The flames produced are the long narrow blow-pipe flames used in blow-pipe a.n.a.lysis, and arranged so as to consist mostly of oxidising flame. The air-supply does not require to be large, nor the pressure high--5 to 10 inches of water will do--but it must be very regular.

The "trick" gla.s.s-blower I referred to employed a foot bellows in connection with a small weighted gasometer, the Westinghouse Company used their ordinary air-blast, and I have generally used a large gas-holder with which I am provided, which is supplied by a Roots blower worked by an engine.

I have also used a "velocity pump" blower, which may be purchased amongst others from Gerhardt of Bonn. The arrangement acts both as a sucking and blowing apparatus, and is furnished with two manometers and proper taps, etc. As I have reason to know that arrangements of this kind work very ill unless really well made, I venture to add that the Gerhardt arrangement to which I refer is No. 239 in his catalogue, and costs about three pounds. It hardly gives enough air, however, to work four blow-pipes, and the blast requires to be steadied by pa.s.sing the air through a vessel covered with a rubber sheet.

In default of any of these means being available, one of Fletcher's foot-blowers may be employed, but it must be worked very regularly. A table mounted with one blow-pipe made on this plan, and worked by a double-acting bellows, is recommended for students' use. For working flint gla.s.s, the air jet may be one-eighth of an inch in diameter and the pressure higher--this will give a brush flame. See Fig. 25.

It will be seen, on looking at the sketch of the blowpipe system, that the pair of blow-pipes farther from the observer can be caused to approach or recede at will by means of a handle working a block on a slide. It often happens that after using all four blow-pipes at once it is necessary to have recourse to one blow-pipe only, and to do this conveniently and quickly is rather an object. Now, in my arrangement I have to turn off both the gas and air from the farther system, and then put in a bit of asbestos board to prevent the nozzles being damaged by the flame or flames kept alight. As I said before, when some experience is gained, gla.s.sblowing, becomes a very simple art, and work can be done under circ.u.mstances so disadvantageous that they would entirely frustrate the efforts of a beginner. This is not any excuse, however, for recommending inferior arrangements.

Consequently, I say that the pipes leading in gas and air should be all branches of one gas and one air pipe, in so far as the two remote and one proximate blow-pipe are concerned, and these pipes should come up to the table to the right hand of the operator, and should have main taps at that point, each with a handle at least 2 inches long.

By this arrangement the operator can instantly turn down all the blow-pipes but one, while, if the inverse operation is required, all the three pipes can be started at once. [Footnote: I find, since writing the above, that I have been antic.i.p.ated in this recommendation by Mr. G. S. Ram, The Incandescent Lamp and its Manufacture, p.

114.]

The separate air and gas taps must be left for permanent regulation, and must not be used to turn the supply on or cut it off. In some respects this blow-pipe will be found more easy to manage than an oxygas blow-pipe, for the gla.s.s is not so readily brought to the very fluid state, and this will often enable a beginner who proceeds cautiously to do more than he could with the more powerful instrument.

Though I have mentioned gla.s.s nozzles for the air supply, there is no difficulty in making nozzles of bra.s.s. For this purpose let the end of a bra.s.s tube of about one-eighth of an inch diameter be closed by a bit of bra.s.s wire previously turned to a section as shown (Fig. 6), and then bored by a drill of the required diameter, say -.035 inch. It is most convenient to use too small a drill, and to gradually open the hole by means of that beautiful tool, the watchmaker's "broach." The edges of the jet should be freed from burr by means of a watchmaker's chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin, 1882, p. 232, -- 342), or by the alternate use of a slip of Kansas stone and the broach.

Fig. 6.

The construction of this blow-pipe is so simple, that in case any one wishes to use a brush flame, he can easily produce one simply by changing his air jets to bits of the same size (say one-eighth to one-sixteenth of an inch) tubing, cut off clean. To insure success, the ends of the tubes must be absolutely plane and regular; the slightest inequality makes all the difference in the action of the instrument. If a jet is found to be defective, cut it down a little and try again; a clean-cut end is better than one which has been ground flat on a stone. The end of a tube may, however, be turned in a manner hereafter to be described so as to make an efficient jet.

Several trials by cutting will probably have to be made before success is attained. For this kind of jet the air-pressure must be greatly increased, and a large Fletcher's foot-blower or, better still, a small double-action bellows worked with vigour will be found very suitable. A fitting for this auxiliary blow-pipe is shown in Fig. 5 at B.

Professor Roentgen's discovery has recently made it necessary to give more particular attention to the working of soft soda gla.s.s, and I have been obliged to supplement the arrangements described by a table especially intended for work with gla.s.s of this character. The arrangement has proved so convenient for general work that I give the following particulars. The table measures 5 feet long, 2 feet 11 inches wide, and is 2 feet 9 inches high.

