METAL BODY REPAIR - OD1653 - LESSON 2/TASK 1
c.
Types of Glass.
(1) Plate Glass. Plate glass is poured on a flat heated surface, cooled
until it becomes plastic, and then rolled to about twice the thickness
intended for the finished plate.
After rolling, it is slowly cooled or
annealed, and when cold it is ground with sand and polished smooth and flat.
Modern glassmaking is a continuous process in which glass comes out between
rollers in a continuous ribbon. The ribbon is not cut until it is cold and
ready for grinding. Grinding and polishing make the surface more porous and
less resistant to attack by acids, alkalies, and (of special importance)
salt water.
It must be handled carefully and kept clean; crayon marks
imbedded in its pores cannot be completely removed.
Plate glass may be
tempered to make it harder and more shock resistant.
(2) Window Glass. Window glass is blown or poured into sheets of the
desired thickness, then annealed and cut to the desired size. Unlike plate
glass, it is never ground or polished.
The surface therefore is somewhat
less porous than plate glass and more resistant to scratches and salt water,
but it is slightly wavy. Like plate glass, it may be tempered. Tests show
that plate glass is markedly superior to window glass in motor vehicles for
reducing fatigue and eye strain and increasing the ability to judge distance
and to read road signs.
(3) Tempered Glass. Tempered glass is made by reheating plate or window
glass until it is somewhat soft, and cooling it quickly in a bath of hot oil
or against a cold metallic surface.
It will then withstand heavy impacts
and great pressures.
However, a comparatively light blow with a pointed
object will break it because of the internal stresses caused by the sudden
cooling, and it may fly apart violently when broken.
Because of these
internal stresses, it cannot be cut or ground, but is shaped when soft.
Rear windows, particularly curved ones, may be tempered glass.
(4) Bulletproof Glass. Bulletproof glass has very limited uses. It is
very thick, usually 3/4 inch, and resists bullets. They may, however, chip
or crack it. Bulletproof glass and curved glass,
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