JEWELS AND STONES FOR INDUSTRIAL PURPOSES

JEWELS AND STONES FOR INDUSTRIAL PURPOSESAuthor(s): H. P. RooksbySource: Journal of the Royal Society of Arts, Vol. 94, No. 4722 (JULY 19th, 1946), pp. 508-525Published by: Royal Society for the Encouragement of Arts, Manufactures and CommerceStable URL: http://www.jstor.org/stable/41362423 .

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5<э8 journal of the royal society of arts July 19, 1946

Goodwin, Miss Aileen Mary, London. Green, Percy Albert, Coleshill, Warwick-

shire. Hall, Instructor Rear- Admiral Sir Arthur

Edward, k.b.e., c.b., London. Hall, Daniel George Edward, M.A.,

D.Lit., Caterham, Surrey. Hardingham, Trevlyn John, Darjeeling,

India. Heilbron, Sir Ian Morris, D.s.o., D.sc.,

LL.D., F.R.S., London. Hibberd, Andrew Stuart, Bickley, Kent. Hoppé, Emil Otho, London. Horstmann, Francis Cecil, Beckenham,

Kent. Innes, John Hosking, Newcastle-on-

Tyne. Johnson, Norman William, Clifton,

Bristol. Kidd, Franklin, m.a., d.sc., Great

Shelford, Cambridgeshire. Lambert, The Hon. Michael John,

London. Lo vat Fraser, Mrs. Grace, London. Lyon, Percy Hugh Beverley, м.с., m.a.,

Rugby, Warwickshire. Mackenna, Frederick Severne, m.a., m.b.,

Droitwich, Worcestershire. Morgan, Walter Charles, London. Nicolson, The Hon. Harold, c.m.g.,

London. Pearn, Professor Bertie Reginald, Ran-

goon, Burma. Polak, Henry Salomon Leon, London. Procter, Irwin, Nairobi, Kenya. Queensberry, The Most Hon. the

Marchioness of, London. Rees, Llewellyn, m.a., London. Roberts, Raymond William, Portsmouth,

Hants. Stocks, Mrs. Mary Danvers, b.sc.,

London. Stoye, Walter, Old Headington, Oxford. Straker, Ethelred Jean, London. Strudwick, Miss Ethel, o.b.e., m.a.,

London. Sumner, Fred, Welling, Kent. Sykes, Major-General the Right Hon.

Sir Frederick Hugh, p.c., g.c.s.i., G.C.I.E., G.B.E., K.c.B. , c.m.g. , London.

Udom, George Essien, Ibadan, Nigeria, British West Africa.

The following candidate was duly elected an Associate of the Society : -

Evans, Laurence E., London. The various Standing Committees were

appointed for 1946-47. Sir Harry A. F. Lindsay was elected a

Vice-President to fill the vacancy created by the resignation of Sir Geoffrey de Havilland.

In view of the rising cost of running the examinations, it was agreed, after consultation with the Examinations Committee, to increase the entrance fees of candidates.

It was reported that the total number of entries for the July series of examinations was 32,119.

It was decided that the meetings of the Society, other than those of the India & Burma and Dominions & Colonies Sections, should next Session commence at 5 p.m.

A quantity of formal and financial business was transacted.

PROCEEDINGS OF THE SOCIETY

SEVENTEENTH ORDINAR Y MEETING Wednesday, April 3RD, 1946

W. T. Gordon, m.a., d.sc., f.g.s., Professor of Geology y University of London, in the Chair

The Chairman : I suppose it is something of a cliché to say that this age is the most wonderful that has been. I have not the slightest doubt that prehistoric man said the same thing when he began making and exchanging stone tools. I think it is fair to say, however, that the age we are living in has shown more rapid progress than any other. To-day we are going to hear about some of the progress which has been made in precision work. Precision work is very characteristic of our times, and the particular line of precision work about which Mr. Rooksby is going to speak has reached a very high state of perfection. I shall not stand any longer between you and your lecturer, but call on him to address us.

The following paper was then read : -

JEWELS AND STONES FOR INDUSTRIAL PURPOSES

By H. P. Rooksby, b.sc., f.inst.p., of the Research Laboratories of The General Elect tic

Co. Ltd., Wembley y England

Introduction Since the beginning of the age of chemistry

and of scientific discovery many must have dreamed of the possibility of making precious stones and jewels. Certainly there are



July 19, 1946 JEWELS AND STONES FOR INDUSTRIAL PURPOSES 509

several records during the nineteenth century of attempts to make both sapphires and diamonds. These were real attempts to synthesise crystals, chemically and physically identical with those found naturally, and should be distinguished from efforts directed merely to imitate the genuine article.

It may perhaps be useful first of all to ask ourselves what are the factors that make a mineral valuable as a gem stone. One obvious reason for prizing crystals of a mineral highly is rarity, but this would not by itself place the mineral in the category of precious stones. The mineral must possess certain physical properties as well. All gems will not necessarily have all the desirable physical properties, but the most valued stones will have several.

Clarity, hue, high refractive index for light, and hardness are probably the most important of the properties that a gem stone should have. Of these, the last-mentioned is of considerable importance, because, once cut and polished, or otherwise finished, a jewel should not be easily scratched or blemished and a hard or tough stone will have advantages in this respect. Of natural minerals the diamond is the hardest known, and sapphire, though much less hard than diamond, is nevertheless near the head of the list.

Now for industrial applications, besides chemical stability, high hardness is the all- important quality required, so that the original objective to produce diamonds and sapphires synthetically for the gem trade has been reinforced by a much stronger incentive to produce for use in industry. It was not until the beginning of this century that the problem of the manufacture of synthetic sapphire was solved, and the attempts to make diamond have all except one met with complete failure. Indeed, even with this one exception there is still a difference of opinion as to the authenticity of the results, and attempts to repeat the original experiments have been unsuccessful.

In this lecture I shall endeavour to tell you of the manufacture of synthetic sapphire and its subsequent processing for industrial use. It will also be necessary to discuss some of the. characteristic features of the synthetic crystals. Finally mention will be made of the attempt to synthesise diamonds.

