There have been a few postings lately regarding tempered and
laminated glass. Both type of glass are called "safety glass".
Standard glass such as in the typical house window will break into
long shards that are very sharp and can inflict serious wounds.
Laminated glass is two pieces of standard window glass (non-tempered)
with a plastic film laminated between them. When this glass breaks,
the plastic center holds the broken pieces of glass together.
Tempered glass is standard glass that has been put into a tempering
furnace. This process is a heating up of the glass to a high
temperature and then quenching the top and bottom surface with air
blown very quickly. This causes the top and bottom surface portions
of the glass to shrink more than the center. This puts the surface
in compression similar to the steel rods that are in prestressed
concrete beams. Glass, like concrete, is strong in compression but
weak in tension. When the surface of tempered glass is pushed (or
struck), the bending of the glass away from the force puts tension on
the surface away from the force. If that surface is pre-stressed (in
compression) the force relieves some of the compression and the glass
can bend quite a lot before breaking. I have seen people walk across
a piece of tempered glass and it bent about 6 inches down without
breaking. When it does break it breaks into many, many small pieces
that are not as sharp as non-tempered glass.
By law, all windshields of motor vehicles must be laminated safety
glass so that broken pieces do not blow back onto the driver. Side
and rear windows are usually tempered glass because it is much
stronger than laminated glass.
All tempered glass will have a tempering mark in the corner. This is
usually in the lower left corner. On my 77 Kingsley rear window it
shows LOF, solid plate, tempered. This means that it was made by
Libby Owens Ford and it tempered. The side windows show PPG tempered
(made by Pittsburg Plate Glass Company).
As someone pointed out, tempered glass can be broken easily by
hitting a small spot on the surface with a sharp object. When not
mounted it must be handled very carefully as bumping the edge very
lightly can cause it to burst since there is a compression/tension
profile between the surface layers and the center on the exposed edge.
Another fact about laminated and tempered is that laminated glass
will block close to 100% of the ultraviolet light (UV) which is what
causes sunburn and skin cancer. This is why you can drive for many
hours with the sun shining though your windshield into your face and
not get sunburned but at the same time get a sunburn on your arm from
the tempered side window in just an hour or two. In your home many
modern windows now have what is called UV glass which has a coating
to block UV but these only block about 60 or 70%. If you want to
protect valuable paintings or furniture in your house from fading
from UV you want to use laminated glass but it is more expensive and
thicker than UV glass.
Emery Stora
77 Kingsley
Santa Fe, NM
laminated glass. Both type of glass are called "safety glass".
Standard glass such as in the typical house window will break into
long shards that are very sharp and can inflict serious wounds.
Laminated glass is two pieces of standard window glass (non-tempered)
with a plastic film laminated between them. When this glass breaks,
the plastic center holds the broken pieces of glass together.
Tempered glass is standard glass that has been put into a tempering
furnace. This process is a heating up of the glass to a high
temperature and then quenching the top and bottom surface with air
blown very quickly. This causes the top and bottom surface portions
of the glass to shrink more than the center. This puts the surface
in compression similar to the steel rods that are in prestressed
concrete beams. Glass, like concrete, is strong in compression but
weak in tension. When the surface of tempered glass is pushed (or
struck), the bending of the glass away from the force puts tension on
the surface away from the force. If that surface is pre-stressed (in
compression) the force relieves some of the compression and the glass
can bend quite a lot before breaking. I have seen people walk across
a piece of tempered glass and it bent about 6 inches down without
breaking. When it does break it breaks into many, many small pieces
that are not as sharp as non-tempered glass.
By law, all windshields of motor vehicles must be laminated safety
glass so that broken pieces do not blow back onto the driver. Side
and rear windows are usually tempered glass because it is much
stronger than laminated glass.
All tempered glass will have a tempering mark in the corner. This is
usually in the lower left corner. On my 77 Kingsley rear window it
shows LOF, solid plate, tempered. This means that it was made by
Libby Owens Ford and it tempered. The side windows show PPG tempered
(made by Pittsburg Plate Glass Company).
As someone pointed out, tempered glass can be broken easily by
hitting a small spot on the surface with a sharp object. When not
mounted it must be handled very carefully as bumping the edge very
lightly can cause it to burst since there is a compression/tension
profile between the surface layers and the center on the exposed edge.
Another fact about laminated and tempered is that laminated glass
will block close to 100% of the ultraviolet light (UV) which is what
causes sunburn and skin cancer. This is why you can drive for many
hours with the sun shining though your windshield into your face and
not get sunburned but at the same time get a sunburn on your arm from
the tempered side window in just an hour or two. In your home many
modern windows now have what is called UV glass which has a coating
to block UV but these only block about 60 or 70%. If you want to
protect valuable paintings or furniture in your house from fading
from UV you want to use laminated glass but it is more expensive and
thicker than UV glass.
Emery Stora
77 Kingsley
Santa Fe, NM
