A Handbook of Laboratory Glass-Blowing Part 4

If, in the early stages of his study of gla.s.s-blowing, the student should attempt to work with the very simplest appliances, it is probable that his progress will be hindered; the use of the apparatus will require an undue amount of care and his attention will be distracted from the actual manipulation of the gla.s.s. The case is widely different after he has acquired a certain facility in gla.s.s-blowing.

_A Simple Form of Blowpipe._--Although there are even more simple forms than that described here, we are not concerned with them. The form described is the simplest with which any considerable amount of gla.s.s-blowing can be carried out with certainty.

This form consists of a tube through which air may be blown with the mouth, a condensation chamber in which any moisture from the breath can condense, a blowpipe jet, a supporting piece and a source of flame.

The tube, condensation chamber, and jet are combined in the ordinary Black's blowpipe, such as is used for blowpipe tests in qualitative a.n.a.lysis; it consists of a conical tin tube having a mouthpiece at the small end and a side tube which carries a bra.s.s jet. A support for such a blowpipe may be cut out of a piece of bra.s.s or tin-plate, and should be fastened to a small, flat, wooden board. A source of flame may consist of an ordinary bra.s.s elbow, such as is used on gas fittings, and into which a piece of thin bra.s.s tube (the body of a fish-tail burner from which the perforated non-metallic plug has been removed will serve quite well) has been fitted. It is an advantage to flatten the bra.s.s tube somewhat and to file the flattened end to a slope which corresponds with the angle at which the blowpipe jet enters the burner. The whole source of the flame should be mounted on a separate base, in order that it may be moved while adjusting the apparatus to the best relative positions of flame and blowpipe jet. The complete apparatus is shown by _a_, Fig. 16.

[Ill.u.s.tration: Fig. 16]

In order to take full advantage of this blowpipe, it is desirable that the student should learn to maintain a steady steam of air with his mouth and, at the same time, be able to breathe. This requires a little practice.

As a first exercise in breathing, before trying to breathe while using the mouth blowpipe, the student should close his mouth and inflate his cheeks with air; now, still keeping his cheeks tightly inflated, he should attempt to breathe through the nose. At first, this may be found rather difficult, but it becomes remarkably easy after a little practice. When he has mastered this, the student may practise the same operation, but with the blowpipe. It is important to bear in mind that the cheeks, not the lungs, form the reservoir for air used in maintaining the blowpipe flame. After a while, the student will find that he can maintain a steady air pressure and yet breathe with complete comfort.

In adjusting the flame, care should be taken not to blow so hard as to produce a ragged and noisy cone of fire. A small jet, such as that commonly used on a mouth blowpipe, will with care give a pointed and quiet flame, having an appearance similar to that shown in the ill.u.s.tration.

With a blowpipe like this, it is quite easy to seal gla.s.s tubes up to an inch in diameter, to join tubes up to half an inch in diameter, to bend tubes, to blow small bulbs, and to make the simpler forms of internal seal; but the provision for condensation of moisture is not ideal, and prolonged use of such a blowpipe also tends to produce undue fatigue.

_A Mouth Blowpipe With an Expanding Reservoir._--This form of blowpipe can be made to give most excellent results; it is highly portable, and does not produce nearly so much fatigue when used continuously as the blowpipe described in the last section. Various slight modifications have been made in its construction during the last eighty years, but that described below will be found quite satisfactory.

The apparatus consists of a tube through which air is blown from the mouth, a valve through which the air pa.s.ses into an expanding reservoir, and a blowpipe jet in communication with the reservoir.

In making the valve, several essentials have to be remembered; it must allow a free pa.s.sage of air into the reservoir, it must open easily, and must close quickly. A satisfactory form of valve is that shown by _b_, Fig. 16. The moving part consists of a light gla.s.s bulb of about three-eights of an inch diameter and having a gla.s.s stem of rather under one-eighth diameter and about an inch and a half long. This stem rests in a guide at the end of a bra.s.s tube, the bulb contacting against the other end which is approximately shaped. The bulb and its seating are ground air-tight. A very light spring holds the bulb in position.

