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Introduction
This months newsletter is the second of a two part series on glass.
The articles are designed to better educate you on glass itself; i.e.
how it is made, types of glass, and glass safety.
Types of
Glass:
Ordinary clear glass will allow approximately 85% of
the light that falls upon it to pass through the glass. Glass is
inherently strong but minor, microscopic flaws that are a portion of
the glass itself allow the glass to crack and break. The thickness of
the glass is dependent upon the size of the glass that is needed and
the wind loads that the glass may come in contact with it.
Annealed
Annealed, or “ordinary” glass, is glass that
when it is manufactured is cooled slowly and under carefully
controlled conditions. This “annealing” procedure removes most
ordinary locked in stresses that might cause the glass to act
unpredictably in its use or create conditions in its installation.
Plate
A heavier form of annealed glass is “plate
glass.” Plate glass is thicker version of ordinary glass and is formed
between high pressure rollers. The additional thickness increases the
glasses strength.
The increase in strength, though, does not
make it a safety glass.
Multi-Paned Windows
Single panes of glass, used in windows, are
extremely poor insulators. A single pane glass window conducts, or
moves, heat from one direction to another (remember heat always flows
towards cold), from ten to twenty five times more than a properly
insulated wall. A second sheet of glass applied to the same window,
with airspace between the two sheets, cuts this rate of heat loss in
half. The thickness of the airspace plays a relatively minor roll
compared to the mere presence of the airspace. When a low conductivity
gas is inserted into the airspace it further increases the thermal
efficiency of the window.
Argon and sulfur hexafluoride are the
gases most commonly used. The thermal efficiency of the window can be
even further improved by the addition of a coating on the glass that
blocks conductivity.
Heat Tempered
Heat tempered glass is
annealed glass which has been re-heated to a temperature of
approximately 1200 degrees, near to its re-softening (liquid) point,
and then forced to cool rapidly by cooling both sides of its surfaces
with a blast of air, while its core is allowed to cool more slowly.
This heat treating process produces additional strength and resistance
to impact. The resulting glass is about four times as strong as
annealed glass. When “tempered” glass does break it reduces the
damaged surfaces to small square edged “granules or fragments” rather
than long sharp edged shards like ordinary glass.
These qualities make tempered glass useful
for windows exposed to potential impact, heavy wind pressures, and
intense heat or cold. These qualities make tempered glass particularly
useful for exterior doors, interior partitions, floor to ceiling
sheets of glass, and glass doors that have no frame like shower doors.
Tempered glass is substantially more
costly and does have some optical distortions.
Heat Strengthened
For some applications, where additional
strength is needed, but not to the degree of “tempered” glass, a “heat
strengthened glass” might be used. Heat strengthening the glass
doubles the strength of the glass.
Heat strengthened glass also falls between
annealed and tempered in its distortions and its breakage qualities.
Laminated
Laminated glass is made by sandwiching a
transparent vinyl film in between two sheets of glass. This vinyl
sheet bonds the three layers together and holds the shards of glass,
in a breakage situation, in place preventing a risk of injury. The
types of glass used in laminated glass can be annealed, heat
strengthened or tempered. In the United States,
laminated glass is used in automobile windshields.
It also has applications in skylights
where the vinyl lamination prevents glass from falling to the
space/room below. Where the transmission of sound may be an issue,
like hospital rooms, libraries or classrooms laminated glass also has
applications. When two or more layers are installed it can be used as
a safety glass for banks, and other facilities where security is an
issue.
Wired
Before the introduction of
tempered and laminated glass “wired glass” was used as a safety glass
to prevent human injury. Though wired glass had its advantages
numerous injuries still occurred in conjunction with its use. Wired
glass is produced by rolling a mesh of small wires into a sheet of hot
glass.
Wired glass, though, has only half the
strength of ordinary annealed glass due to the internal stresses from
different rates of contraction of the wire and the glass during the
cooling process. Its use as a safety glass has all but been abandoned
since the early 1970’s. Wired glass is still commonly used in small
door windows to allow someone to see persons on the other side of the
door. Since the 2006 building codes wired glass is no longer allowed
in locations subject to “impact.”
Tinted
Tinted glass is made by adding amounts of
certain chemicals to the molten glass mixture to produce a desired
“hue” to the glass.
Safety
Legislation
As of the early 1960’s few
building code guidelines addressed glass, and the estimated 320,000
annual injuries caused by people impacting glass. The three most
common forms of accidents were caused by; failure to see the glass,
slips and falls (even when it was previously known that the glass was
present, like with a glass shower door or patio door), and accidents
caused during the intentional breakage of the glass.
In 1972, The U.S. Congress
enacted the Consumer Product Safety Act, and one of the first acts of
the Consumer Product Safety Commission was to address the hazards of
glass. I will not address any specific “codes” of the act, but I will
mention that when it comes to issues like safety glass there is no
grand fathering of existing conditions.
The age of the property can not be a
determining factor as regards safety glass.
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