GLASS - SUPER COOL LIQUID - - Engineering Nepal, The complete engineering website.




Most of us think as glass as a solid material, but it is actually a supercooled liquid. Molecular units have a disordered arrangement yet still have sufficient cohesion that mechanical rigidity is produced.

The glass was first made in the Middle East, approximately during the third millennium BC. Early uses were primarily for vessels or decoration. Glass did not come into use for windows until the first century AD, and was made at that time by casting or hand blowing the glass.
Today, glass is a highly engineered material with many different varieties and countless uses. There is float glass, annealed glass, wired glass, tempered glass, safety or laminated glass, leaded glass, heat absorbing glass, low e glass, etc.

Common Types of Glass

The types of glass most commonly encountered in property and casualty claims would include float glass, tempered glass, laminated glass, and wired glass. Safety glass is a general term which can refer to tempered, laminated or wired glass.

Float glass is manufactured by rolling molten glass into a sheet than "floating" it on top of a bath of molten tin. This produces a flat sheet of glass with smooth and even surfaces.

Tempered glass typically floats glass that has been heat treated. The heat treating produces a denser outer layer on both faces of the glass which increases the strength of the glass and causes it to shatter into many small fragments when broken. This is desirable where safety is a concern, as regular float glass, when broken, can form long sharp dagger-like shards which can cause significantly more injury.

Laminated glass is actually two layers of glass sandwiching a thin plastic sheet. One or each of those layers of glass could also be tempered. The middle plastic sheet serves to carry the general glass sheet assembly along once broken. Laminated glass is employed in automotive windshields and in buildings wherever safety could be a concern.

Wired glass is float glass that has been shaped with a layer of fine wire mesh within the center of its crosswise. This layer of mesh is visible once the pane of glass is cooked through. Wired glass is extremely sturdy and is employed for security functions and conjointly in locations wherever safety could be a concern.


Cost for the various types of glass varies significantly, for a number of reasons. Float glass is the easiest glass to manufacture, is available in many thicknesses and sheet sizes, can be tinted, textured, etc., and is easily cut. Hence, float glass is the cheapest type.

Tempered glass, by the nature of its manufacture, cannot typically be "bought off the shelf". Once tempered, it cannot be cut, therefore, tempered glass is bought for a specific size and application. For example, in a building application, say in a door, the tempered glass is ordered to fit. The glass manufacturer then will cut a piece of float glass, slightly oversize, and temper it. In the tempering process, the glass shrinks both in length and width as well as in thickness.

Replacement of plain float glass windows can usually be done the same day. Unless the required tempered glass pane is a standard, readily stocked size, this is not possible with the replacement of tempered glass panes. This is true also for laminated glass.

Thermal Windows

Almost all windows used in buildings today are constructed with thermally insulated glass units. A thermal unit can be double or triple glazed.

A double glazed unit comprises two glass panes separated by an air space, typically ½ in. A perimeter seal/spacer keeps the two glass panes apart and provides a seal which prevents outside air from entering the space between the panes. The air space significantly increases the insulating ability of the window compared to single pane windows.

More energy efficient double glazed windows will substitute an inert gas such as Argon for the air between the panes. Further increases in efficiency can be obtained through the use of "low e coatings" which reduce heat transfer through the window due to radiation.
The perimeter seal contains a desiccant which removes any moisture from the air contained in the space which may have been present during manufacture. When the seal fails, moisture can be drawn into space and can condense on cold interior surfaces. Repeated cycles of condensation and drying will leave a film on the interior (sealed) surface of the panes, resembling the appearance of a dirty window.


Failed Thermal Unit Seals

Under normal circumstances, most thermal windows will last at least 10 years before the seal loses its integrity. However, the loss of the seal can occur due to other factors, such as a crack in the glass, twisted or warped window framing, fire, or tight-fitting blinds.

One thing to consider when considering the validity of claims regarding failed thermal units is the degree of staining on the sealed glass surfaces. Significant staining would suggest seal failure had occurred months or even years prior to the alleged incident cause.

Cracked Glass

Improper window installation or other construction deficiencies can cause the window frame to twist which, in turn, can produce stresses in the glass which cause cracking. Vibration claims often include cracked windows as a part of the claim. Finally, settlement of the building, either brought on by a recent incident or simply as an ongoing movement within the building, can cause shifting of the window unit and again, stresses to develop within the glass which causes cracking.

Personal Injury

As noted previously, significantly worse injuries usually result from broken float glass as compared to safety glass (tempered, laminated or wired), due to the ease of breakage and the dangerous nature of the resulting shards.
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