Laboratory Glass-Working for Scientists - Sciencemadness Dot Org

I
DEVITRIFICATION
of silica. The crystals which are most likely to separate
soda-lime-silica glass are those of calcium silicate, tridymite
id cristobalite. Calcium silicate occurs usually as the monoclinic
inn wollastonite, but sometimes in a hexagonal form. The monoic
form tends to appear as very long, thin crystals bunched tolgBther
to give a brush-like appearance. The hexagonal form is not
pcicular. Tridymite is hexagonal, and forms hexagonal stars, like
mow, by twinning at 60°. The cristobalite forms twins at 90°. Devitrification
on the surface of glass seems to depend upon loss of alkaloxides,
and may be assisted by dust particles.
Transparent vitreous silica (transparent Vitreosil) is liable to devitrify
if potassium or sodium compounds are present. The surface
of the material must be thoroughly cleaned, and the part to be
worked should then not be touched with the fingers.
When devitrification is observed in the soda and borosilicate types
of glass, the crystals may sometimes be removed by fusion of the
glass in the flame; alternatively the semi-molten devitrified portion
may be removed with tongs, and replaced by fresh glass added as thin
rod. If there is a large extent of devitrification the portion of glass
should be completely cut out and replaced. Old soda glass apparatus
is very liable to devitrify when repaired: F. C. FRARY, C. S. TAYLOR
and J. D. EDWARDS (1928) advise, in such cases, a preliminary washing
of the glass with dilute hydrofluoric acid to remove the surface
layer.
Thermal Strain in Glass
When a block of glass is suddenly heated on all its faces, the outer
layers are under compression and the inner layers are under tension,
as explained on p. 12. This strain is temporary, however, since it
vanishes as soon as the temperature gradient vanishes. Temporary
strain is similarly produced when the surface of a block of rigid hot
glass is cooled. Above the upper annealing temperature (p. 11) a
stress can only exist in glass for a short time, because the glass flows
to relieve the stress. Stress is relieved only very slowly at the lower
annealing temperature. Thus over a certain temperature range glass
changes from a viscous to a rigid body. Consider a block of glass to
be rapidly cooled through this temperature range, so that there is
always a temperature gradient. In the viscous region the glass is
strain-free, and thus when it first becomes rigid it is also strain-free.
Thus there is a rigid block of glass containing a temperature gradient
but free from strain. When this temperature gradient is removed, the
inner layers of the glass are in tension and the outer layers in compression.
There is then a permanent strain in the glass. Clearly, when
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