The Nature of Sapphire Let us understand immediately the nature

of the material we call sapphire. Sapphire,

ruby, corundum are all chemically and structurally similar, consisting of the a-crystal form of aluminium oxide. The term sapphire is an old one, probably Greek or Hebrew in origin ; it means " the scratching stone." It was in ancient times used to describe the characteristically blue and clear natural crystals of alumina valued as gem stones. The word corundum is more modern and is preferably reserved for other natural forms, white or opaque or otherwise less attractive. It has now become customary to describe as white sapphire single crystals of alumina of a high standard of perfection.

So blue sapphires and rubies are similar to white sapphires, but coloured by the presence of certain specific impurities which enter the crystal lattice.

In the applications with which we are concerned the single crystal character of sapphire stones, whether natural or synthetic, is important. For satisfactory working of the material the perfection of the crystal structure must be high, and stones that comprise a heterogeneous assemblage of crystal fragments are unacceptable.

The hardness of sapphire or corundum is 9 on Moh's scale, higher than quartz at 7, but lower than diamond at 10. It should not be imagined, though, that sapphire is nine-tenths as hard as diamond, for Moh's scale is not quantitative, and is simply an old accepted means of placing materials in order of hardness.

But, other than diamond, there are very few minerals harder than alumina ; because of this, powdered corundum is made and used extensively as an abrasive and polishing medium. Its high melting point of 2озо°С. has made it one of the most important refractories for high-temperature furnace work.

The hardness of natural stones suggests immediately that instrument jewels made of sapphire would give particularly good wearing properties. To-day the jewels of watches and the bearings of all good-quality instruments are cut from sapphire synthetically grown to be crystallographically and chemically similar to the best examples of the natural mineral. Obviously the ideal material to use for instrument and meter bearings would be diamond. The hardness of sapphire, however, though not as high as diamond, is adequate to give a very long life, and sapphire jewels have an enormous advantage in cost over diamonds-



510 JOURNAL OF THE ROYAL SOCIETY OF ARTS July 19, 1946

The Verneuil Process for the Synthesis of Crystals of Sapphire

Our aim then is to make large and homogeneous single crystals of alumina that can subsequently be cut and shaped as required. It might be imagined that the process of manufacture would not be very difficult. Would not, for instance, fusion of the alumina in a crucible and slow cooling produce suitable material ? Unfortunately the answer is not so simple. To be successful the growth of good single crystals of most substances can only be done if conditions are very carefully controlled, and with alumina there is the added difficulty of a very high melting-point.

Although fusion in a crucible does produce ■a corundum of coarsely crystalline structure, the product is multicrystalline and not homogeneous enough to be suitable for the purpose for which sapphires are employed. However carefully the cooling of the melt is controlled, it is practically impossible to pre- vent several crystal nuclei arising at different parts of the solidifying mass, and only one is allowable if the solid product is to be a single crystal.

It was Verneuili1) in 1902 who found the solution to the problem. He conceived the idea of building up the crystal from a single small nucleus and arranging matters so that no fresh nuclei could be formed.

He dropped finely-powdered alumina through the flame of an oxy-hydrogen torch on to a refractory support rod. The contact with the rod was reduced to the minimum possible area consistent with mechanical strength. From this point contact the single crystal was built up very slowly by feeding the powder through the flame in small increments, which were individually melted and merged with the parent crystal.

The basic form of Verneuil's apparatus has not been modified to any great extent to this day. Improvements have been made so that the crystals that can be grown now are much larger than the small ones with which Verneuil was content, but the principal features are unchanged.

The crystals are called boules , from the French word for " ball," because the original specimens made by Verneuil were spherical in shape. A diagram showing the chief features and arrangement of a modern form of the Verneuil furnace is shown in Fig. 1.

The finely-powdered raw material is held in a canister whose bottom is a 40-mesh sieve ; the container is like an inverted

pepper-pot. When the apparatus is in operation the canister is periodically shaken by taps from a small hammer located above it. The hammer is actuated by a cam mechanism, and the intervals between taps can be altered according to requirements.

With each tap a puff of powder passes through the sieve. Oxygen is fed into the annular space surrounding the powder container by means of a pipe, and the gas stream carries the powder downwards through a long tube to the furnace proper. Hydrogen is led into a larger concentric tube and mixes with the oxygen at the jet.

The furnace itself consists of two semi- cylindrical refractory bricks, recessed to form a vertical cylindrical tube about i£ in. in diameter. Through the lower end of this tubular furnace the refractory support enters, and the flame directs the powder down on to this support. The support can be raised or lowered by turning a handwheel. Small portions of the front of the bricks are cut away to form a narrow window for inspection purposes. The temperature of the flame can be changed within certain limits by altering the proportions of hydrogen and oxygen in the gas mixture.

The procedure in making an alumina boule is as follows. After the powder is placed in the canister and the flame is lit, the tapping mechanism is started and a small pile of powder is allowed to accumulate on the support. Next the temperature of the flame is raised until fusion occurs at the top of the pile. It is then necessary to allow this narrow fused portion to grow upwards a little to form the pip or stem of the boule. This takes a few minutes, after which the temperature of the flame is modified to cause the stem to enlarge and grow outwards. When a sufficiently large diameter has been reached, the boule continues to grow upwards slowly, and the furnace needs comparatively little attention apart from an occasional adjustment of the height of the refractory support. Fig. 2 is a view of the furnace with the bricks separated to expose a finished boule still attached to the support rod.

It should be understood that the mechanism of growth is such that just a very thin layer is built on to the crystal at a time. Only the top of the boule is in a molten condition in the furnace and, with each increment of powder from the canister, the underlying thin layer solidifies and assumes the structure and crystallographic orientation of the sapphire boule as a whole. For each



July 19, 1946 JEWELS AND STONES FOR INDUSTRIAL PURPOSES 51I

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512 JOURNAL OF THE ROYAL SOCIETY OF ARTS July I9, 1946

increment, time must be sufficient for fusion to occur and a general merging with the underlying material to take place before the next increment arrives. The rate of growth, therefore, cannot be taken above a certain value without impairing the homogeneity of the boule. It is found that boules that are grown too fast are liable to develop cracks and gross flaws, faults that are absent from specimens grown at the proper rate. The

Fig. 2. - Furnace for synthesis of sapphire crystals . The bricks have been separated to show finished boule attached to refractory Support ( Reptoduced by courtesy of G.E.C. Journal.)

maximum rate of growth for a good homogeneous crystal is approximately 15 grs. an hour, so that boules weighing between 45 and 60 grs. will occupy some three to four hours' furnace time.

