CUTTING AND BENDING GLASS—FORMING GLASS APPARATUS BEFORE THE BLOW-PIPE—MAKING AND GRINDING STOPPERS TO APPARATUS, ETC.
In the later pages of this Chapter it will be assumed that the operations first described have been mastered. The beginner should therefore practise each operation until he finds himself able to perform it with some degree of certainty. Generally speaking, however, after the failure of two or three attempts to perform any operation, it is best to give up for a few hours, and proceed to the work next described, returning to that upon which you have failed subsequently. If, unfortunately, it should happen that the work next in order involves the performance of the operation in which the failure has occurred, it is best to pass on to some later work which does not demand this particular accomplishment, or to rest a while, and re-attack the difficulty when refreshed.
Cutting Glass Tubes.—The simplest method of cutting a glass tube is to make a sharp scratch with a file across the glass at the point where it is desired to cut it, and on pulling apart the two ends, it will break clean off. It is important that the file be sharp. In pulling apart the ends the scratch should be held upwards, and the pull should have a downward direction, which will tend to open out the scratch. In the case of a large tube, a scratch will not ensure its breaking clean across. The tube must be filed to some depth, half-way, or even all round it. A goodway of breaking a tube is to place the file in the table after scratching the glass, to hold the glass tube above its edge with one hand on each side of the scratch, and to strike the under side of the tube a sharp blow upon the edge of the file, directly beneath the scratch. In this way very even fractures of large and moderately thin tubes may be made. It answers particularly well for removing short ends of tube, not long enough to hold; the tube is held firmly upon the file, and a sharp blow given to the short end with a piece of large tube or a key.
A file whose faces have been ground till they are nearly smooth, so as to leave very finely-serrated edges, will be found useful for cutting glass tubes. Such a file should be used almost as a knife is used for cutting a pencil in halves.
The simple methods just described are too violent to be applied to delicate apparatus, too tedious when employed upon the largest tubes, and very difficult to apply when the tube to be cut is very thin, or too short to permit the operator to get a good grip of it on either side of the file mark. In such cases, one or other of the following methods will be useful:—
1. Make a scratch with a file, and touch it with the end of avery smallpiece of glass drawn out and heated at the tip to its melting point. It is important that the heated point of glass be very small, or the fracture is likely to be uneven, or to spread in several directions. Also, it is best to use hot soda glass for starting cracks in tubes of soda glass, and lead glass for doing so in lead glass tubes. If the crack does not pass quite round the tube, you may pull it asunder, as previously described, or you may bring the heated piece of glass with which the crack was started to one end of the crack, and slowly move it (nearly touching the glass) in the required direction; the crack will extend, following the movements of the hot glass. Instead of hot glass, pastils of charcoal are sometimes employed for this purpose. They continue to burnwhen once lighted, and there is no need to re-heat them from time to time. They should be brought as close to the glass as is possible without touching it, and, when no longer needed, should be extinguished by placing the lighted end under sand, or some other incombustible powder, for they must not be wetted.
2. A method much practised by the makers of sheet glass, and suitable for large objects, is to wrap a thread of hot glass round the tube, at once removing it, and touching any point of the glass which the thread covered with water or a cold iron, when a crack will be started and will pass round the glass where it was heated by the thread.
3. Tubes which are large and slightly conical may have a ring of red-hot iron passed over them till it comes into contact with the glass, then, the iron being removed, and a point on the heated glass being at once touched with cold iron as before, it will break as desired. Or a string, moistened with turpentine, may be loosely twisted round the tube, and the turpentine ignited, afterwards the application of sudden cold to any point on the zone of hot glass will usually start a crack, which, if necessary, may be continued in the usual manner. The last three methods are chiefly useful in dealing with the largest and thickest tubes, and with bottles.
A fairly stout copper wire, bent into the form of a bow so that it can be applied when hot to a considerable surface of a glass tube, will be found superior to the point of hot glass or metal usually employed, for leading cracks in glass tubes. With such a wire a tube can be cut so that the cross section of the end is at any desired angle to the axis of the tube, with considerable precision. I am indebted for this suggestion to Mr. Vernon Boys and Dr. Ebert.
Bending Glass TubeFig. 6.
Fig. 6.
