As in the case of lead glass, so with soda glass. A thorough acquaintance with the effect of the various parts of the flame upon it should be gained before further work is entered upon, for which purpose an hour or more spent in observing its behaviour in the flame will be fully repaid by increased success subsequently.
The Use of Combustion Tube.—It is often necessary to construct apparatus of what is known as hard glass or combustion tube. It is almost as easy to work combustion tube as to deal with lead and soda glass if the oxy-hydrogen flame be employed.
It is not necessary to set up a special apparatus for this purpose; many of the ordinary blow-pipes can be used with oxygen instead of with air. It is only necessary to connect the air-tube of the blow-pipe with a bottle of compressed oxygen instead of with the bellows. The connecting tube should not be too wide nor too long, in order to avoid the accumulation in it, by accident, of large quantities of explosive mixtures.
Two precautions are necessary in manipulating hard glass in the oxy-hydrogen flame. The glass must not be overheated. At first one is very apt to go wrong in this direction. The supply of oxygen must not be too great; a small hissing flame is not what is wanted. If either of these precautions are neglected most glass will devitrify badly. With a little care and experience, devitrification can be absolutely avoided. Ordinary combustion tube can be used, but I find that the glass tube (Verbrennungsröhr) made by Schott & Co. of Jena, which can be obtained through any firm of dealers in apparatus, is far better than the ordinary tube.
By following these instructions, any one who has learned how to work with lead or soda glass will find it easy to manipulate hard glass.
[2] For details of the composition of the various glasses, some work on glass-making may be consulted.
[3] The presence of silicates of calcium and aluminum are considered to promote a tendency to devitrification in glass; and glasses of complex composition are more apt to devitrify than the simpler varieties. See Glass-making, by Powell, Chance, and Harris, Chap. IV.
[4] See Principles of Glass-making, p. 31.
[5] Nevertheless the supply of air must not be so excessive as to reduce the temperature of the flame sufficiently to prevent the thorough softening of the glass, which will occur if the bellows is worked with too much zeal.