Fig. 7.

It is provided with a single gas socket, into which either a large or small gas tube may be screwed. The larger tube is 5.5 inches long and 0.75 of an inch in diameter. The smaller tube is the same length, and half an inch in diameter. The axis of the larger tube is 3.5 inches above the table at the point of support, and is inclined to the horizontal at an angle of 12. The axis of the smaller tube is 2.5 inches above the surface of the table, and is inclined to the horizontal at the same angle as the larger one.

The air jets are simply pieces of gla.s.s tube held in position by corks. The gas supply is regulated by a well-bored tap. The air supply is regulated by treading the bellows--no tap is requisite.

The bellows employed are ordinary smiths' bellows, measuring 22 inches long by 13 inches wide in the widest part. They are weighted by lead weights, weighing 26 lbs. The treadle is connected to the bellows by a small steel chain, for the length requires to be invariable. As the treadle only acts in forcing air from the lower into the upper chamber of the bellows, a weight of 13 lbs. is hung on to the lower cover, so as to open the bellows automatically.

The air jets which have hitherto been found convenient are: for the small gas tube

(1) a tube 0.12 inch diameter drawn down to a jet of 0.032 inch diameter for small work;

(2) plain tubes not drawn down of 0.14 inch, 0.127 inch, and -0.245 inch diameter, and for the large gas tube, plain tubes up to 0.3 inch in diameter.

The table is placed in such a position that the operator sits with his back to a window and has the black calico screen in front of him and to his right. The object of the screen is to protect the workman against draughts. The table is purposely left unscreened to the left of the workman, so that long tubes may be treated.

-- 17. Other appliances which will be required for gla.s.s-blowing are of the simplest character.

(1) Small corks for closing the ends of tubes.

(2) Soft wax--a mixture of bees' wax and resin softened by linseed oil to the proper consistency, easily found by trial, also used for temporarily closing tubes.

(3) A bottle of vaseline for lubricating.

(4) An old biscuit tin filled with asbestos in shreds, and an asbestos towel or cloth for annealing gla.s.s after removal from the flame. As asbestos absorbs moisture, which would defeat its use as an annealing material, it must be dried if necessary.

(5) A Gla.s.s-Cutter's Knife. This is best made out of a fine three-cornered file, with the file teeth almost ground out, but not quite; it should be about 2 inches long. After the surface has been ground several times, it may be necessary to reharden the steel. This is best done by heating to a full red and quenching in mercury. The grindstone employed for sharpening the knife should be "quick," so as to leave a rough edge. I have tried many so-called gla.s.s knives "made in Germany," but, with one exception, they were nothing like so good as a small French or Sheffield file. In this matter I have the support of Mr. Shenstone's experience.

(6) A wire nail, about 2 inches long, mounted very accurately in a thin cylindrical wooden handle about 5 inches long by one-quarter of an inch diameter, or, better still, a bit of pinion wire 6 inches long, of which 1.5 inches are turned down as far as the cylindrical core, An old dentists' chisel or filling tool is also a very good form of instrument.

(7) A bit of charcoal about 3.5 inches long and 2 wide, and of any thickness, will be found very useful in helping to heat a very large tube. The charcoal block is provided with a stout wire handle, bent in such a manner that the block can be held close above a large gla.s.s tube on which the flames impinge. In some cases it is conveniently held by a clip stand. By the use of such a slab of charcoal the temperature obtainable over a large surface can be considerably increased.

I have seen a wine-gla.s.s (Venetian sherry-gla.s.s) worked on a table with four blow-pipes, such as is here described, with the help of a block of hard wood held over the heated gla.s.s, and helping the attainment of a high temperature by its own combustion.

(8) Several retort stands with screw clips.

(9) Some blocks of wood about 5" X 2" X 2" with V-shaped notches cut in from the top.

(10) A strong pair of pliers.

(11) An apparatus for cleaning and drying the breath, when blowing directly by the mouth is not allowable. The apparatus consists of a solid and heavy block of wood supporting a calcium-chloride tube permanently connected with a tube of phosphorus pentoxide divided into compartments by plugs of gla.s.s wool. Care should be taken to arrange these tubes so as to occupy the smallest s.p.a.ce, and to have the stand particularly stable. The exit tube from the phosphorus pentoxide should be drawn down to form a nozzle, from, say, half an inch to one-eighth of an inch in diameter, so as to easily fit almost any bit of rubber tube. The entry to the calcium chloride should be permanently fitted to about a yard of fine soft rubber tubing, as light as possible. The ends of this tube should terminate in a gla.s.s mouthpiece, which should not be too delicate.

