CHAPTER VII.

_VITREOUS SILICA._

=Introductory.=--Vitreous Silica was made in fine threads by M. Gaudin in 1839,[22] and small tubes of it were made in 1869 by M. A. Gautier, but its remarkable qualities were not really recognised till 1889, when Professor C. V. Boys rediscovered the process of making small pieces of apparatus of this substance, and used the torsion of "quartz fibres" for measuring small forces. More recently the author of this book has devised a process for preventing the "splintering" of quartz which gave so much trouble to the earlier workers, and jointly with Mr. H. G. Lacell, has produced a variety of apparatus of much larger dimensions than had been attempted =previously=. At the time of writing we can produce by the processes described in the following pages tubes 1 to 1.5 cm. in diameter and about 750 cm. in length, globes or flasks capable of containing about 50 cc., masses of vitreous silica weighing 100 grams or more, and a variety of other apparatus.

[22] A brief summary of the history of this subject will be found in _Nature_, Vol. 62, and in the Proceedings of the Royal Institution, 1901.

=Properties of Vitreous Silica.=--For the convenience of those who are not familiar with the literature of this subject, I may commence this chapter with a brief account of the properties and applications of vitreous silica, as far as they are at present ascertained. Vitreous silica is less hard than chalcedony, but harder than felspar. Tubes and rods of it can be cut with a file or with a piece of sharpened and hardened steel, and can afterwards be broken like similar articles of glass. Its conducting power is low, and Mr. Boys has shown that fine fibres of silica insulate remarkably well, even in an atmosphere saturated with moisture. The insulating qualities of tubes or rods of large cross sections have not yet been fully tested; one would expect them to give good results provided that they are kept scrupulously clean. A silica rod which had been much handled would probably insulate no better than one of glass in a similar condition. The density of vitreous silica is very near to that of ordinary amorphous silica. In the case of a small rod not absolutely free from minute bubbles it was found to be 2.21.

Vitreous silica is optically inactive, when homogeneous, and is highly transparent to ultraviolet radiations.

The melting point of vitreous silica cannot be definitely stated. It is plastic over a considerable range of temperature. Professor Callendar has succeeded in measuring the rate of contraction of fine rods in cooling from 1200 deg. to 1500 deg. C., so that its plasticity must be very slight below the latter temperature. If a platinum wire embedded in a thick silica tube be heated from without by an oxy-hydrogen flame the metal may be melted at temperatures at which the silica tube will retain its form for a moderate length of time, but silica softens to a marked extent at temperatures a little above the melting point of platinum.

It has been observed by Boys, Callendar, and others that fine rods of silica, and also the so-called "quartz fibres," are apt to become brittle after they have been heated to redness. But I have not observed this defect in the case of more massive objects, such as thick rods or tubes; and as I have repeatedly observed that mere traces of basic matter, such as may be conveyed by contact with the hand, seriously injure the surface of silica, and have found that silica quickly becomes rotten when it is heated to about 1000 deg. in contact with an infusible base such as lime, I am disposed to ascribe the above-mentioned phenomenon to chemical rather than to purely physical causes.[23] It is certain, however, that silica apparatus must never be too strongly heated in contact with basic substances. Silica is easily attacked by alkalis and by lime, less readily by copper oxide, and still less by iron oxide.

[23] In a recent communication Professor Callendar tells me that the devitrification commences at the outside and is hastened by particles of foreign matter.

The rate of expansion of vitreous silica has been studied by H. le Chatelier, and more recently by Callendar. The former found its mean coefficient of expansion to be 0.0000007 between 0 deg. and 10000 deg.,[24] and that it contracted when heated above 700 deg..

[24] The silica blocks used were prepared by fusion in an electric furnace; it is therefore probable that they were not quite pure.

Professor Callendar used rods of silica prepared by the author from "Brazil crystal"; these were drawn in the oxy-gas flame and had never been heated in contact with solid foreign matter, so that they consisted, presumably, of very pure silica. His results differ in some respects from those obtained by Le Chatelier, for he finds the mean coefficient of expansion to be only 0.00000059, _i.e._ about one seventeenth as great as that of platinum. Callendar found the rods of silica expanded very regularly up to 1000 deg. but less regularly above that temperature. Above 1200 deg. they contracted when heated.

The behaviour of vitreous silica under sudden changes of temperature is most remarkable. Large masses of it may be plunged suddenly when cold into the oxy-gas flame, and tubes or rods at a white heat may be thrust into cold water, or even into liquid air, with impunity. As a consequence of this, it is in one respect much more easily worked in the flame than any form of glass. Difficult joints can be thrust suddenly into the flame, or removed from it, at any stage, and they may be heated unequally in different parts with impunity. It is safe to say that joints, etc., in silica never crack whilst one is making them nor during the subsequent cooling. They may be set aside in an unfinished state and taken up again without any precautions. Therefore it is possible for an amateur to construct apparatus in silica which he would be quite unable to produce from glass.