A characteristic of alumina boules grown in the Verne uil furnace is that they can be readily split longitudinally into two portions. A boule may be split by tapping the stem lightly with a hammer whilst holding it in the palm of the hand. The split surface containing the axis of the boule is not a

crystallographic cleavage ; the splitting appears to be purely a question of the release of strain caused by the thermal distribution in the furnace. A satisfactory boule splits with a smooth surface showing the minimum of imperfections. (Fig. 3.)

Preparation of the Alumina Powder Control of growing conditions is not by

any means the complete answer to the production of good white sapphire crystals. The quality of the powder from which the crystal is grown has a most profound influence on the quality of the boule. It is not sufficient even to have powder of irreproachable purity, for certain physical properties are required as well. For instance, the powder must be of low bulk density, must have free- flowing properties, and must not pack down solidly in the container.

When manufacture was first begun in Britain it was anticipated that the appropriate powder would be obtainable from the Continent, but the German invasion of France cut off supplies of Swiss powder, and we had to commence an investigation into the physics and chemistry of the powder, with very little information from the original source. It was known that ammonium alum was used by Continental manufacturers, (2) but it was soon found that no commercial product would give alumina of sufficiently high purity to give colourless sapphire of the required quality.

To obtain alumina of the correct texture and consistency, the alum crystals are fired in silica trays in an electric or gas-fired muffle furnace at iooo°C. for about two hours. During decomposition the charge swells up quite spectacularly to a meringue-like cake (Fig. 4). Anhydrous alumina of a sponge-like consistency and in the y crystalline form remains. The sponge-like cakes of у alumina are finally broken down to fine powder by a tumbling process. (8)

A photomicrograph of the powder is shown in Fig. 5. It will be seen that each particle has a sponge-like texture, and this physical condition has to be retained if the material is to be suitable for use. The particles seen on the photomicrograph are not individual crystals, but aggregates of a highly porous structure. The true crystal size is very many times smaller, and X-rays show that the material is in fact у alumina, which, although fired at a temperature of iooo°C., still has not grown crystals larger than some 50 A.U. or 50X10-8 cm. across.




Fig. 3. - Typical sapphire boules , showing " splitting " ( Reproduced, by courtesy of G.E.C. Journal

Fig. 4. - Dish containing y alumina prepared by decomposition of ammonium alum at 1000 °C.



514 JOURNAL OF THE ROYAL SOCIETY OF ARTS July 1 9, 1 946

Properties of the Sapphire Boule Let us now direct our attention to some

of the crystallographic properties of the boule as grown in the Verneuil type of furnace. It has already been mentioned that the boule is a single crystal, and that it can be split into two halves by giving the stem a sharp blow with a hammer. This splitting does not mean that the boule has a twinned

Hexagonal crystals have four axes. Three of these are equal and intersect at 6o° to define the base of a hexagonal prism, whilst the fourth is the height of the prism (Fig. 6). The prism axis is commonly referred to as the optic axis, and it has three-fold symmetry. Its direction in a crystal of unknown orientation can be found either by optical means using polarised light or by X-rays.

Fig. 5. - Photomicrograph ( X275) of alumina powder used for synthetic sapphires (. Reproduced by courtesy of G.E.C. Journal.)

structure ; the two portions are simply parts of a single crystal which splits to release strain resulting from the manner of growth.

One of the most important features that has to be studied is the crystallographic orientation. Orientation in the boule is significant because, as will be discussed later, the wearing properties of finished jewels are thought to be considerably affected by the directions of the crystal axes in relation to the bearing surface. If the orientation in the boule is known, cutting can be so directed as to secure a favourable orientation in the finished jewels.

The crystal structure of a alumina or sapphire can, for our purposes, be thought of as being based upon the hexagonal system.

Fig. 6. - Diagram showing principal axes in hexagonal crystal system




The direction of the other axes, which have only two -fold symmetry and are therefore described as diad axes, can only be determined conveniently by X-ray methods. It will be seen that a hexagonal crystal would be completely defined by specifying one of the diad axes and the optic axis.

Orientation data have been collected for a large number of boules and it may be of interest to summarise the conclusions that have been reached. The first generalisation that may be made is that the optic axis invariably lies in the plane of splitting. On the other hand the direction does not seem

What is surprising is that the optic axis invariably lies in the splitting plane, and it is this direction we want to know when we come to make jewel bearings from the boule, as will be indicated later.

We may next consider how truly the synthetic boule conforms to the single crystal ideal. It is difficult to establish scientifically-based criteria of quality, and what is usually done is to base acceptance largely on the perfection and smoothness of the split surface. It is a general experience that boules showing rough or striated surfaces have a greater tendency towards

FIG. 7. - Half boules of synthetic sapphire , showing variations in character of split surface

to bear any steady relation to the long axis or direction of growth of the boule ; it lies in the plane of splitting, but may be oriented at any angle between o° and 90o to the direction of growth.

No such preferential orientation has been found for the directions of the diad axes. All angles to the split surface of the boule seem to be equally probable, though according to Winchell(4) some directions are more likely to be associated with rough fractures. Random disposition of the diad axes is not surprising, when it is remembered that the orientation of the boule as a whole must be largely a matter of chance. It is entirely determined by the chance orientation of the first crystal nucleus formed at the tip of a pile of polycrystalline material.

cracking during subsequent processing than boules exhibiting smooth and even split surfaces. In the worst cases the boule does not split regularly at all and is cracked in several places.

This generally happens if the rate of growth is excessive, although it is not the only factor, and the purity of the powder is of enormous importance. But there is little doubt that, other things being equal, there is a limiting rate of growth which cannot be exceeded if a homogeneous crystal showing a clean split is to be obtained with a reasonable degree of consistency. Fig. 7. is a photograph of several half boules with split surfaces of varying character and smoothness.

Another imperfection observed in some



5 16 JOURNAL OF THE ROYAL SOCIETY OF ARTS July 1 9, 1 946

synthetic crystals is that bubbles occur in parts of the specimen. These are thought to be due to insufficient refining of the layers of molten alumina during the merging with the main body of the structure. Gases are naturally very likely to be trapped as new material is added to the growing surface, and once the liquid material has solidified the remaining bubbles cannot escape. The presence of some bubbles can be tolerated, but large numbers are objectionable as they are likely to result in flaws in the bearing surface of the finished jewels.