This valve is fitted into a metal or gla.s.s T piece, one limb of which leads to the air reservoir and the other limb leads to the blowpipe jet; the limb containing the valve leads to the tube through which the air is blown in.

A convenient reservoir may be made from a fairly large football bladder.

A network of string should be fitted over the outside of the bladder and the strings should terminate in a hook on which a weight can be hung, in order to provide a means of adjusting the pressure at which the air is delivered to the jet. This bladder should be washed out and allowed to drain after use.

The air tube which pa.s.ses from the valve to the mouth may conveniently be made of bra.s.s, but, in order to avoid the continued contact of metal with the lips of the operator, it should be fitted with a non-metallic mouthpiece. It is an advantage from the point of view of portability to have the air tube easily detachable from the T piece containing the valve.

The blowpipe jets, of which there may be several with advantage, may be made of gla.s.s tubing, bent to the most convenient angle and having an enlargement or bulb at some point in the tube. This bulb serves as a final condensing place for any traces of moisture that may escape from the larger reservoir.

The whole device, blowing tube, reservoir, and T piece may be fastened to a clamp, so that it can be secured on the edge of any table where blowpipe work is to be carried out. If the blowpipe is to be used with gas, the form of burner described under. "A Simple Form of Blowpipe"

will be found quite satisfactory.

_The Use of Oil, or Other Non-Gaseous Fuels._--Although gas, when available, is usually preferred on account of its convenience, there are several other fuels which give a hotter flame. They have, also, the additional advantage of not requiring any connecting pipes; but each has its own disadvantage.

One liquid fuel deserves special mention as being rather less desirable than the others; this is alcohol. Although very convenient in use, it has the disadvantage of being rather too highly inflammable and capable of burning without a wick, thus involving a certain fire risk; the flame is scarcely visible in a bright light, and the heat given by either the ordinary flame or the blowpipe flame produced from alcohol is considerably less than that from a similar flame in which coal gas is used. For small work, however, the facility with which a spirit lamp may be lighted may more than counterbalance these disadvantages at times.

_Paraffin Wax._--Where there is no coal gas available and the blowpipe is only required at intervals, and especially where high portability is required, there are few fuels so convenient as paraffin wax. This may be obtained in pieces of a satisfactory size by cutting paraffin candles, from which the wick has been withdrawn, into lengths of about half an inch. These cut pieces have the advantage over any oily fuel, such as colza oil, that they can be wrapped in paper or carried in a cardboard box; further they will keep indefinitely, even in the presence of air, without undergoing any perceptible change.

_Forms of Lamp for Paraffin Wax._--Probably, the best form is that devised by Thomas Bolas, and described by him in the _Journal of the Society of Arts_, December 2nd, 1898. This lamp consists of a small open tray of iron, through which pa.s.s three or more flat tubes, and between these tubes are placed small flat pieces of wick, the fit being such that the pieces of wick may be adjusted easily by means of a pair of pointed tweezers.

The flame thus obtained, instead of having one large hollow, is broken or divided so that the combustion is concentrated into a smaller area, and the air blast, which is directed across the flame, carries the flame with it in a more complete manner than is the case with the ordinary flame; a more thorough combustion being realised by this arrangement.

Another advantage is the ease with which the wick may be changed and a larger or smaller wick inserted to suit the flame to any size of air jet.

This form of lamp may be used for oily fuel, although it is specially suitable for paraffin wax.

Two small pieces of bent tin-plate may be used as side covers, and these serve to adjust the flame within certain limits. A tin-plate cover which fits easily over the whole lamp serves as an extinguisher. The complete lamp is shown by _d_, Fig. 16, and this figure shows also a quick-change air-jet device, the whole arrangement forming a blowpipe for use where a non-gaseous fuel is to be employed.

Although the lamp just described is desirable when complete control over the size of the flame is necessary, and if the ideal conditions and maximum heat are to be obtained, yet a simpler form of lamp will be found to give very good results. Such a lamp may consist of a flat tin tray, having a diameter of about three and a half inches and a depth of about one inch. In this tray is a tin support for the wick, and the wick itself may consist of a bundle of soft cotton, for example, a loosely rolled piece of cotton cloth, but in either case the top of the wick should be cut to approximately the same angle as that at which the blowpipe jet meets the flame.

