METAL BODY REPAIR - OD1653 - LESSON 2/TASK 1
2.
Glass Composition and Characteristics
Glass at ordinary temperatures is rigid, not easily corroded except by
hydrofluoric acid, and is comparatively resistant to abrasion. Like wax, it
becomes softer and more pliable as the temperature rises. Salt water has a
chemical or abrasive action on glass that causes a frosted appearance after
sufficient exposure. It is only slightly porous and, therefore, easily
cleaned. Glass is not entirely colorless. The ability of glass to transmit
light and offer protection against air, water, and dust makes glass one of
the most important automotive materials. Glass is only slightly less strong
than some grades of cast iron. But, it is far more brittle, and any
external force will set up stresses within it that will be retained for some
time. Any force added to these retained stresses may cause a break. This
property is a great aid in cutting sheet glass. Glass is made by combining
a silicate such as common sand, and alkali such as sodium carbonate, and
either lead or lime. It is commonly classified as lead glass or lime glass.
a. Lead Class. This glass has the higher luster but is more brittle and
more easily scratched. It is used principally for optical and decorative
purposes.
b. Lime Glass. Only lime glass is used for automotive work. Because the
available supply of silicate, alkali, and lead or lime is not usually pure
or readily usable, carbon, arsenic or other substances must be added to
purify them and assist them in combining. Substances such as iron, copper,
properties. These solid components, melted together, form liquid glass.
When its impurities rise to the surface, the molten glass is drawn off and
allowed to harden. A sheet of glass is generally a large, flat piece having
no particular shape. When the term is applied to laminated glass, it means
one of the layers. Laminated glass is sold in cut, rectangular sheets from
4 by 18 to 24 by 56 inches, called blocks. About 50 square feet, weighing
approximately 200 pounds, are ordinarily packed together and are known as a
box.
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