Cutting and Slicing Boule for Instru- ment Jewels

We can next pass on to the cutting of the boule for practical use. The boule is first of

utilised for jewels for watches, hole jewels being a common form. The cavity is half- spherical, with a small-diameter hole of special shape, to act as an oil reservoir, passing right through the jewel at the bottom of the cavity. (Fig. 8.)

In examining the wearing properties of sapphire jewels there are two main , factors to be considered. One factor that appears to be of supreme importance is that of surface polish ; the other concerns the crystallographic orientation of the bearing surface.

The preparation of a highly-polished surface in the cavity of the jewel not only influences the wearing properties, but it also affects the initial value of the pivot-bearing friction. Polishing is usuälly done with diamond dust>

Fig. 8. - Diagrams representing three typical forms of sapphire jewel bearings all sliced perpendicular to its long axis. These slices are then cut so that pieces of square section are obtained. A third cutting operation gives segments of the required thickness.

The next step is to prepare the bearing surface. Originally the recesses or cavities were made by hand, but to-day machines effect a great economy of labour, and the complete mechanisation of the recessing and finishing processes is a practical proposition.

Besides the making of the cavity the jewel must be turned and the outside surface ground on a centre, so that the final shape is a short cylinder or disc. Dimensions will be determined by the particular application for which the jewels are intended, and a wide range of sizes is made.

The cavity may take a number of different forms. For instrument bearings Vee jewels, in which the cavity has the shape of a reversed cone, are used. For meter bearings the cup jewel, in which the cavity is spherical in outline, is employed. Other designs are

which is carefully selected and graded for the purpose.

Not a great deal has been established about the nature of the polish layer. G. F. Shotter,(5) reporting some electron diffraction investigations conducted by Professor Finch, suggests that there is a difference between the polish produced on gem stones by jewellers and that produced by experienced manufacturers of instrument and meter jewels. It would appear that with the most durable polish there is a flowed layer of polycrystalline structure, but with the polycrystalline material recrystallised or reoriented in alignment with the parent crystal.

Surfaces polished by jewellers, on the other hand, give little indication, so far as electron diffraction measurements can show, of departure from the single crystal condition.

Whatever the final interpretation may be, it is a matter of general agreement that the surface of the bearing recess must be perfectly smooth and show no flaws or imperfections. Inspection is usually carried




out under a binocular microscope with oblique illumination.

Early work done by Verney Stott(6) at the National Physical Laboratory pointed to the significance of crystallographic orientation in the jewel crystals. Stott's work was particularly directed to wear on meter jewels, for which the requirements are perhaps most stringent. Stott found that for maximum life and least wear it was necessary to select jewels whose orientation was such that the optic axis lay in, or near to, the tangent plane to the bottom of the bearing surface. If the optic axis was perpendicular or at an obtuse angle to the tangent plane, wear was much more severe and the life of the jewels was impaired. (Fig.

Fig. 9. - Diagram illustrating crystallographic orientation in sapphire meter jewels. The angle в should be between 80 0 and 90 o for long life

The probable explanation is as follows. Although crystals of alumina are said not to exhibit cleavage, they do have a weakness known as parting. The most prominent parting is along the basal plane, the crystal plane perpendicular to the optic axis. In other words, crystals of alumina tend to break by separation along the basal plane rather than along any other crystallographic plane. This tendency to a laminated structure is indicated in the diagrams of Fig. 10, which represent meter cup jewels of different orientations. When the optic axis lies perpendicular to the tangent plane at the base of the jewel cup, the o° jewel, the weak planes are in an unfavour- able orientation, and laminae from the surface may be most easily torn away by a revolving pivot. In the perpendicular orientation, the 90o jewel, the parting planes intercept the

bearing surface at a steep angle, and conse- quently fragmentation is not so likely to occur.

The photograph shown in Fig. 1 1 of some wooden models, originating with Mr. Shotter of the North Metropolitan Power Supply Co., gives another picture of the laminated structure in meter jewels and illustrates from another viewpoint the effects of varying orientation.

Once wear has begun, of course, the effect is cumulative, for debris collects in the cup, the friction rises and further abrasion of the surface occurs.

The effects of orientation, and the importance of ensuring correct location of the optic axis in particular, have been studied and confirmed in America by Goss.(7) More recently still, Shotter(6) has produced additional practical evidence from an elaborate series of tests. It would seem that a specified orientation must therefore be accepted as an essential feature of meter jewel selection. There is no real difficulty in meeting such a specification in the case of boule, because, as has been previously explained, owing to the peculiarities of boule orientation established by optical and X-ray methods, cutting procedure can be devised so as to ensure a satisfactory orientation in the finished jewel.

Synthetic Sapphire in Other Forms Although for some applications the boule

is a suitable form to handle, in the case of instrument and meter jewels a considerable proportion of the boule is wasted in cutting, and the cutting operations themselves are very expensive. If a single crystal of small circular cross-section could be synthesised, the cutting might be reduced to a relatively simple process of slicing off discs of the correct thickness, and a great economy of labour and material would be effected. These considerations have influenced development work in both this country and America, with the result that sapphire crystals can now be produced in rod form. It should be emphasised that the rods that can be made are just as much single crystals as the boule form itself. This may seem rather strange if we tend to think of crystals primarily in terms of external shape and symmetry. But when it is remembered that the faces of a naturally-occurring crystal are but an outward manifestation of an inner and more fundamental symmetry of atomic architecture, it will be understood that the external appearance of a synthetic



5*8 JOURNAL OF THE ROYAL SOCIETY OF ARTS July 19 , I946

Fig. 10.- Diagram illustrating " laminated " structure of sapphire jewels of different crystallographic orientations.

Fig. li. - Wooden models of sapphire meter jewels, illustrating changes in direction of" laminated " structure with crystal orientation. The orientations correspond with those in the diagrams of Fig. 10




crystal may be influenced to take on a wide variety of shapes and sizes.

The technique of growing single -crystal sapphire rod is similar to that of growing boules.

The rate of increase of height of rod is found to be limited to much the same rate as for boule synthesis. Whatever the cross - sectional dimensions of the crystal being grown, there seems to be a finite time required for each increment of melted alumina to merge with the underlying crystal, which must not be exceeded if a homogeneous structure is to be achieved. With small - diameter rods there is also a greater con- sumption of powder in proportion to the size than in the case of boule, because of wastage around the growing crystal. But this wastage is more than offset by the economy effected in the working processes, so that there is a final net gain in the manufacture of rod for a specific size of jewel bearing.

