The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame Part 3

[Ill.u.s.tration: FIG. 8.]

When the end of the tube presents the appearance shown in the diagram _D_, and the ma.s.s of gla.s.s at _d_ is small, the small lump that remains must be removed by heating it till it softens, and _gently_ blowing with the mouth, so as to round the end and distribute the gla.s.s more regularly, as shown in _E_. The whole end, from the dotted line _e_, must then be heated with constant rotation in the flame. If this final heating of the end _e_ be done skilfully, the gla.s.s will probably collapse and flatten, as at _F_. The end must then be gently blown into the form shown at _G_.

If a flat end to the tube be desired, the tube may be left in the condition shown by _F_, 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.

In each case, _immediately_ after the tube is completed, it must be closely wrapped in cotton wool and left to cool. With good lead gla.s.s this last process, though advantageous, is not absolutely necessary; and as gla.s.s cools slowly when enveloped in cotton wool, this precaution may frequently be neglected in the case of apparatus made from lead gla.s.s.

[Ill.u.s.tration: FIG. 9.]

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 gla.s.s tube _A_ which is to be sealed, and one end of a piece of waste tube _E_ of about the same diameter, and when they are fused to bring them together as at _DD_ (Fig. 9). _E_ will then serve as a handle in the subsequent operations on _A_. Such a rough joint as that at _D_ must not be allowed to cool too much during the work in hand, or _E_ and _A_ may separate at an inconvenient moment. Or the gla.s.s at the end of the tube may be pressed together to close the tube, and the ma.s.s of gla.s.s may be seized with a pair of tongs and drawn away.

=Choking, or Contracting the Bore of a Gla.s.s 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 on pp. 32-35.

This may be done as shown at _A_ or _B_ in Fig. 8, according to the use to which the contracted tube is to be put.

[Ill.u.s.tration: FIG. 10.]

Greater strength and elegance will be secured by preserving the external diameter of the tube unchanged throughout, as shown in Fig. 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 gla.s.s softens and the sides show an inclination to fall together, when this occurs, push the two ends gently towards _A_. If the tube should become too much thickened at _A_, 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.

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 (see Bordering, 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 gla.s.s 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 pieces of narrow tube _AA_ to the ends of a piece of wider tube _B_ of the desired dimensions. The method of performing this operation is described under welding, on pp. 39-47.

[Ill.u.s.tration: FIG. 11.]

[Ill.u.s.tration: FIG. 12.]

=Piercing Tubes.=--The gla.s.s-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 point _a_ in _A_ (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 at _a_ till an area corresponding to the desired size of the opening is thoroughly softened. Then expand the softened gla.s.s by blowing to the form shown at _B_. Re-heat _a_, blow a small globe as at _C_, and carefully break the thin gla.s.s, then smooth the rough edges by rotating them in the flame till they form a mouth like that of _D_. Instead of leaving the bulb to be broken at the third stage _C_, it is a good plan to blow more strongly, so that the bulb becomes very thin and bursts, the removal of the thin gla.s.s is then accompanied by less risk of producing a crack in the thicker parts of the gla.s.s. Openings may be made in a similar manner in the sides of tubes or in globes, in fact, in almost any position on gla.s.s 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.

[Ill.u.s.tration: 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 tube _A_ of Fig. 13 in order to insert a platinum wire at _a_; direct the smallest pointed flame that will heat a spot of gla.s.s to redness on the point _a_. When the gla.s.s is viscous, touch it with the end of a platinum wire _w_, to which the gla.s.s will adhere; withdraw the wire and the viscous gla.s.s will be drawn out into a small tube, as shown at _B_; 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 gla.s.s at that point; it will melt and close round the wire.

Before the hot gla.s.s 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 gla.s.s is about to soften, and let it cool in cotton wool. Unless this is done, I find that gla.s.s 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 small piece of white enamel should be attached to the gla.s.s at _a_ before sealing in the wire.

