Chemistry Reference
In-Depth Information
temperature range. For glazing and enameling, however, these properties
are virtually irrelevant. The
coefficient of linear expansion
, a measurement of
how much a material expands or contracts in length when heated or cooled
per degree of temperature (see Textbox 76), is relatively unimportant for
glass. For glazing and enameling, however, differences between the coeffi-
cient of linear expansion of the coating (glaze or enamel) and the coated
ceramics or metals may lead to breakup of the coating, known as
crazing
; in
extreme cases when the difference is great, the glaze or enamel may even
come apart and be detached altogether (Shafer 1976). Table 26 summarizes
the properties required from glass, glaze, and enamel.
TABLE 26
Properties Required from Glass, Glaze, and Enamel
Material
Property
Glass
Enamel or glaze
Ease to work
Low melting point plasticity
Determined by properties of
over a wide range of
substrate
temperatures
Transparency
Usually desirable
Seldom desired since opaque
layer obliterates underlying
surface defects
Coefficient of
Unimportant
Differences between substrate and
linear
coating layer lead to craze and
expansion
peel
TEXTBOX
27
SUPERCOOLED LIQUIDS; GLASS AND VITREOUS MATERIALS
When a molten material cools down slowly and reaches its melting point,
it solidifies by a process known as
crystallization
; the amorphous, fluid melt
turns into solid, rigid crystals having a characteristic and distinctive inter-
nal structure, symmetry, and shape (see Textboxes 3 and 21). This is how
for example, all primary minerals in the earth's crust, are formed. If the
melt cools down rapidly, however, the material does not crystallize: as it
cools its
viscosity
(its property to flow) gradually increases; as it cool still
further, it turns rigid, solidifies over a range of temperatures (not at a sharp
melting point) and remains internally amorphous, lacking internal struc-
ture (Feltz 1993).