The behaviour of vitreous silica with solvents has not yet been fully investigated, but Mr. H. G. Lacell has this subject in hand. If it behaves like the other forms of anhydrous silica it will withstand the action of all acids except hydrofluoric acid. It is, of course, very readily acted upon by solutions of alkalis and alkaline salts.

As regards the use of silica in experiments with gases, it must be remarked that vitreous silica, like platinum, is slightly permeable to hydrogen when strongly heated. One consequence of this is that traces of moisture are almost always to be found inside recently-made silica tubes and bulbs, however carefully we may have dried the air forced into them during the process of construction. Owing to the very low coefficient of expansion of silica, it is not possible to seal platinum wires into silica tubes. Nor can platinum be cemented into the silica by means of arsenic enamel, nor by any of the softer glasses used for such purposes. I have come near to success by using kaolin, but the results with this material do not afford a real solution of the problem, though they may perhaps point to a hopeful line of attack. Possibly platinum wires might be soldered into the tubes (see _Laboratory Arts_, R. Threlfall), but this also is uncertain.

The process of preparing silica tubes, etc., from Lumps of Brazil Crystal may be described conveniently under the following headings. I describe the various processes fully in these pages, as those who are interested in the matter will probably wish to try every part of the process in the first instance. But I may say that in practice I think almost every one will find it advantageous to start with purchased silica tubes, just as a glass-worker starts with a supply of purchased glass tubes. The manufacturer can obtain his oxygen at a lower price than the retail purchaser, and a workman who gives much time to such work can turn out silica tube so much more quickly than an amateur, that I think it will be found that both time and money can be saved by purchasing the tube. At the same time the beginner will find it worth while to learn and practise each stage of the process at first, as every part of the work described may be useful in the production of finished apparatus from silica tubes.

This being so, I am glad to be able to add that a leading firm of dealers in apparatus[25] has commenced making silica goods on a commercial scale, so that the new material is now available for all those who need it or wish to examine its properties.

[25] Messrs. Baird and Tatlock.

=Preparing non-splintering Silica from Brazil Pebble.=--The best variety of native Silica is Brazil Pebble, which may be obtained in chips or larger masses. These should be thoroughly cleaned, heated in boiling water, and dropped into cold water, the treatment being repeated till the masses have cracked to such an extent that they may be broken easily by blows from a clean steel pestle or hammer.

The fragments thus produced must be hand-picked, and those which are not perfectly free from foreign matter should be rejected. The pure and transparent pieces must then be heated to a yellow-red heat in a covered platinum dish in a muffle or reverberatory furnace and quickly plunged into a deep clean vessel containing clean distilled water; this process being repeated, if necessary, till the product consists of semi-opaque friable masses, very much like a white enamel in appearance. After these have been washed with distilled water, well drained and dried, they may be brought into the hottest part of an oxy-gas flame safely, or pressed suddenly against masses of white hot silica without any preliminary heating, such as is necessary in the case of natural quartz. Quartz which has not been submitted to the above preparatory process, splinters on contact with the flame to such an extent that very few would care to face the trouble and expense of working with so refractory a material. But after the above treatment, which really gives little trouble, all the difficulties which hampered the pioneer workers in silica disappear as if by magic.

=Apparatus.=--Very little special apparatus need be provided for working with silica, but it is absolutely essential to protect the eyes with very dark glasses. These should be so dark as to render it a little difficult to work with them at first. If long spells of work are undertaken, two pairs of spectacles should be provided, for the glasses quickly become hot enough to cause great inconvenience and even injury to the eyes.

Almost any of the available oxy-gas burners may be used, but they vary considerably in efficiency, and it is economical to obtain a very efficient burner. The 'blow-through' burners are least satisfactory, and I have long since abandoned the use of them. Some of the safety 'mixed-gas jets' have an inconvenient trick of burning-back, with sharp explosions, which are highly disconcerting, if the work be brought too near the nozzle of the burner. I have found the patent burner of Mr. Jackson (Brin's Oxygen Company, Manchester) most satisfactory, and it offers the advantage that several jets can be combined in a group easily and inexpensively for work on large apparatus. The large roaring flames such as are used, I understand, for welding steel are very expensive, and not very efficient for the work here described.

=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 coarse 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 glass.

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 masses 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 quantities 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 glass 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 passing 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 quantity of nitric peroxide. These may be removed by heating the tube and drawing filtered air through it, but not by washing, 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.

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 1.5 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 glass, 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 masses 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. Assn. 1890), and in Mr. Threlfall's Laboratory Arts.