Having in mind that discs for jewel bearings are cut perpendicular to the long axis of the rod, it is relevant to ask how the crystal orientation, and in particular the direction of the optic axis, lies in single - crystal rod. The orientation conditions are similar to those that have been mentioned in connection with the cutting of boule. That is, the orientation is largely a matter of chance, being determined by the orientation of the first nucleus formed. In other words, there is no fixed relation between the optic axis and the longitudinal axis of the rod, and the angle between these two directions may have any value between o° and 90o.

Jewels cut from rod, therefore, must invariably reflect this variability in orientation, unless only a small proportion of the rods produced that happen to be of the required orientation are Used, or some method is found of controlling the orientation of the growing crystal.

An obvious technique for controlling this aspect of the structure is by seeding with a small crystal of the desired orientation and growing the rod on to this. The technique has been developed particularly in America, where it is claimed that, with the seed-crystal technique, rods can be grown consistently of the same predetermined orientation.(8)

Other Applications of Synthetic Sapphire Besides the use of synthetic sapphire for

jewel bearings there are a number of other purposes for which sapphire is or can be used. Sapphire knife edges for balances are

a great improvement on agate, and gramo- phone needles of sapphire have a very long life in suitably -designed equipment. Then there are extrusion and drawing processes for which sapphire dies can be very suitable, and gauges and spinning nozzles are two other applications worthy of mention. Thread guides of sapphire in the shape of pigtails or hooks formed of rod of circular section constitute another modern development^9)

Coloured Sapphire Boule Alumina can take several impurities into

solution , and certain of these impurities confer attractive colours upon the crystals. Coloured boule is perhaps mainly valued for gem stone purposes, but there are some practical industrial uses for coloured jewels. For instance, the colour may be useful for purposes of distinction and differentiation between jewels to be employed in different parts of an instrument, and watchmakers often have a preference for rubies because they are more easily seen than white sapphires.

For gem stones the exact shade or hue of colour must be carefully controlled. With cut stones the gemnologist has very definite views on the precise hue which will be generally accepted as attractive and valuable. The colour may not be equally important in industrial applications, but a good depth of colour is desirable. Red or ruby, and blue, are those most used, though with suitable ingredients a whole range, to include various shades of red, orange, green and blue, may be made.

In red sapphire or ruby the colouring impurity is chromium, and amounts from about one-half of 1 per cent, to as much as 5 per cent, of chromium oxide may be incorporated in the alumina powder. The chromium oxide actually dissolves in the alumina crystal lattice, and a lattice expansion occurs.

With the chromium oxide treated aluminium oxide powder, synthetic rubies are grown in the high-temperature furnace in much the same way as white sapphires.

The best blue sapphire is made from powder in which two impurities are incorporated, namely iron oxide and titania. Iron oxide alone gives a green crystal, whilst a blue of less attractive hue is made with cobalt oxide as the impurity.

Fabrication and working of coloured boules is conducted along thé sáme lines as for white sapphire.



520 JOURNAL OF THE ROYAL SOCIETY OF ARTS July 19, 1946

Synthetic Spinel Besides coloured sapphire boules another

kind of crystal of different composition may be grown similarly in the conven- tional Verneuil furnace. These crystals are known, as spinels and have a composition similar to that of the mineral spinel. Ideally the spinel crystal contains equi- molecular amounts of magnesia and alumina, but, since spinel and alumina are to some extent mutually soluble in one another, the term spinel, when used to describe synthetic crystals, does not necessarily refer to the one-to-one molecular composition. In fact, it is quite common to make spinels containing three or four molecular parts of alumina.

The structure of spinel differs from that of alumina. It conforms to the cubic crystal system, and magnesium aluminate is the analogue of a whole series of crystals of similar structure, amongst which is the well- known magnetite, the magnetic oxide of iron.

y alumina has ideally a spinel-like structure, and it is probably best to regard the spinels containing more than one molecular part of alumina as solid solutions of y alumina with Mg0.Al203.

Powder for growing spinel boules is made in a similar manner to that of sapphires, except that an appropriate amount of magnesia is incorporated in the alum.

The growth of spinel boules of good quality appears to be a little more difficult to control than the growth of sapphires. The boules tend to become opaque if grown too fast, cracks are more frequent, and consistent results are only obtained if the size is kept to comparatively small dimensions. The boules do not usually split, so that single crystals of quite large section result.

The synthesis of the crystals is more easily achieved if colouring impurities are present, and blue spinels are commonly manufactured.

Spinel is appreciably less hard than sapphire. The mineral is put at 8 on Moh's scale, but it is probable that the synthetic crystals vary in hardness according to the amount of alumina in the composition. Because of their comparative softness spinels have the advantage over sapphires of being more easily worked, and when the lower hardness can be tolerated spinel crystals are often employed instead of sapphires. It is possible, for instance, that for some applications there may be a need for a relatively tough crystal which does not exhibit the crystallographic parting characteristic of

synthetic corundum. Single crystals of the spinel composition might fulfil this need.

Synthetic Diamonds The possibility of the synthesis or manu-

facture of diamond is even more attractive than that of the manufacture of sapphire. Quite apart from its gem stone value, for industrial applications the diamond is without parallel in hardness and would thus be very widely used were a plentiful supply of synthetic stones available. The newest synthetic sintered carbides still fall far short of diamond in hardness.

The story of the attempts to make diamond is one of continual frustration and disappointment, except in one single instance. Obviously the kind of technique employed for synthetic corundum would be useless. It is quite evident from the conditions and surroundings in which natural diamonds are found, that crystallisation must have occurred under extreme pressure and probably at very high temperatures.

There are three main historical attempts at synthetic preparations, namely those of Hannay(10) in 1880, of Moissan(n) in 1893, and of Sir Charles Parsons(12) during the 1914-18 war. Both Hannay and Moissan believed that they had succeeded in making small fragments of diamond. Later experiments repeating Moissan's method produced fragments similar to those described by him, but these were shown not to be diamonds. More recently, during the war years, an elaborate investigation was conducted in Germany of the Moissan experiments, but no success was obtained.

Unfortunately all of Moissan's original fragments appear to have been lost, but a collection of fragments, said to constitute the outcome of Hannay's experiments, found its way into the Natural History Museum at South Kensington. This collection has been kept for many years, but until recently the stones have not been subjected to such tests as could conclusively demonstrate their nature.