In using paraffin wax as a fuel, it is necessary to see that sufficient wax reaches the wick to prevent charring during the first few minutes before the bulk of the wax is melted.

_Animal and Vegetable Oils._--Almost any oil may be used as a fuel, but many tend to become hard and gummy if allowed to stand in the air for any considerable time. When this happens, the wick becomes clogged and it is impossible to obtain a good flame. A number of the oils tend, also, to produce rather strongly smelling smoke.

_A Flame-Guard for Use With Non-Gaseous Fuels._--In order to avoid the eye-strain produced by the luminous base of the flame from a wick burning paraffin wax or oil, it is often advantageous to make a small tunnel of tin-plate, which can be rested on the sides of the lamp and rises over the top of the wick. Such a flame guard is shown by _e_, Fig.

16.

_Small Rods and Tubes from Gla.s.s Sc.r.a.p_:--It is scarcely practicable to make small quant.i.ties of good gla.s.s with the blowpipe flame as the only source of heat, but it is less difficult to make small rods or tubes from gla.s.s sc.r.a.p, and the ability to do this is sometimes of considerable value when a small tube has to be joined on to some special piece of apparatus made of gla.s.s of unknown composition. It may be possible to obtain some fragments of similar gla.s.s, either from a broken part of the apparatus or from a similar piece, and from these fragments small tubes or rods can be made.

The fragments of gla.s.s may be melted together on the end of a clay pipe-stem, care being taken to avoid trapping air bubbles as fresh fragments are added to the molten ma.s.s. When a sufficient quant.i.ty of gla.s.s has been acc.u.mulated, the viscous ma.s.s may be drawn out into a rod by bringing another pipe-stem into contact with the hot ma.s.s, rotating both pipe-stems steadily, and separating them until a rod of the desired size has been obtained.

If, on the other hand, it is desired to produce a tube from the ma.s.s of heated gla.s.s, the ma.s.s should be blown hollow before the pipe-stems supporting it are separated.

_Methods of Manufacture._--When the student has familiarised himself with the more common operations and processes used in gla.s.s-blowing, he will be in a position to increase his skill and knowledge of special methods by a critical examination of various examples of commercial work. There are few exercises more valuable than such an examination, combined with an attempt to reconstruct the stages and the methods by which the article chosen for examination was made.

Obviously, it is impossible to give full details of all constructions in a small text-book; but it is easy to give an example of the constructional methods employed in the making of almost any piece of light blown-gla.s.s apparatus, and these methods should prove of special value when apparatus of a new pattern has to be evolved for the purposes of research. That is to say, one designs the apparatus required, applies known methods of construction as far as possible, and, by the examination of commercial apparatus having similar features, evolves the new methods required. For an exercise in such a process of reconstruction we may well take an ordinary commercial vacuum tube, such as that shown by _a_, Fig. 17.

[Ill.u.s.tration: Fig. 17]

In the tube from which this drawing was made, it was found that the spiral in the middle bulb was of a slightly yellowish colour and gave a green fluorescence when the electric discharge was pa.s.sed through the tube; that is to say, the spiral is made of uranium-gla.s.s, which is usually a soda-gla.s.s containing trace of uranium, and hence differing slightly in composition from the ordinary gla.s.ses. The two enclosed tubes which are bent into a series of S bends gave a pink fluorescence, which indicates lead-gla.s.s; and the remainder of the tube fluoresced with an apple-green colour; this suggests ordinary soda-gla.s.s. We have, therefore, a piece of apparatus in which three dissimilar gla.s.ses are joined, while, at the same time, that apparatus contains a number of internal seals, and it is not probable that the dissimilar gla.s.ses will have their coefficients of expansion so nearly alike as to permit of a stable internal seal being made if one part of the seal consists of a gla.s.s differing from that of the other part.

These considerations lead us to a closer examination of the joins where the dissimilar gla.s.ses are introduced, and we find that in no case is the internal seal made between dissimilar gla.s.ses, but that a soda-gla.s.s extension is joined on to both the uranium-gla.s.s tube and the lead-gla.s.s tubes at a point about half an inch before the internal seal commences.