INDEX.

Air-traps, 69. Annealing, 23. Apparatus needed for Glass-working, 11. Appendix, 82.

Beginners, Failures of, 22. Bellows, Position of, 3. ---- Various forms of, 7. _See also_ Blower. Bending Glass Tubes, 28. Blower, Automatic, 8. Blow-pipe, Cheap form of, 4. ---- Dimensions of, 4-5. ---- Fletcher's Automaton, 6. ---- Fletcher's Compound, 6. ---- Gimmingham's, 6. ---- Herapath's, 6. ---- Jets for the, 7. ---- Use of the, 8. _See also_ Flames. Blow-pipes, Use of several in combination, 21. Brush Flame, 9. ---- Oxidising, 20. Bulbs, Methods of blowing, 47.

Calibrating Apparatus, 76-81. Camphorated Turpentine, 11. Cetti's Vacuum Tap, 66. Charcoal Pastils, 11. Choking or Contracting the Bores of Tubes, 35. Combining the Parts of Complicated Apparatus, 61. Combustion Tube, how to work it, 25. Contracting the Bore of a Tube, 35. Cotton Wool for Annealing, 24. Cutting Glass Tubes, 26, 27, 28.

Dividing a Line into Equal Parts, 75.

Electrodes, 38, 55. Electrolysis, Making Apparatus for, 59.

Files for Cutting Glass, 27. Flame, the Pointed, 8. ---- the Brush, 9. ---- the Oxidising Brush, 20. ---- the Smoky, 10. Fletcher's Automaton Blow-pipe, 6. Fletcher's Compound Blow-pipe, 6. Funnels, Thistle-headed, 57.

Gimmingham's Blow-pipe, 6. Gimmingham's Vacuum Tap, 68. Glass, Annealing, 23. ---- Devitrification of, 15. ---- Method of Working with Lead, 17. ---- Method of Working with Soda, 22. ---- Nature of, 12. ---- Presenting to the Flame, 16. Glass Tubes, Bending, 28. ---- Bordering, 31. ---- Characters of good, 14. ---- Choking, 35. ---- Cleaning, 15. Glass Tubes, Cutting, 26, 27, 28. ---- Piercing, 37. ---- Purchase of, 12. ---- Sealing, 32. ---- Sealing Hermetically, 58. ---- Sizes of, 82. ---- Welding or Soldering, 39, 62. ---- Widening the Ends of, 36. Graduating Apparatus, 70. Grinding Stoppers, 51.

Herapath's Blow-pipe, 6. Hofman's Apparatus for Electrolysis, 59.

Inside Joints, 43.

Jets for Blow-pipes, 7. Joints, Air-tight, 64.

Lead Glass, Method of Working with, 17. Lead Glass, Blackening of, 17. Light, Effect of, in Working, 3. Line, to Divide into Equal Parts, 75.

Mercury Joints, Various, 64.

Non-splintering Silica, Preparation of, from Quartz, 88.

Ozone Generator, To Make an, 44.

Pastils of Charcoal, 11. Piercing Tubes, etc., 37. Platinum Electrodes, Sealing in, 38, 55. Pointed Flame, the, 9.

Quartz Fibres, 94.

Rounding Ends of Tubes, 31.

Sealing or Closing Openings in Tubes, 32. Side-tubes, Fixing, 41. Smoky Flame, 10. Soda Glass, Method of Working, 22. Soldering or Welding, 39, 62. Spiral Tubes, 56. Stoppers, Making and Grinding, 51.

Table for Glass-blower, 3. Taps, Vacuum, 65. Thistle-headed Funnels, 57. Traps, Air, 69. Tube, Combustion, how to work it, 25. Tubes. _See_ Glass Tubes. ---- T-, 41. ---- U-, 56. Turpentine, Camphorated, for Grinding, 11.

U-Tubes, 56.

Vacuum Taps, 65-68. ---- Tube, To Make a, 60. Vitreous Silica, Apparatus required for Making, 89. ---- Behaviour under sudden changes of Temperature, 87. ---- Bulbs, etc., Making Joints on, 93. ---- Expansion of, 86. ---- Hardness of, 85. ---- Insulating Power of, 85. ---- Melting Point of, 85. ---- Permeability to Gases, 87. ---- Properties of, 84. ---- Rods, Making Joints on, 94. ---- Tubes, Method of Making, 90. ---- Tubes, Making Joints on, 94.

Welding or Soldering Tubes together, 39, 62. White Enamel, Uses of, 39, 56. Widening the Ends of Tubes, 36. Working-place, 2.

Printed by T. and A. CONSTABLE, Printers to His Majesty at the Edinburgh University Press, Scotland