Bending Glass Tubes.—The blow-pipe flame is not a suitable source of heat for bending tubes, except in certain cases which will be mentioned in a subsequent paragraph.For small tubes, and those of moderate size, a fish-tail burner, such as is used for purposes of illumination, will answer best. Use a flame from one to two inches in breadth—fromAtoA(Fig. 6), according to the size of the tube which is to be bent. If the length of tube that is heated be less than this, the bend will probably buckle on its concave side.
The tube to be heated should be held in the position shown inFig. 6, supported by the hands on each side. It should be constantly rotated in the flame, that it may be equally heated on all sides. In the figure the hands are represented above the tube, with their backs upwards. A tube can be held equally well from below, the backs of the hands being then directed downwards, and this, I think, is the more frequent habit. It is difficult to say which position of the hands is to be preferred. I lately observed how a tube was held by three skilful amateurs and by a professional glass-blower. All the former held the tube with the hands below it. The latter, however, held it from above, as inFig. 6. He, however, was working with a rather heavy piece of tube, and I am inclined myself to recommend that position in such cases. During a long spell of work, the wrist may be rested from time to time by changing the position of the hands.
When the tube has softened, remove it from the flame, andgently bend it to the desired angle. The side of the tube last exposed to the flame will be slightly hotter, and therefore softer, than that which is opposite to it. This hotter side should form the concave side of the bent tube.
Bent Glass TubeFig. 7.
Fig. 7.
The exact condition in which the glass is most suitable for bending can only be learned by making a few trials. If it is too soft in consequence of being overheated, the sides will collapse. If, in the endeavour to heat the sideAofFig. 7a little more thanB,Bis insufficiently heated, the tube will be likely to break on the convex sideB. If the bent tube be likely to become flattened, and this cannot always be prevented in bending very thin tubes, the fault may be avoided by blowing gently into one end of the tube whilst bending it, for which purpose the other end should be closed beforehand. A tube already flattened may, to some extent, be blown intoshape after closing one end and re-heating the bent portion, but it is not easy to give it a really good shape.
When making a bend like that inFig. 7, to secure that the arms of the tubeCandD, and the curve atB, shall be in one plane, the tube should be held in a position perpendicular to the body, and brought into the position shown in the figure during bending, by which means it will be found easy to secure a good result. Lead glass tubes must be removed from the flame before they become hot enough to undergo reduction. If they should become blackened, however, the stain may be removed by re-heating in the oxidising flame (seep. 18).
When a very sharp bend is to be made, it is sometimes best to heat a narrow zone of the glass rather highly in the blow-pipe flame, and to blow the bend into shape at the moment of bending it, as previously described, one end having been closed for that purpose beforehand. Lead glass should be heated for this purpose in the oxidising flame (pp. 17to22).
The processes of bending large tubes, makingU-tubes and spiral tubes, are more difficult operations, and will be explained inChap. IV.
Rounding and Bordering the Ends of Tubes.—After cutting a piece of glass tube in two pieces, the sharp edges left at its ends should be rounded by holding them in a flame for a few moments till the glass begins to melt. The oxidising point of a pointed flame may be used for both kinds of glass. The flame will be coloured yellow by soda glass at the moment of melting. This indication of the condition of soda glass should be noted, for it serves as a criterion of the condition of the glass. The ends of soda glass tubes may also be rounded in the flame of a common Bunsen’s burner.
When the end of a tube is to be closed with a cork orstopper, its mouth should be expanded a little, orbordered. To do this, heat the end of the tube by rotating it in the flame till it softens, then remove it from the flame, at once introduce the charcoal cone (Fig. 5,p. 11), and rotate it with gentle pressure against the softened glass till the desired effect is produced. In doing this it is very important that the end of the tube shall be uniformly heated, in order that the enlargement shall be of regular form. If the tube cannot be sufficiently expanded at one operation, it should be re-heated and the process repeated.
Borders, such as are seen on test-tubes, are made by pressing the softened edge of the tube against a small iron rod. The end of the rod should project over the softened edge of the tube at a slight angle, and be pressed against it, passing the rod round the tube, or rotating the tube under the rod.
Sealing, that is closing the ends of tubes, or other openings, in glass apparatus.
In performing this and all the other operations of glass blowing, the following points must be constantly kept in mind:—
(a.) That it is rarely safe to blow glass whilst it is still in the flame, except in certain special cases that will be mentioned subsequently. Therefore always remove apparatus from the flame before blowing.
