Laboratory Glass-Working for Scientists - Sciencemadness Dot Org

GENERAL TECHNIQUE FOR WORKING FUSED SILICA
light is necessary. The recommended glass for best protection is
green Protex grade C, f inch thick (Chance Bros. Ltd).
For hand work with a bench torch, tubing is rotated in the flame
to secure even heating and hence even shaping; while when clamping
of the various tubes is employed and the hand torch is used, all the
dangers of cracking, which are present when borosilicate or soft
glasses are being worked, are negligible. The Vitreosil must be blown
while still in the flame; and therefore, when a bench torch is used and
the apparatus is manipulated by hand, a blowing tube must be
attached at one end. It is preferable then to use a glass-blower's
swivel (see p. 42).
For tooling of molten Vitreosil TS inch Nichrome wire may be
employed; though if much tooling is required, then carbon rods or
plates are necessary. In general, most of the operations described in
earlier chapters can be modified for working with Vitreosil and, if a
sufficiently hot flame is available, less manipulative skill is usually
necessary because the material stays fairly viscous.
When operating with tubes it is well to take care to prevent the
flame licking the insides; the vapours can cause contamination which
results in devitrification when more work is done.
Large Diameter and Thick- Walled Tubing
With very large silica tubing, over 4 cm bore, the walls are usually
very thick. Special methods are required when the walls are 5 mm
thick or more. The wall edges, at the ends to be joined, are cut or
ground to sharp knife-edges, so that a V-shaped notch is left all round
when the ends are butted together. The ends are fused together,
using a hand torch and 3-5 mm welding rod to fill in the V-shaped
notch. With very large tubes the flame must be manipulated to keep
the tubes hot all round, maintaining an even heat, and the apparatus
must be annealed—preferably in a furnace for one hour at 1050°C.
Translucent and transparent Vitreosil rods of the proper size for
the above operations are obtainable.
Silica Torsion Fibres
These fibres can be made by drawing by hand or by simple mechanical
means, and by blowing them out with a flame. Here we describe
only simple methods for making fibres by drawing them out with a
weight falling under gravity. With these methods fibres with a range
of sizes can be made quite quickly. T. C. KEELEY (1924) fixed a small
weight to a silica rod and heated this at a point until the silica
softened sufficiently for the weight to fall and draw out a fibre. V. J.
CLANCEY (1950), during an investigation of the evaporation and
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