As an additional precaution against dust, I sometimes add a tube containing a long plug of gla.s.s wool, between the phosphorus pentoxide and the delivery tube, and also a tube containing stick potash on the entry side of the calcium chloride tube, but it may safely be left to individual judgment to determine when these additions require to be made. In practice I always keep the affair set up with these additions. The communication between all the parts should be perfectly free, and the tubes should be nearly filled with reagents, so as to avoid having a large volume of air to compress before a pressure can be got up.

The arrangement will be clear by a reference to Fig. 8, which ill.u.s.trates the apparatus in use for joining two long tubes. I have tried blowing-bags, etc, but, on the whole, prefer the above arrangement, for, after a time, the skill one acquires in regulating the pressure by blowing by the mouth and lips is such an advantage that it is not to be lightly foregone.

Fig. 8.

-- 18. The Table.

The system of four blow-pipes is, of course, a fixture. In this case the table may be about a yard square, and may be covered with asbestos mill-board neatly laid down, but this is not essential. The table should have a rim running round it about a quarter of an inch high.

The tools should be laid to the right of the worker, and for this purpose the blow-pipes are conveniently fixed rather to the left of the centre of the table, but not so far as to make the leg of the table come so close to the operator as to make him uncomfortable, for a cheerful and contented spirit ought to be part of the gla.s.s-worker's outfit.

The most convenient height for a blow-pipe table--with the blow-pipes about 2 inches above the table top--is 3 feet 2 inches. Nothing is so convenient to sit upon as a rough music-stool with a good range of adjustment. The advantage of an adjustable seat lies in the fact that for some operations one wants to be well over the work, while in others the advantage of resting the arms against the table is more important.

-- 19. Special Operations.

The preliminary to most operations before the blow-pipe, is to draw down a tube and pull it out to a fine point. This is also the operation on which a beginner should exercise himself in the first instance. I will suppose that it is desired to draw out a tube about one-quarter of an inch in diameter, with the object of closing it, either permanently or temporarily, and leaving a handle for future operations in the shape of the point, thin enough to cool quickly and so not delay further work.

For this simple operation most of the gla.s.s-blower's skill is required. The tube must be grasped between the first finger and thumb of both hands, and held so that the part to be operated on lies evenly between the two hands. The distance between the operator's thumbs may conveniently vary from 2.5 to 4 inches. Releasing the grip of the left hand, let the operator a.s.sure himself of his ability to easily rotate the tube about its axis--by the right thumb and finger--he will incidentally observe by the "feel" whether the tube is straight or not.

A good deal of progress can be made from this point before the tube is heated at all. The operator can acquire a habit of instinctively rotating the tube by both hands, however the tube itself be moved about in s.p.a.ce, or however it be pushed or pulled. The habit of constant and instinctive rotation is literally about one-third of the whole art of gla.s.sblowing. It is unlikely, however, that the beginner will discover that he has not got this habit, until a few failures draw his attention to it.

The gla.s.s tube being held in position lightly yet firmly, and the operator being sure that he feels comfortable and at his ease, and that the blow-pipe flame (a single flame in this instance) is well under control, the preliminary heating may be commenced. With a tube of the dimensions given this is a very simple affair. Turn the air partly off, or blow gently, to get a partly luminous gas flame; hold the tube about an inch from the end of this flame, and turn it round and round till it commences to soften.

In the case of soda gla.s.s it is usual to employ the gas flame only, but I find that it is better in most cases to use the hot air of a gently-blown flame, rather than have the disadvantage of the soot deposited in the alternative operation. When the gla.s.s begins to soften, or even before, it may be moved right into the blow-pipe flame, and the latter may be properly urged.

It is not possible to give quite explicit and definite instructions, applicable to every case, as to when the time is ripe for pa.s.sing the work into the flame, but the following notes will indicate the general rules to be observed:-

(1) A thick tube must be warmed more slowly and raised to a higher temperature than a thin tube.

(2) The same remark applies to a tube of large diameter, as compared with one of small diameter, whatever the thickness.

(3) In the case of very large or thick tubes the hot air is advantageously employed at first, and to complete the preliminary heating, the luminous flame alone may be used. The object of this is to enable the operator to judge, by the presence of soot, its inability to deposit--or its burning off if deposited--of the temperature of the gla.s.s, and of the equality of this temperature all over the surface, for a large and thick tube might be heated quite enough to enable it to be safely exposed to the full heat before it is appreciably yielding to the fingers. In general, when the soot burns off freely, or lead gla.s.s begins to show the faintest sign of reduction, or soda gla.s.s begins to colour the flame, it is more than safe to proceed.

In order to turn on the full flame the operator will form a habit of holding the work in the left hand only, and he will also take care not to let anything his right hand may be doing cause him to stop rotating the tube with his left thumb and finger.