(b.) That when heating glass tubes, unless it is specially desired to heat one portion only, the tube must be constantly rotated in the flame to ensure that it shall be uniformly heated, and to prevent the tube or mass of glass from assuming an irregular form.
(c.) Always blow gently at first, and slowly increase the force applied till you feel or see the glass giving way. It is a good plan to force the air forward in successive short blasts rather than in one continued stream.
Sealing End of TubeFig. 8.
Fig. 8.
(d.) When it is necessary to force air into tubes of fine bore, such as thermometer tubes, the mouth must not be used, for moisture is thereby introduced into the tube, which it is very difficult to remove again in many cases. All tubes of very small bore should be blown with the aid of an india-rubber blowing-bottle, such as are used for spray-producers, Galton’s whistles, etc. The tube to be blown must be securely fixed to the neck of the bottle, which is then held in one hand, and air is forced from it into the tube as it is required. These bottles are frequently of service to the glass-blower—e.g., when tubes of very fine bore have to be united, it is necessary to maintain an internal pressure slightly exceeding that of the air throughout the operation, in order to prevent the viscous glass from running together and closing the tube. An india-rubber blowing-ball is very convenient for this purpose.
To seal the end of a glass tube (Fig. 8), adjust the flame so that it will heat a zone of glass about as broad as the diameter of the tube to be sealed (seeA,Fig. 8). Hold the tube on each side of the point where it is to be sealed in the manner described in the description of bending glass tubes (p. 28). Bring the tube gradually into the flame, and heat it with constant rotation, till the glass softens (for lead glass the oxidising flame must be used, as has been already explained).[6]When the glass begins to thicken, gently pull asunder the two ends, taking care not to pull out the softened glass too much, but to allow the sides to fall together, as shown atA. When this has occurred, heat the glass at the narrow part till it melts, and pull asunderthe two ends. The closed end should present the appearance shown atD. If the glass be drawn out too quickly its thickness will be unduly reduced, and it will present the appearance shown atB. In that case apply a pointed flame atb, and repeat the previous operation so as to contract the tube as atc, taking care not to allow the glass to become much increased nor decreased in thickness.
If a considerable mass of glass be left atd, it may be removed by heating it to redness, touching it with the pointed end of a cold glass tube, to which it will adhere, and by which it may be pulled away.
When the end of the tube presents the appearance shown in the diagramD, and the mass of glass atdis small, the small lump that remains must be removed by heating it till it softens, andgentlyblowing with the mouth, so as to round the end and distribute the glass more regularly, as shown inE. The whole end, from the dotted linee, must then be heated with constant rotation in the flame. If this final heating of the endebe done skilfully, the glass will probably collapse and flatten, as atF. The end must then be gently blown into the form shown atG.
If a flat end to the tube be desired, the tube may be leftin the condition shown byF, or a thin rounded end may be flattened by pressure on a plate of iron.
If a concave end be wished for, it is only necessary to gently suck air from the tube before the flattened end has become solid.
Waste Tube as HandleFig. 9.
Fig. 9.
In each case,immediatelyafter the tube is completed, it must be closely wrapped in cotton wool and left to cool. With good lead glass this last process, though advantageous, is not absolutely necessary; and as glass cools slowly when enveloped in cotton wool, this precaution may frequently be neglected in the case of apparatus made from lead glass.
In order to draw out tubes for sealing, close to one end, and thus to avoid waste of material, it is a good plan to heat simultaneously the end of the glass tubeAwhich is to be sealed, and one end of a piece of waste tubeEof about the same diameter, and when they are fused to bring them together as atDD(Fig. 9).Ewill then serve as a handle in the subsequent operations onA. Such a rough joint as that atDmust not be allowed to cool too much during the work in hand, orEandAmay separate at an inconvenient moment. Or the glass at the end of the tube may be pressed together to close the tube, and the mass of glass may be seized with a pair of tongs and drawn away.
Choking, or Contracting the Bore of a Glass Tube.—If it be not desired to maintain the uniformity of external dimensions of the tube whilst decreasing the diameter of the bore, the tube may be heated and drawn out as described in the description of sealing tubes onpp. 32-35. This may be done as shown atAorBinFig. 8, according to the use to which the contracted tube is to be put.
Unchanged External DiameterFig. 10.
Fig. 10.
