tubing--Spirals--Ground joints--Sealing in platinum wire--Sealing
vacuum tubes--Closed tubes for heating under pressure.

INDEX 59

LABORATORY
MANUAL OF GLASS-BLOWING

CHAPTER I

MATERIALS AND APPARATUS

One of the most important factors in the success of any piece of
glass-blowing is the glass employed. As is well known, there are two
general varieties of glass: Lead glass and soda glass. Formerly much
apparatus was made of lead glass, but at present it is very seldom met
with, except in the little drops of special glass used to seal platinum
wires into the larger sizes of tubes. Lead glass is softer and more
readily fusible than soda glass, but has the disagreeable property of
growing black in a few seconds unless worked in a strong oxidizing
flame. This may be prevented by using a "hissing" flame, with a large
excess of air, and working in the extreme end of the flame; or the black
lead formed may thus be reoxidized, and the glass restored to its
original clearness.

Almost all the soft glass on the market is a soda glass, although
sometimes part of the soda is replaced by potash. Most of the hard glass
appears to be a potash glass. The following qualities are desirable in a
glass for ordinary working: (1) moderately low working temperature, (2)
freedom from air bubbles, striations and irregularities, (3) proper
composition, so that the glass will not devitrify or crystallize while
being handled at its working temperature, (4) ability to withstand rapid
heating without cracking.

The working temperature of different samples of so-called "soft glass"
varies a good deal, and is best determined by trial. The glass should
become almost soft enough for blowing in a flame that still shows a
little yellow near the tip, so that at the highest temperature of the
flame it may flow fairly freely and thus easily eliminate irregularities
in thickness. If the glass is too hard, the shrinking of the glass,
collection of material for a bulb, and in fact most of the working
processes will be slower, and the glass will not stay at its working
temperature long enough after its removal from the flame to permit it to
be properly blown.

Air bubbles in the original batch of glass are drawn out into long
hair-like tubes during the process of manufacture. When such tubing is
worked, the walls of these microscopic tubes collapse in spots, and the