Indeed, it has been a difficult problem to provide proof of identity without destruction, but it is possible to do so to-day by employing X-ray diffraction technique.

Mr. F. A. Bannister and Mrs. Kathleen Lonsdale(14) in 1943 made an X-ray examination of the Hannay stones and proved very decisively that eleven of the twelve are diamonds. Moreover, they were able to show that at least one of the stones had a




structure characteristic of the rare type II class. Type II diamonds have a mosaic, less perfect, structure than the more numerous type I diamonds.

The two types were originally differentiated by Sir Robert Robertson(15) during an examination of the absorption characteristics in the infra-red. Bannister and Lonsdale venture the opinion that the occurrence of type II diamonds in the Hannay collection is additional confirmation of their synthetic origin. They argue that crystals formed under laboratory conditions are much more likely to be mosaic than perfect in crystallographic structure.

There is no doubt then that the fragments in the Hannay collection are true diamonds. The only remaining point at issue is whether the fragments are those that were actually formed in Hannay's experiments. Bannister and Lonsdale feel that the weight of evidence supports the view that they were, but others have seen fit to question the conclusion. Until Hannay's or Moissan's experiments are successfully repeated, or another method is devised that produces similar identifiable stones, the matter cannot be settled unequi- vocably. It has to be remembered that Sir Charles Parsons, after a great amount of effort and expenditure on Hannay's technique, failed to obtain anything identifiable as diamond, and the report of the German efforts by Moissan's method is not encouraging.

Natural Diamonds for Industrial Use There are an immense number of different

varieties of natural diamond. Indeed, one is almost justified in claiming that no collection of natural stones will contain two alike in every respect. There are manifest differences in size, shape, colour and perfection that will be apparent on mere visual inspection, and other tests will reveal differences in density, chemical purity, homogeneity of structure and so on. Under ultra-violet light also, differences in behaviour are seen ; some stones fluoresce strongly, whilst others do so but feebly, or not at all.

Let us examine for a moment some of the properties that should be possessed by diamonds for industrial applications, and in particular by stones to be made into dies for wire-drawing. The largest and whitest diamonds are naturally enough selected and reserved for use as gem stones. For a given application, moreover, the question of cost must be a foremost consideration, and it

would be uneconomic to use stones of high gem value. But there is no hard-andrfast division between gem and industrial diamonds.

To be of practical use for fine wire-drawing dies the diamonds must have certain physical properties. Shape is of primary importance, because one wants only to do the minimum amount of preliminary work before com- mencing drilling. This is partly a question of economics, as processing and shaping diamonds is slow and costly. It is most convenient, therefore, to handle stones of roughly even dimensions in all directions, and a useful form is that known as a rhombic dodecahedron. Colour does not matter,

Fig. 12. - Electron micrograph of diamond dust for polishing purposes.

unless it is so deep as to affect transparency. The colour must not hinder close observation by microscopic inspection of the progress of drilling and correctly shaping the die hole. Stones having foreign inclusions or internal cracks or flaws are not acceptable.

No special care is taken to orient the direction of drilling at a particular angle to the crystallographic axes. With fine dies it does not seem that wear is much influenced by the direction of the die hole in relation to the crystal axes. It may of course be convenient to utilise stones of a particular orientation because of considerations of handling and observation of the course of drilling. With large -diameter dies, in which the die hole may form a large cavity in the stone, leaving the walls relatively thin, the crystal orientation may become of greater importance. It may then be necessary to avoid directions that will make the stone



522 JOURNAL OF THE ROYAL SOCIETY OF ARTS July 19, I946

liable to " burst " by splitting along cleavage directions.

Probably the largest use of diamonds is for the manufacture of diamond tools, utilised for a wide variety of purposes, including the cutting of metals, glass and other materials. No rock-drilling operations of any size are done without diamonds, and the drilling of oil-wells is an important application. Powdered diamond is the essential ingredient in bonded wheels and discs used for cutting and grinding purposes ; for example, in the fabrication of quartz and sapphire. It is the material which has to be used in drilling and shaping diamonds themselves and a specially fine grade of powder (Fig. 12) is an incomparable medium for surface-polishing such hard materials as sapphire.

It is an arresting fact that three-fourths of the world's yearly production of diamonds are used for industrial purposes, only one- fourth being utilised for gem stones.

Conclusion I have attempted in this discourse to tell

you about the synthesis and uses of single crystals of sapphire. We have also glanced at the failures to make diamonds, and seen that industrial use is for a long time to come likely to be based upon the natural stone.

It is worth recalling that before the 1939 war synthetic-jewel production was largely in the hands of Swiss, German and French manufacturers. During the War years a British and later an American industry of considerable size have been developed. The rod form has more recently received particular attention in the United States (ie). Russian production of synthetic sapphire boules has also been successfully established, the technique being essentially that of Verneuil, although little detailed information is available.

In this country no synthesis of sapphire had been attempted prior to 1939, but to ensure the supply of jewel bearings an early decision was taken by the Ministry of Aircraft Production to establish manufacture here. Arrangements were made for setting up plant similar to that employed in Switzerland. There were many difficulties to be overcome, not the least of which Was the severance in 1940 of access to Swiss technical information. It was only by careful co-ordination of research and manufacturing experience that successful progress was made,(17) and the

British sapphire boule is now the equal of that of Swiss origin.

From the scientist's point of view synthetic sapphires are much to be preferred to natural stones. Naturally-occurring material must inevitably be liable to wide variations in quality. There is a much better guarantee of uniformity of quality in crystals manufactured under controlled conditions.

Consistency of quality and precisely predictable physical characteristics constitute the considerable advantages that would result should diamonds be successfully synthesised. I feel confident that the problem will be solved eventually. With the release of atomic energy almost anything seems possible, and practical difficulties have never really frightened the scientist once he has made the decision to overcome them.

Finally, may I give some very necessary acknowledgments .

In the first place, I should like to make it perfectly clear that I am merely the spokes- man for a group of people who have been working on this subject. Without their generous help this talk could not have been given. In particular, I would like to mention my colleagues, Mr. Chirnside and Mr. Dauncey, who are largely responsible for the investigations on sapphire, and Mr. Leeds who conducts the activities on diamonds.

I have also to thank Miss Anthony for the time and effort she has spent on the preparation of the film.