Greater strength and elegance will be secured by preserving the external diameter of the tube unchanged throughout, as shown inFig. 10. For this purpose heat the tube with the pointed flame, if it be small, or in the brush flame if it be of large size, constantly rotating it till the glass softens and the sides show an inclination to fall together, when this occurs, push the two ends gently towardsA. If the tube should become too much thickened atA, the fault may be corrected by removing it from the flame and gently pulling the two ends apart till it is of the proper size. If the bore at the contracted part of the tube should become too much reduced, it may be enlarged by closing one end of the tube with a small cork, and blowing gently into the open end after sufficiently heating the contracted part. The tube should be rotated during blowing or the enlargement produced may be irregular.
Wide Tube with Narrow ExtremitiesFig. 11.
Fig. 11.
When the external diameter of the tube is to be increased as well as its bore diminished, press together the ends of a tube heated at the part to be contracted, as already described, and regulate the size of the bore by blowing into the tube if at any time it threatens to become too much contracted.
Widening Tubes.—Tubes may be moderately expanded at their extremities by means of the charcoal cone (seeBordering,p. 31). They may be slightly expanded at any other part by closing one end and gently blowing into the open end of the tube, after softening the glass at the part to be widened before the blow-pipe. But the best method of obtaining a wide tube with narrow extremities (Fig. 11) is to join piecesof narrow tubeAAto the ends of a piece of wider tubeBof the desired dimensions. The method of performing this operation is described underwelding, onpp. 39-47.
Piercing a TubeFig. 12.
Fig. 12.
Piercing Tubes.—The glass-blower very frequently requires to make a large or small opening in some part of a tube or other piece of apparatus. This is known as piercing. Suppose it is desired to make a small hole at the pointainA(Fig. 12). When the tube has been brought to the flame with the usual precautions, allow the end of the pointed flame to touch it atatill an area corresponding to the desired size of the opening is thoroughly softened. Then expand the softened glass by blowing to the form shown atB. Re-heata, blow a small globe as atC, and carefully break the thin glass, then smooth the rough edges by rotating them in the flame till they form a mouth like that ofD. Instead of leaving the bulb to be broken at the third stageC, it is a good plan to blow more strongly, so that the bulb becomes very thin and bursts, the removal of the thin glass is then accompanied by less risk of producing a crack in the thicker parts of the glass. Openings may be made in a similar manner in the sides of tubes or in globes, in fact, in almost any position onglass apparatus. If another tube is to be attached at the opening, it is a good plan to proceed to this operation before the tube has cooled down.
Inserting Platinum WireFig. 13.
Fig. 13.
The openings obtained by the method above described are too large when platinum wires are to be sealed into them. Suppose that it is necessary to pierce the tubeAofFig. 13in order to insert a platinum wire ata; direct the smallest pointed flame that will heat a spot of glass to redness on the pointa. When the glass is viscous, touch it with the end of a platinum wirew, to which the glass will adhere; withdraw the wire and the viscous glass will be drawn out into a small tube, as shown atB; by breaking the end of this tube a small opening will be made. Introduce a platinum wire into the opening, and again allow the flame to play on the glass at that point; it will melt and close round the wire. Before the hot glass has time to cool, blow gently into the mouth of the tube to produce a slightly curved surface, then heat the neighbouring parts of the tube till the glass is about to soften, and let it cool in cotton wool. Unless this is done, I find that glass tubes into which platinum wires have been sealed are very apt to break during or after cooling.
To ensure that the tube shall be perfectly air-tight, a smallpiece of white enamel should be attached to the glass atabefore sealing in the wire.
Uniting Pieces of Glass to Each Other, known as Welding, or Soldering.—The larger and more complicated pieces of glass apparatus are usually made in separate sections, and completed by joining together the several parts. This is therefore a very important operation, and should be thoroughly mastered before proceeding to further work.
In order to produce secure joints, the use of tubes made of different kinds of glass must be avoided. Soda glass may be joined securely to soda glass, especially if the tubes belong to the same batch, and lead glass to lead glass. But, though by special care a joint between lead glass and soda glass, if well made, will often hold together, yet it is never certain that it will do so.
