glass. The surface chosen is the score
side, because the tin side does not accept silver properly. The reflective
quality of the miror depends upon the thickness of the silver layer,
glass thickness, and glass color. High quality mirrors can have a copper
backing. The copper is deposited over the layer of silver, and offers
the greatest amount of protection. Mirrors deteriorate rapidly when
exposed to air, so the metal films must be protected immediately by
a coating. The glass is usually preheated to a range of 120 - 140 degrees
Fahrenheit, before the coating is applied. This paint type coating can
be applied in one coat, or consist of two coats of dissimilar, but compatible
material.
If the metal backing of a mirror deteriorates, the silver turns black.
This condition is known as black edge. Not much definite is known about
the causes of black edge. Some experts believe black edge is caused
by chloride in water. Others believe it is caused by industrial and
household solutions that come into contact with the mirror. Whatever
the cause, most agree that sealing the edges of mirrors helps to guard
against black edge.
Spandrel glass is not specifically a type of glass. Instead, it refers
to the use of obscure glass in non-vision areas of a building. One of
these areas is the spandrel of a building. The spandrel is the part
of the wall between the head of one window and the sill of the window
above it. For aesthetic reasons, this area is often covered with glass.
Spandrel glass is made using several methods. One method is to fuse
a colored ceramic material, called a frit, to one surface of the glass.
The glass must be either heat strengthened or fully tempered because
the frit causes the glass to absorb heat. Spandrel glass may or may
not be insulated. Some codes require an open-weave glass fiber cloth
or special tape to be attached to the back of the spandrel panel to
ensure that the panel stays in the opening if it breaks. If reflective
glass is used as the spandrel application, the back surface must be
obscured in some way so that the building structure does not become
visible under certain light conditions.
As mentioned above, laminated glass is used in safety glazing applications.
However, it has many other applications:
· Burglar resistance
· Bullet resistance
· Sound reduction
· Sloped glazing
· Space enclosures
Laminated glass is used as the inboard lite in skylights. By varying
the thickness and color of the PVB, laminated glass can be used to reduce
the transmission of solar energy, control glare, and screen out ultraviolet
radiation.
A new process, resin laminating, is being used to laminate curved glass
and other specialized applications. In this process, two lites are spaced
.030" to .060" apart. Three sides of the perimeter are dammed. With
the assembly vertical, a liquid chemical mixture is poured into the
space and allowed to cure at room temperature for two to ten hours.
Art glass goes by many names: opalescent, cathedral or stained glass.
Art glass is usually produced in small batch operations. Thicknesses
vary within each sheet and from sheet to sheet. Generally, colors do
not match from sheet to sheet. Art glass is available in a maximum thickness
of inches. It cannot be tempered.
Source: Basic Guide to Glass and Glazing.
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