The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame Part 8

=The method of making Silica Tubes.=--Before commencing to make a tube a supply of vitreous silica in rods about one or two millimetres in diameter must be prepared. To make one of these, hold a fragment of the non-splintering silica described above in the oxy-gas flame by means of forceps tipped with platinum so as to melt one of its corners, press a small fragment of the same material against the melted part till the two adhere and heat it from below upwards,[26] till it becomes clear and vitreous, add a third fragment in a similar manner, then a fourth, and so on till an irregular rod has been formed. Finally re-heat this rod in sections and draw it out whilst plastic into rods or coa.r.s.e threads of the desired dimensions. If one works carefully the forceps do not suffer much. I have had one pair in almost constant use for several years; they have been used in the training of five beginners and are still practically uninjured.

[26] This is to avoid bubbles in the finished gla.s.s.

The beginner should work with a gauge and regulator on the bottle of oxygen, and should watch the consumption of oxygen closely. A large expenditure of oxygen does not by any means necessarily imply a corresponding output of silica, even by one who has mastered the initial difficulties.

When a supply of the small rods of vitreous silica has been provided, bind a few of them round a rod of platinum (diameter say, 1 mm.) by means of platinum wires at the two ends and heat the silica gradually, beginning at one end after slightly withdrawing the platinum core from that end, till a rough tube about four or five centimetres in length has been formed. Close one end of this, expand it, by blowing, into a small bulb, attach a silica rod to the remote end of the bulb, re-heat the bulb and draw it out into a fine tube. Blow a fresh bulb on one end of this and again draw it out, proceeding in this way till you have a tube about six or eight centimetres in length. All larger tubes and vessels are produced by developing this fine tube suitably.

=Precautions.=--The following points must be carefully kept in mind, both during the making of the first tube and afterwards:--

(1) The hottest spot in the oxy-gas flame is at a point very near the tip of the inner cone of the flame, and silica can be softened best at this hot spot. The excellence of a burner does not depend on the size of its flame, so much as on the temperature of its "hot spot," and the success of the worker depends on his skill in bringing his work exactly to this part of the flame. Comparatively large ma.s.ses of silica may be softened in a comparatively small jet if the hot spot is properly utilised.

(2) Silica is very apt to exhibit a phenomenon resembling devitrification during working. It becomes covered with a white incrustation, which seems to be comparatively rich in alkali.[27] This incrustation is very easily removed by re-heating the whitened surface, provided that the material has been kept scrupulously clean. If the silica has been brought into the flame when dusty, or even after much contact with the hands of the operator, its surface is very apt to be permanently injured. _Too much attention cannot be given to cleanliness by the workman._

[27] The rock crystal exhibits a yellow flame when first heated in the oxy-gas flame, and most samples contain spectroscopic quant.i.ties of lithium.

(3) When a heated tube or bulb of silica is to be expanded by blowing, it is best not to remove it from the flame, for if that is done it will lose its plasticity quickly unless it be large. The better plan is to move it slightly from the "hot spot" into the surrounding parts of the flame at the moment of blowing.

It is best to blow the bulb through an india-rubber tube attached to the open end of the silica tube. At first one frequently bursts the bulbs when doing this, but holes are easily repaired by stopping them with plastic silica applied by the softened end of a fine rod of silica and expanding the lump, after re-heating it, by blowing. After a few hours'

practice these mishaps gradually become rare.

I find it a good plan to interpose a gla.s.s tube packed with granulated potash between the mouth and the silica tube. This prevents the interior of the tube from being soiled. The purifying material must not be packed so closely in the tube as to prevent air from pa.s.sing freely through it under a very low pressure.

It may be mentioned here that a finished tube usually contains a little moisture, and a recognisable quant.i.ty of nitric peroxide. These may be removed by heating the tube and drawing filtered air through it, but not by was.h.i.+ng, as it is difficult to obtain water which leaves no residue on the silica.

=Making larger tubes and other apparatus of Silica.=--In order to convert a small bulb of silica into a larger one or into a large tube, proceed as follows:--Heat one end of a fine rod of silica and apply it to the bulb so as to form a ring as shown in the figure. Then heat the ring and the end of the bulb till it softens, and expand the end by blowing. If this process is repeated, the bulb first becomes ovate and then forms a short tube which can be lengthened at will, but the most convenient way to obtain a very long tube is to make several shorter tubes of the required diameter, and say 200 to 250 mm. in length, and to join these end to end. It does not answer to add lumps of silica to the end of the bulb, for the sides of the tube made in this way become too thin, and blow-holes are constantly formed during the making of them.

These can be mended, it is true, but they spoil the appearance of the work.

[Ill.u.s.tration]

Tubes made in the manner described above are thickened by adding rings of silica and blowing them when hot to spread the silica. If a combination of several jets is employed, very large tubes can be constructed in this way. One of Messrs. Baird and Tatlock's workmen lately blew a bulb about 5 cm. in diameter, and it was clear that he could have converted it into a long cylindrical tube of equal diameter had it been necessary to do so.

Very thin tubes of 15 cm. diameter, and tubes of considerable thickness and of equal size, are easily made after some practice, and fine capilliaries and millimetre tube can be made with about equal readiness.

If a very fine tube of even bore is required, it may be drawn from a small thick cylinder after a little practice.

When a tube becomes so large that it cannot be heated uniformly on all sides by rotating it in the flame, it is convenient to place a sheet of silica in front of the flame a little beyond the object to be heated, in order that the former may throw back the flame on those parts of the tube which are most remote from the jet. A suitable plate may be made by sticking together small lumps of silica rendered plastic by heat.

The silica tubes thus made can be cut and broken like gla.s.s, they can be joined together before the flame, and they can also be drawn into smaller tubes when softened by heat.

In order to make a side connection as in a T piece, a ring of silica should be applied to the tube in the position fixed upon for the joint.

This ring must then be slightly expanded, a new ring added, and so on, till a short side tube is formed. To this it is easy to seal a longer tube of the required dimensions. It is thus possible to produce Geissler tubes, small distilling flasks, etc. Solid rods of silica are easily made by pressing together the softened ends of the fine rods or threads previously mentioned. Such rods and small ma.s.ses can be ground and polished without annealing them.

=Quartz Fibres.=--These were introduced into physical work by Mr. Boys in 1889. They may be made by attaching a fine rod of vitrified quartz to the tail of a small straw arrow provided with a needle-point; placing the arrow in position on a cross-bow, heating the rod of silica till it is thoroughly softened and then letting the arrow fly from the bow, when it will carry with it an extremely fine thread of silica. A little practice is necessary to ensure success, but a good operator can produce threads of great tenacity and great uniformity. Fuller accounts of the process and of the various properties and uses of quartz fibres will be found in Mr. Boys' lectures (Roy. Inst. Proc. 1889, and Proc.

Brit. a.s.sn. 1890), and in Mr. Threlfall's Laboratory Arts.