To join two Tubes of Equal Diameters.—Close one end of one of the tubes with a small cork. Heat the open end of the closed tube, and either end of the other tube in a small flame until they are almost melted, taking care that only the ends of the tubes are heated, and not to let the glass be thickened; bring the two ends together with sufficient pressure to make them adhere, but not sufficient to compress the glass to a thickened ring. Before the joint has time to cool too much, adjust your blow-pipe for a pointed flame, if you are not already working with that kind of flame, and allow the point of the flame to play on any spot on the joint till it is heated to redness; rotate the tube a little so as to heat the glass adjacent to that which is already red-hot, and repeat this till the whole circumference of the rough joint has beenheated.[7]Repeat the operation last described, but, when each spot is red-hot, blow gently into the open end of the tube so as to slightly expand the viscous glass. Finally, rotate the whole joint in the flame till the glass is softened, and blow gently as before into the open end of the tube, still rotating it, in order that the joint may be as symmetrical as possible. If in the last operation the diameter of the joint becomes greater than that of the rest of the tube, it may be cautiously re-heated and reduced by pulling it out, or this may be secured by gently pulling apart the two ends, whilst the operator blows it into its final shape.
Joining Small TubesFig. 14.
Fig. 14.
When small tubes, or tubes of fine bore, are to be joined, in order to prevent the fused glass from running together and closing the tube, it is a good plan to border and enlarge the ends that are to be united, as atA(Fig. 14). Some glass-blowers prefer to border all tubes before uniting them.
When a narrow tube is to be joined to one that is only slightly wider, expand the end of the narrow tube till it corresponds in size to the larger tube. If the tube be too narrow to be enlarged by inserting a charcoal cone, seal one end and pierce it as directed (onp. 37).
For joining small thin-walled tubes Mr. Crookes recommends the use of a small Bunsen flame.
Joining Unequal Size TubesFig. 15.
Fig. 15.
In welding pieces of lead glass tube, take care that the heated glass is perfectly free from reduced lead at themoment when the two ends of viscous glass are brought into contact.
To join Tubes of Unequal Sizes End to End(Fig. 15).—Draw out the larger tube and cut off the drawn-out end at the part where its diameter is equal to that of the smaller tube, then seal the smaller tube to the contracted end of the larger according to the directions given for joining tubes of equal size. When a good joint has been made, the tube presents the appearance ofA,Fig. 15, the union being at aboutbb. Next heat the whole tube between the dotted linesaa, and blow it into the shape ofBin which the dotted lineddshould correspond to the actual line of junction of the two tubes.
In making all joints it is important to leave no thick masses of glass about them. If the glass be fairly thin and uniformly distributed, it is less likely to break during or after annealing under any circumstances, and especially if it has to bear alternations of temperature.
Joining a Tube to the Side of another Tube(Fig. 16).—One of the tubes must be pierced as atAinFig. 16(for the method, seep. 37), and its two ends closed with small pieces of cork. The edges of the opening, and one end of the other tube,must then be heated till they melt, and united by pressing them together. The joint may then be finished as before.
Making T-jointFig. 16.
Fig. 16.
A properly blown joint will not present the appearance ofB(Fig. 16), but rather that ofC. This is secured by directing the pointed flame upon the glass ataa(B) spot by spot, and blowing out each spot when it is sufficiently softened. If the tubes are large, the whole joint should subsequently be heated and blown, but in the case of small tubes this is of less importance. Finally it is to be wrapped whilst hot in cotton wool for the annealing process.
If a second tube has to be joined near to the first one, say atb, it is well to proceed with it before the joint first made cools down, and the joint first made, especially if soda glass be used, must be held in the flame from time to time during the process of making the second joint to keep it hot; if this be not done the first joint is very likely to break. A joint previously made may, however, be re-heated, if well made and well annealed.
A three-way tube, like that inFig. 17, is made by bendingA(Fig. 16) to an angle, and joiningBto an opening blownon the convex side of the angle; or,AofFig. 16may be bent as desired after attachingBin the ordinary way.
Three-Way TubeFig. 17.
Fig. 17.
Tubes may also be joined to openings made in the sides of globes or flasks; great care must be taken, however, especially if the walls of the globe be thin, to secure that the tube is well attached to the mouth of the opening when the melted ends are first brought into contact, for, with thin glass, any hole that may be left will probably increase whilst the joint is being blown into shape, owing to cohesion causing the glass to gather in a thickened ring round an enlargement of the original opening.[8]
In order to unite a tube of soda glass to a tube of lead glass, the end of the soda glass tube must be carefully covered with a layer of soft arsenic glass.[9]This must be done so perfectly that when the ends to be united are brought together the lead and soda glass are separated by the enamel at every point.
