A Handbook of Laboratory Glass-Blowing Part 5

_Blowing a Bulb or Expanding a Join._--Prolonged heating is necessary in order that the thick parts may be heated completely through. Blowing should take place by stages, in order that the thin parts, which tend to expand first, have time to cool. The thick parts can then be expanded by further blowing and thus a bulb or expansion of even thickness can be obtained.

_Cutting Gla.s.s._--The most useful method for general use is by means of the file or gla.s.s-blowers' knife. Either file or knife must be kept sharp by grinding. Neither file nor knife should be used on hot gla.s.s.

The diamond and wheel cutter are useful for cutting sheet-gla.s.s, and when the diamond is employed a singing noise is an indication of a satisfactory cut.

_Leading a Crack._--A crack may be led in any desired direction by means of a bead of hot gla.s.s or a small gas flame. The gla.s.s which it is desired to crack should be heated at a point slightly in advance of the crack, which will extend in the direction of the source of the heat.

_Turning Out the End of a Tube._--This is done by heating the end of the tube and rotating it against an iron rod. The rod must be kept polished and free from rust, and it must not be allowed to become too hot while in use, otherwise the gla.s.s will stick to it.

_Joining Unlike Gla.s.ses._--Joints between unlike gla.s.ses are often unstable. When such joints are made it is desirable to blow them as thin as possible, and to avoid the junction of unlike gla.s.ses in any complex joint, such as an internal seal. A transitional portion of tubing may be built up by the successive addition and interfusion of beads of one of the gla.s.ses to the end of a sealed tube consisting of the other gla.s.s.

_Joining a Tube to a Very Thin Bulb._--The bulb may be thickened at the point of union by fusing on a bead of gla.s.s and expanding this slightly.

A small central portion of the expanded part may then be perforated by bursting and the tube joined on.

_Insertion of One Bulb Within Another._--A bulb may be divided into two halves by leading a crack round it and the inner bulb is then introduced. The two halves of the outer bulb may be fitted together (care being taken to avoid any damage to the edges), and the bulb may be completed by rotating the contacting edges before the blowpipe until they are soft, and then expanding slightly by means of air-pressure.

_Annealing._--For most purposes, in the case of thin, blowpipe-made or lamp-blown gla.s.s apparatus, it is sufficient to cool slowly by rotating the finished article over a smoky flame and setting it aside in a place free from draughts, and where the hot gla.s.s will not come in contact with anything.

Simple bulbs and joints do not even need this smoking; but thick articles, and especially those that are to be subjected to the stress of grinding, need more prolonged annealing in a special oven.

_Use of Lead-Gla.s.s._--When lead-gla.s.s is to be used, the blowpipe flame should be in good adjustment and the gla.s.s should not be allowed to approach so near to the blue cone as to be blackened. Slight blackening may often be removed by heating the gla.s.s in the extreme end of the flame.

Lead-gla.s.s articles tend to be rather more stable than similar articles of soda-gla.s.s.

_Combustion-Gla.s.s._--This may be worked more easily if a small percentage of oxygen is introduced into the air with which the blowpipe flame is produced. If the air is replaced entirely by oxygen there is a risk of damaging the blowpipe jet, unless a special blowpipe is employed.

_Internal Seal._--There are two ways of making these, one, in which the inner portion of the tube is fused on to the inside of the bulb or tube through which it is to pa.s.s, an opening is made by bursting and the outer tube is joined on. This is a quick and in some ways more satisfactory method than the other, in which there is no separate inner piece.

_Rubber Blowing Tube._--In complicated work it is often convenient to use a thin rubber blowing-tube which is connected with the work either by a cork and piece of gla.s.s tubing or by fitting over a drawn-out end.

The use of such a blowing-tube avoids the inconvenience of raising the work to the mouth when internal air-pressure is required. One end of the rubber tube is retained in the mouth during work.

_General Notes._--A large amount of gla.s.s-blowing is spoiled through carelessness in arranging the work beforehand. The student should have every detail of his manipulation clearly in mind before he commences the work; he should not trust to evolving the method during the actual manipulation.

Undue haste is another fruitful source of failure. Practically every operation in gla.s.s-blowing can be carried out in a perfectly leisurely manner, and it is better to err rather on the side of deliberation than on the side of haste.

If, as will doubtless happen at times, a piece of work gives trouble and it is necessary to pause and consider the whole question, or if for any other reason it is necessary to stop during the construction of a partially finished join or other operation, great care should be taken not to allow the work to cool. A large, brush-like flame may be produced by increasing the amount of gas admitted to the blowpipe, and the work should be held just in front of the current of hot air produced by such a flame.

It will then be possible to continue work on this without causing it to crack when further heat is applied.

As time goes on, the student will find an increasing confidence in his ability to manipulate the soft gla.s.s, and with increasing confidence will come rapidly increasing power of manipulation. Perhaps the greatest obstacle to success in gla.s.s-blowing is undue haste in manipulation.