REFERENCES i. M. A. Verneuil, Annales de Chimie et de Physique , 3, 20 (1904). 2. E. G. Sandmeier, Jour. Inst . Elee. Eng., 72, 505 (i933)- 3. British Patent No. 563636. 4. H. Winchell, Amer. Mineralogist, 29, 399 (i944)- 5. G. F. Shotter, Rep. Brit. Elee. Allied Indust. Res. Assoc., Ref. T/T39 (1944). 6. Verney Stott, Collected Researches of the National Physical

Laboratory, England, 24, 3 (193 1). 7. J. H. Goss, Elee. Eng. Trans., 60, 811 (1941). 8. British Patent No. 17914 (1944). 9. Product Engineering 16, 429 (1945). 10. J. B. Hannay, Proc. Roy. Soc., 30, 450 (1880), 32, 407 (1881). li. H. Moissan, Der elektrische Ofen, Berlin (1900). 12. С. A. Parsons, Phil. Trans. Roy. Soc., 220, 67 (1919). 13. P. L. Guenther, P. Geselle, W. Rebentisch, Zeits fur anorg. Chemie, 250, 357 (1943), see also Industrial Diamond Review 6, 42 (1946). ' ' 14. F. A. Bannister, K. Lonsdale, Mineral. Mag. Lond.f 26, 315 (1943). 15. R. Robertson, J. J. Fox, A. E. Martin, Phil. Trans .

Roy. Soc., 232, 463 (1934), and Proc. Roy. Soc., 157, 579 (1936). 16! Industrial Diamond Review 6, 80 (1946). ' 17. K. W. Brown, R. C. Chirnside, L. A. Dauncey and H. P. Rooksby, G.E.C. Journal, 13, 53 (1944).




The Chairman : Before I offer any remarks on the subject of the paper I should like to propose a hearty vote of thanks to Mr. Rooksby for his very interesting address. I am glad that he did not specialise in the decoration side of the industry, because I think that the precision aspect of this subject is more important. My view is that sapphires should be made for use, rather than beauty, and I make no apology for holding that view.

The vote of thanks was carried with acclamation.

DISCUSSION The Chairman : There were one or two

points in the paper which were of particular interest to me. I watched the film showing the growing of the " boule " with special interest, because some time ago I suggested the making of a similar film to a cinema company. I do not know whether it was ever made, but I am glad to see that one has now been made, and that by a scientist.

Another feature which interested me was the question of the " parting " in the corundum jewels. There are three other partings in corundum besides the basal parting illustrated, and it was found some time ago that the jewels which had a bad relationship to the rhombohedral parting were not good. I wonder whether Mr. Rooksby has considered that in dealing with the matter.

With the "rod " boule which the Americans have studied, there was the same vertical plane of parting. It is true that when a rod boule splits, each half is the same crystal, but along that plane the parting is definite. I have suggested that it is possibly due to a twin film. We find in other minerals such as augite and hornblende that there is some- times a similar state of affairs. The two halves of the crystal are in the same orientation, but there is just a thin film down the middle of the crystal which is in twin relationship to the two halves. I should like to know Whether that point has been considered.

Lastly, with regard to the splitting of the boule. In the cinema film which was shown we saw the boule being split with a hammer. The procedure adopted in Switzerland is not to use a hammer but a pair of pliers. Taking the little stalk in the jaws of the pliers they " nip " it in various directions. When the correct position is reached, the boule splits. I should like to know whether that method has been tried here.

Mr. H. P. Rooksby : I am afraid that the question of orientation is rather more complex than the outline which I gave in the paper ; I do not wish to go into any complex considerations of crystallographic orientations at this particular meeting. As far as we know the most important parting is the basal plane parting ; but the rhombohedral parting is prominent in sapphires, and I believe that the orientations specified for jewels, particularly for meter jewels, are such as to preclude that particular parting from playing a very important part in the wear. It is now thought that the meter jewel orientation should be confined to the 90o specimen as closely as possible.

With regard to rod crystals, I do not know what the results of American investigations are, but I understand that the crystallographic orientations involved are completely random unless the seed crystal technique is used. I cannot think that the splitting principles are any different in rod or boule. We know that the boule splits, while some rods are made which do not split. There is, therefore, a thermal question involved which I do not think I can go into now. With regard to the Chairman's theory about the existence of a hair plane between the two parts of the boule, I feel that that is rather unlikely, but it is a personal observation and I have no experimental data to offer.

We do know that boules can be split with pliers and we have done that at Wembley. The technique of splitting the boule with a hammer was chosen for the film because we happen to use it more often.

Major Cadman, m.b.e., b.sc., f.r.i.c., F.C.S., M.i.CHEM.E. : I was pleased to hear the lecturer end his paper on an optimistic note with regard to the production of synthetic diamonds. Considering the magnificent work of the research chemists in the General Electric Company's research laboratories, I venture to predict that if these same people took up the problem of the production of synthetic diamonds they would soon solve it themselves. One thing is certain, that any- one who cares to use the same technique as that described so brilliantly by the lecturer would be able to produce the same result. Yet we are told - and told correctly - that all attempts made so far to reproduce synthetic diamonds by the methods claimed to have been successful have failed. I do not know what raw materials were used except in one case, that of Professor Moissan. He made his experiments with lump sugar.



524 JOURNAL OF THE ROYAL SOCIETY OF ARTS July IQ, IQ46

Every housewife knows that a lump of sugar begins to melt when heated, then turns brown and finally becomes a black mass. That black mass is almost pure carbon, and it was that carbon, treated with intense heat and pressure, from which Moissan claimed to have produced diamonds, admittedly very small ones.

I should like to congratulate the lecturer for showing us to what use modern precision instruments have been put in this work. We have been told that X-ray technique, the electron microscope - which is only in its infancy - and other instruments which have been known for a longer time, were all used. I think that if these same instruments were brought into use in attempts to produce other synthetic gems some success might be obtained.

I have one question to ask the lecturer. Have any other gems been produced synthetically ?

Mr. H. P. Rooksby : I think the scientists are generally optimistic about the possibilities of producing synthetic diamonds, principally, I think, because when scientists are pessi- mistic about anything they are usually proved to be wrong !

In reply to Major Cadman's question, spinels have been made by the process devised by Verneuil, but these are the only stones which have been synthesised by the technique I have described. I do not know if garnets have been made ; the only other stone which comes to mind is the emerald, but I do not know the process. In Kraus and Slawson's book there is a reference to synthetic emeralds, but no indication is given of the technique of manufacture.