=Uniting Pieces of Gla.s.s to Each Other, known as Welding, or Soldering.=--The larger and more complicated pieces of gla.s.s 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 gla.s.s must be avoided. Soda gla.s.s may be joined securely to soda gla.s.s, especially if the tubes belong to the same batch, and lead gla.s.s to lead gla.s.s. But, though by special care a joint between lead gla.s.s and soda gla.s.s, 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 gla.s.s be thickened; bring the two ends together with sufficient pressure to make them adhere, but not sufficient to compress the gla.s.s 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 gla.s.s adjacent to that which is already red-hot, and repeat this till the whole circ.u.mference of the rough joint has been heated.[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 gla.s.s. Finally, rotate the whole joint in the flame till the gla.s.s 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.

[7] Some gla.s.s-blowers at once work on the gla.s.s as next described, without this preliminary treatment. I find that some gla.s.s, usually soda gla.s.s, will not always bear the necessary movements without breaking unless first heated all round.

[Ill.u.s.tration: FIG. 14.]

When small tubes, or tubes of fine bore, are to be joined, in order to prevent the fused gla.s.s from running together and closing the tube, it is a good plan to border and enlarge the ends that are to be united, as at _A_ (Fig. 14). Some gla.s.s-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 (on p. 37).

For joining small thin-walled tubes Mr. Crookes recommends the use of a small Bunsen flame.

In welding pieces of lead gla.s.s tube, take care that the heated gla.s.s is perfectly free from reduced lead at the moment when the two ends of viscous gla.s.s are brought into contact.

[Ill.u.s.tration: FIG. 15.]

_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 of _A_, Fig. 15, the union being at about _bb_.

Next heat the whole tube between the dotted lines _aa_, and blow it into the shape of _B_ in which the dotted line _dd_ should correspond to the actual line of junction of the two tubes.

In making all joints it is important to leave no thick ma.s.ses of gla.s.s about them. If the gla.s.s be fairly thin and uniformly distributed, it is less likely to break during or after annealing under any circ.u.mstances, 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 at _A_ in Fig. 16 (for the method, see p. 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.

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

A properly blown joint will not present the appearance of _B_ (Fig. 16), but rather that of _C_. This is secured by directing the pointed flame upon the gla.s.s at _aa_ (_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 at _b_, it is well to proceed with it before the joint first made cools down, and the joint first made, especially if soda gla.s.s 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 in Fig. 17, is made by bending _A_ (Fig. 16) to an angle, and joining _B_ to an opening blown on the convex side of the angle; or, _A_ of Fig. 16 may be bent as desired after attaching _B_ in the ordinary way.

[Ill.u.s.tration: 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 gla.s.s, any hole that may be left will probably increase whilst the joint is being blown into shape, owing to cohesion causing the gla.s.s to gather in a thickened ring round an enlargement of the original opening.[8]

[8] If such an opening be observed, it may usually be closed by touching its edges with a fused point of gla.s.s at the end of a drawn out tube.

In order to unite a tube of soda gla.s.s to a tube of lead gla.s.s, the end of the soda gla.s.s tube must be carefully covered with a layer of soft a.r.s.enic gla.s.s.[9] This must be done so perfectly that when the ends to be united are brought together the lead and soda gla.s.s are separated by the enamel at every point.

[9] This can be obtained from Messrs. Powells, Whitefriars Gla.s.sworks.

_To Seal a Tube inside a Larger Tube or Bulb._--Suppose that an air-trap (3 of Fig. 18) is to be constructed from a small bulb (_A_) blown on a gla.s.s tube (1).

[Ill.u.s.tration: FIG. 18]

Either cut off the tube close to the bulb at _B_, or better, remove the end by melting the gla.s.s and pulling it away from _B_, and then pierce _A_ at _B_, No. 2, by heating the gla.s.s there and blowing out a small bulb as described under Piercing.

Prepare a tube (4) drawn out at _E_ with a bulb blown at _D_. Insert _E_ into the opening _B_, press _D_ well against the mouth _B_ and slowly rotate before the blow-pipe till _D_ adheres to _B_. Then heat and blow the joint spot by spot as in other cases, taking care that the gla.s.s is blown out on each side of the joint; lastly, heat the whole joint between _aa_, and blow it into its final shape.

These joints are very apt to break after a few minutes or hours if the gla.s.s of _D_ be much thicker than that of the bulb _A_. They should be wrapped in cotton wool for annealing as soon as possible, as the rate at which the tube _E_ cools 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.