To Seal a Tube inside a Larger Tube or Bulb.—Suppose that an air-trap (3 ofFig. 18) is to be constructed from a small bulb (A) blown on a glass tube (1).
Sealing a Tube Inside a Larger TubeFig. 18
Fig. 18
Either cut off the tube close to the bulb atB, or better, remove the end by melting the glass and pulling it awayfromB, and then pierceAatB, No. 2, by heating the glass there and blowing out a small bulb as described under Piercing.
Prepare a tube (4) drawn out atEwith a bulb blown atD. InsertEinto the openingB, pressDwell against the mouthBand slowly rotate before the blow-pipe tillDadheres toB. Then heat and blow the joint spot by spot as in other cases, taking care that the glass is blown out on each side of the joint; lastly, heat the whole joint betweenaa, and blow it into its final shape.
These joints are very apt to break after a few minutes or hours if the glass ofDbe much thicker than that of the bulbA. They should be wrapped in cotton wool for annealing as soon as possible, as the rate at which the tubeEcools is likely to be less rapid than that of the parts of the apparatus which are more freely exposed to the air; therefore all such internal joints require very careful annealing, and they should always be made as thin as is consistent with the use to which they are to be put.
Tubes may also be sealed into the ends or sides of larger tubes by piercing them at the point at which the inserted tube is to be introduced, and proceeding as in the case of the air-trap just described.
Ozone generators of the form shown on next page (Fig. 19), afford an interesting example of the insertion of smaller tubes into larger.
On account of the small space that may be left between the inner and outer tubes of an ozone generator, and of the length of the inner tube, its construction needs great care. I find the following mode of procedure gives good results. Select the pieces of tube for this instrument as free from curvature as possible. For the inner tube, a tube 12 mm., or rather more, in external diameter, and of rather thin glass, is drawn out, as for closing, until only a very narrow tube remains atC, the end ofCis closed the arearoundCis carefully blown into shape, so that by melting offCthe tubeAwill be left with a well-rounded end. A small bulb of glass is next blown onAatB. This bulb must be of slightly greater diameter than the contracted endEof the larger tube (II.), so thatBwill just fail to pass throughE. The length fromBtoCmust not be made greater than fromEtoGon the outside tube. The end atCis then to be cut off so as to leave a pin-hole in the end ofA.
Ozone GeneratorFig. 19.
Fig. 19.
The outer tube (II.), whose diameter may be 5 or 6 mm. greater than that ofA, is prepared by sealing a side tube on it atF, after previously contracting the endE. For this purpose the endEshould be closed and rounded, and then re-heated and blown out till the bulb bursts. To ensure that the diameter of the opening is less than that of the tube, care must be taken not to re-heat too large an area of the end before blowing it out. It is very important that the crosssection atEshall be in a plane at right angles to the axis of the tube.
Wrap a strip of writing paper, one inch in breadth, closely round the end ofAatCtill the tube and paper will only just pass easily into the mouthDof the outer tube, push the inner tubeA, with the paper upon it, intoD, and when the paper is entirely withinD, withdrawA, and cautiously push the paper a little further into the outer tube. InsertAintoDEthroughE, so that the bulbBis embraced byE. CloseDwith a cork. Ascertain that the paper does not fit sufficiently tightly between the two tubes to prevent the free passage of air, by blowing into the mouthKofA. Air should escape freely fromEwhen this is done. Gradually bring the line of contact ofBandEand the surrounding parts of the tube before a pointed flame, after previously warming them by holding near a larger flame, and rotate them before the flame so that the glass may soften and adhere. Then heat the joint spot by spot as usual. In blowing this joint, take care that the glass on each side of the actual joint is slightly expanded. It should present the form shown by the dotted lines in III. (these are purposely exaggerated, however). Finally, heat the whole joint between the linesJItill it softens, and simultaneously blow and draw it into its final shape as seen at III.
The side tubeFshould not be too near the endE. If, however, it is necessary to have them close together, the jointFmust be very carefully annealed when it is made; it must also be very cautiously warmed up before the construction of the joint atHis begun, and must be kept warm by letting the flame play over it from time to time during the process of making the latter joint.
A good joint may be recognised by its freedom from lumps of glass, its regularity of curve, and by a sensibly circular line atH, where the two tubes are united.