Mr. Robert Webster : The lecturer has said that cobalt has been used for colouring sapphires. Is that something new ? I understand that originally the use of cobalt was a failure so far as gemmological experiments were concerned.

Mr. H. P. Rooksby : I am not quite sure of my facts with regard to corundum. I know that a very good blue colour is produced in spinels with cobalt. I have no reason to doubt that a blue colour can be produced in alumina with cobalt, but I think that it is of a different hue from that produced by iron oxide and titania. The specimen which is on the table is actually a spinel crystal.

Mr. Thorold Jones : Whilst serving in the war-industrial production of synthetic sapphires we were familiar with variation in

the optic axis and fractures, and whilst we relieved the stresses which were troublesome during cutting by annealing in a tunnel-kiln, it often occurred to me that it might be possible to make an isotropic corundum. The end of the war, however, made that experiment out of the question.

With regard to the question of whether the rod will split, I can say that the rod does split along a fairly definite plane and usually when it is bent. I should also like to say that I share the lecturer's optimism with regard to the future production of synthetic diamonds.

Mr. Ashley Lumby : I do not think that the lecturer gave any indication of the cost of producing synthetic sapphires. I should like to have some idea of how the cost of the synthetic sapphire compares with the cost of the natural stone and what limits there are to the size of the gems made. Perhaps if a larger gem could be made it would be more economic commercially.

Mr. H. P. Rooksby : I think that the cost of the material used is relatively small compared with the cost of fabrication. A large part of the cost of the natural stone is incurred in the cutting and finishing processes. There is, of course, a considerable wastage in these processes. With the natural stone it is also necessary to throw quite a lot of material out which would be tolerated in the case of gems for decorative purposes. For industry a very careful method of selection is used and a very large proportion of natural material may be useless. I think it would be fair to say that the cost of synthetic sapphires is similar to that of natural stones if properly selected for industrial purposes.

Dr. H. D. H. Drane : I was glad to hear Major Cadman introduce the spice of adventure into the discussion, because I am very interested in this question of the production of synthetic diamond. I think there is an enormous potential demand for the use of the diamond as an abrasive and, at the moment, its use for that purpose is limited only by the shortage of the material and its cost. The diamond mines have been worked to capacity and they require new plant. Meantime, the fascinating possibility of producing diamonds synthetically by novel methods has occurred to a number of people as a result of the sudden advent of the atomic bomb. Clearly, enterprise is afoot and it is satisfying to know that there is



July 19, 1946 CONTROL OF LEPROSY IN THE BRITISH EMPIRE 525

genial competition in this new field. But although novel experimental conditions may be devised involving the release of atomic energy, a close consideration of their possible application to the purpose of making synthetic diamond shows that they cannot be expected to lead to any greater practical success than was achieved by the late Sir Charles Parsons in his extensive researches on this subject.

Mr. R. J. Low : I was very interested in the furnace described in the paper, but the film did not show whether there was an excess of hydrogen or oxygen. Also, in so small a furnace how are the nozzles kept cool ? What are the walls of the furnace made of and has the use of the atomic- hvdrogen arc and the electric arc been tried ? With silica ware, there are two varieties, the opaque and the clear. The clear is made by making the opaque first, then grinding it up and re-fusing it. Has that process been attempted in the making of sapphires ?

Mr. H. P. Rooksby : The silica is quite different because it is entirely non-crystalline, whereas we have been discussing a preparation of single crystals. The difference between the opaque and the transparent variety is largely a question of the elimination of trapped gases. Apparently the method of crushing and re-grinding the opaque material does allow of a refining process so that a transparent fused silica is finally obtained. It is, however, without crystalline structure.

As far as I know, neither the electric arc nor the atomic-hydrogen arc has been used in place of oxygen and hydrogen. The walls of the furnace are made of specially prepared refractory bricks which have a high alumina content. I cannot say more than that, but they are specially prepared for this type of furnace. The relative proportions of oxygen and hydrogen can be adjusted to alter the temperature of the flame, and that is done in the actual making of the sapphire boule because different temperatures are used for different stages in the growth of the crystal. The small stem grows and then the temperature is raised by altering the proportions of oxygen and hydrogen to enable the boule to grow outwards.

I think that the main heat zone is well below the nozzle, although it must reach a fair temperature.

On the motion of Mr. R. O. Porter, a vote of thanks was accorded to the Chairman for presiding, and the meeting then terminated.

DOMINIONS AND COLONIES SECTION

Tuesday, April дтн, 1946

Sir John Cumming, k.c.i. е., c.s.i., in the Chair

The Chairman : At the outset I ought to mention that Sir Bernard Bourdillon, who was to have taken the chair this afternoon, is unfortunately ill and unable to be present. I am sure you share my disappointment at his absence, because he, as Chairman of the Executive Committee of what is known as B.E.L.R.A., the British EmpireLeprosy Relief Association, would have been the appropriate person to take the chair on such an occasion as this, when we are to hear Sir Leonard Rogers. I acceded to the desire of the lecturer that I should take Sir Bernard's place this afternoon, partly because of our long-standing friendship and partly because, when I was in the East, I took much interest in this very subject.

Sir Leonard requires no introduction to an audience such as the present one. Suffice it to say that he has an international reputation on account of his brilliant research into the causes of and remedies for various tropical diseases. His reputation is so high that he is one of the very few members of the Indian Medical Service who have been made Fellows of the Royal Society. I shall not stand between you and him any longer, but will ask him to deliver his lecture.

The following paper was then read : -

PROGRESS IN THE CONTROL OF LEPROSY IN THE BRITISH EMPIRE

By Major-General Sir Leonard Rogers, K.C.S.I., C.I.E., M.D., F.R.C. P., F.R.C.S., F.R.S., late President , Medical Board , India Office

In 1923 I had the privilege of reading a paper before this Society on " Recent Advances towards the Solution of the Leprosy Problem." After describing an improved treatment of not too advanced cases of the disease, together with researches on epidemiology with a view to finding cases in early amenable stages, I concluded that these advances should greatly facilitate the control of the disease in future. To-day I am glad to have this opportunity to report some progress in British possessions towards that desirable end.



Documents

JEWELS AND STONES FOR INDUSTRIAL PURPOSES