Careful examination of these joins shows that the change from one gla.s.s to another is not abrupt but gradual. Such a transitional joint may be made by taking a length of soda-gla.s.s tubing, sealing the end and fusing a minute bead of the other gla.s.s on to the sealed end, the end is then expanded and another bead of the other gla.s.s added, this bead is expanded and the operation is repeated, thus building up a tube, and, finally, the tube of the other gla.s.s is joined on to the end of this.

We are now concerned with the question of the insertion of the uranium-gla.s.s spiral into the bulb (see p. 38). Obviously the spiral is too large to pa.s.s through the necks of the bulb, and it is difficult to imagine that the spiral was obtained by the insertion of a length of straight tubing which was bent after entering the bulb; therefore, the only remaining method is that the spiral was made first and the soda-gla.s.s extensions fastened on, and that the bulb was blown, cut in halves and the spiral inserted, and the two halves were then rejoined.

That this was actually the case is confirmed by traces of a join which are just visible round the middle of the bulb. The insertion of the spiral and the making of the first internal seal are shown by _b_, and _c_.

There is one detail in making the second join of the spiral to the bulb which calls for attention, and the small branch, similar to an exhaustion branch, at the side of the bulb provides a clue to this. If an attempt were made to complete the second internal seal through a closed bulb it would be impossible to obtain a good result, as the air-pressure in the bulb would not be under control when once union was effected, and further heating of the air in the bulb would cause expansion and perforate the wall near the second internal seal; we therefore make a small branch which can be left open and through which such air-pressure as may be found necessary can be maintained.

The third join, by which the lead-gla.s.s tube is joined to the soda-gla.s.s is made in stages similar to those in which the soda-gla.s.s and uranium-gla.s.s were joined; but the internal seal is most conveniently made by sliding a length of tubing over the lead-gla.s.s and fusing this tubing to the large diameter soda-gla.s.s tube to which the lead-gla.s.s is already joined. The first stage of this operation is ill.u.s.trated by _d_.

When this seal is completed, the end of the soda-gla.s.s tube is drawn off and sealed as shown in _e_, and at this stage a side tube or branch is joined on. The sealed end of the outer and large diameter soda-gla.s.s tube is heated until it contracts and fuses to the enlargement that has previously been joined to the lead-gla.s.s tube, and the end is burst out as shown in _f_. Another length of soda-gla.s.s is then joined on to the burst-out end, and this length of soda-gla.s.s tubing is drawn out to a thin-walled contraction; the non-contracted part is expanded to form the bulb, and a small exhaustion branch made on the side, the drawn-out portion being cut off, and an electrode, previously prepared by coating a part of its length with a suitable enamel, is introduced. The tube is tilted to keep the electrode away from the drawn-out end, which is melted off and sealed. A small perforation is made with a hot platinum or iron wire in the sealed end, the electrode is shaken into position, and the sealing is completed as explained on page 42.

The remainder of the tube, that is to say the lead-gla.s.s tube and the bulb on the other side of the middle bulb, is completed in a similar manner.

SUMMARY OF CONDITIONS NECESSARY FOR SUCCESS IN GLa.s.s-BLOWING.

For the convenience of the student, it may be well to summarise the chief essentials for success in gla.s.s-blowing, and at the same time to add such brief notes on the various methods as may seem desirable.

_Adjustment of Blowpipe._--The air jet should be clean internally, and so centered as to give a flame having a well-defined blue portion, the tip of the flame should not be only slightly luminous but purple in colour. In the case of a blowpipe burning oil or wax fuel the flame may be a trifle more ragged without disadvantage.

_Bellows and Blowing._--The bellows should be adjusted to deliver air at constant pressure, either by insertion of a tap or, better, by attention to the wind reservoir if necessary. The movement of the foot in blowing should be steady, not jerky.

_Heating Gla.s.s._--The tube or rod should be heated cautiously until it has reached its softening point in its thickest part. Steady rotation of the gla.s.s during the heating is almost essential.