When the joint after annealing has become quite cold, the pin-hole atCon the inner tube may be closed, after removing the paper support, by warming the outer tube, and then directing a fine pointed flame throughDon toC. And the endDof the outer tube may be closed in the ordinary manner, or a narrow tube may be sealed to it. As the end of glass atDwill be too short to be held by the fingers when hot, another piece of tube of similar diameter must be attached to it to serve as a handle (seep. 35,Fig. 9).
Blowing a Bulb or Globe of Glass.—For this purpose it is very important that the glass tube employed shall be of uniform substance. The size and thickness of the tube to be employed depends partly on the dimensions of the bulb desired, and partly on the size of neck that is required for the bulb. It is easier to blow large bulbs on large-sized tubes than on those of smaller size. When it is necessary to make a large globe on a small tube, it can be done, however, if great care be taken to avoid overheating that part of the small tube which is nearest to the mass of viscous glass from which the bulb is to be formed. For the purpose of blowing a very large bulb on a small tube, it is best to unite a wide tube to that which is to serve as the neck, as it will save some time in collecting the necessary mass of glass from which to form the globe.
Blowing a Bulb or GlobeFig. 20.
Fig. 20.
To blow a Bulb at the End of a Tube.—Select a good piece of tube, say 1·5 cm. in diameter, and about 30 cm. long; draw out one end to a light tail (a,Fig. 20) about 3 inches in length. Then heat up ashortlength of the tube atb, with a small brush flame, by rotating the glass in the flame, and gently press it together when soft to thicken it; blow into it if necessary to preserve the regularity of its figure. Repeat this process on the portion of tube nearest to that whichhas been first thickened, and so on, till as much glass has been heated and thickened as you judge will serve to make a bulb of the size desired. You should have a mass of glass somewhat resembling that shown atB(Fig. 20), but probably consisting of the results of more successive operations than are suggested in that diagram. Apply the flame as before to the narrower partsccofB, gently compress and blow until all the small bulbs first made are brought together into a mass still somewhat resembling the enlarged end ofB, but more nearly cylindrical, with the glass as regularly distributed as possible, and of such length fromdto the contracted part that the whole of it may easily be heated simultaneously with the large brush flame of your blow-pipe. Take great care in the foregoing operations not to allow the sides of the mass of glass to fall in and run together, and, on the other hand, do not reduce the thickness of the glass needlessly by blowing it more than is necessary to give the glass as regular a form as possible. When you are satisfied with the mass of glass you have collected, melt off the taila, andremove the pointed end of glass that remains, as directed on page 33. Turn on as large a brush flame as is necessary to envelop the whole mass of glass that you have collected, and heat it with constant rotation, so that it may gradually run together to the form seen atC(Fig. 20), taking care that it does not get overheated neard, or the tube which is to form the neck will soften and give way.
The position in which the mass of heated glass is to be held will depend upon circumstances; if the mass of glass be not too great, it is best to keep it in a nearly horizontal position. If the mass of glass be very large, it may be necessary to incline the endBdownwards; but as that is apt to result in an excess of glass accumulating towardsd, avoid doing so if possible by rotating the glass steadily and rapidly. If at any time the glass shows indications of collapsing, it must be removed from the flame and gently blown into shape, during which operation it may be rotated in the perpendicular position; indeed, to promote a regular distribution of the glass by allowing it plenty of time to collect, it is well from time to time to remove the heated mass of glass from the flame, and slightly expand it by blowing. Finally, when a regular mass of glass, such as is shown atC(Fig. 20) has been obtained, remove it from the flame, and blow it to its final dimensions. A succession of gentle puffsquicklysucceeding each other should be employed, in order that the progress of the bulb may be more easily watched and arrested at the right moment. During the process of blowing, the hot glass must be steadily rotated.
To collect the glass for blowing a bulb of lead glass, employ the flame described onpp. 17-22for heating lead glass.
If the tube be held horizontally whilst the globe is blown, its form will most nearly approach that of a true globe. If it be held in the perpendicular position, with the mass of glass depending from it, the form of the bulb will usually besomewhat elongated. If it be held perpendicularly, with the mass of glass upwards, the resulting bulb will be flattened.
When a bulb is not of a sufficiently regular form, it may sometimes be re-made by re-collecting the glass, and re-blowing it. The greatest care is needed at the earlier stages of re-heating to prevent the glass from collapsing into a formless and unworkable mass. This is to be prevented in all such cases by gently blowing it into shape from time to time whilst gathering the glass.