434 



NATURE 



[May 25, 191 1 



properties which make thcin extraordinarily beautiful, or 

 that they have remarkable hardness and durability, but 

 tht!y have hoih, and it is the impossibility of reproducin({ 

 this combination in any other material that renders the 

 detection of imitations a matter of ease in the hands of 

 anyone familiar with the facts. 



The most important point to remember about paste is 

 its lack of durability ; it is not only too soft to stand 

 much wear, but its composition is so unstable that it 

 rapidly deteriorates and loses its brilliancy on exposure. 

 You will see, therefore, that although there is a certain 

 legitimate scope for such paste imitations, they are very 

 unsatisfactory substitutes for the genuine article. This 

 being the case, as scientific knowledge has advanced, 

 attention has been more and more concentrated on the 

 problem of producing by artificial means the actual 

 minerals found in nature, and thus obtaining what I have 

 defined as artificial in contradistinction to imitation jewels, 

 having both the beauty and durability of the natural 

 article without the objectional concomitant of enormous 

 cost. 



The first point to be considered in attacking this problem 

 is the composition of the stone, as it is obvious that, other 

 things being equal, the possibilities of success are greater 

 with a stone of simple than one of comparatively compli- 

 cated composition. The economic aspect has also to be 

 considered — it is not much use devoting time and 

 ingenuity to the production of an artificial stone when the 

 natural one is so common that the cost of the two would 

 be practically identical. 



Commercially, we are as far from being able to produce 

 artificin! diamonds ns in thf dnvs of th^ nlrhemists. It is, 



perhaps, a bold thing to say that no such thing as an 

 artificial diamond will ever be placed on the market, but 

 one can safely assert that, so far as our knowledge stands 

 at present, it is impracticable. In saying this, I am quite 

 aware that statements as to the commercial production of 

 synthetic diamonds being an accomplished fact have quite 

 recently api>eared broadcast in the public Press, but those 

 who are responsible for such statements are (shall we say?) 

 under a misapprehension as to the meaning generally con- 

 veyed by the term "synthetic," and are unable to follow 

 the distinction I have drawn between an artificial gem 

 and an imitation. 



The chief problem to be faced is that of attaining the 

 necessary temperature, and it is not surprising that crystal- 

 line alumina was produced as a scientific curiosity so far 

 back as the commencement of the nineteenth century. It 

 is at this time that we first begin to hear of the oxy- 

 nydrogen blow-pipe (or the gas blow-pipe, as it was then 

 called). The process of producing reconstructed rubies by 

 means of the oxy-hydrogen blow-pipe is, roughly, as 

 follows : — The residue from cutting rubies and small worth- 

 less stones is broken into coarse sand, a small quantity of 

 which is placed on the centre of a disc of platinum ; this 

 is then carefully brought to the fusion point, care being 

 taken at this stage not to raise the temperature to such 

 an extent as to melt the platinum support. So soon as 

 this mass is fused it serves to protect the platinum, and 

 the reconstructed ruby can be built up on it by adding the 

 fragments of ruby one at a time by means of small 

 platinum forceps. These pieces have to be dropped on 

 with great care in order to secure incorporation with the 

 mass and prevent, so far as possible, the formation of air 

 bubbles. It will be readily understood that this process is 

 a tedious and laborious one, and, in fact, the formation 

 of masses of sufficient size to yield large stones on cutting 



NO. 2169, VOL. 86] 



if a matter of Mich difficulty that the coat of productk>n i» 

 vcr>' high. 



Just about Mven year* ago, however. Verneuil ' over- 

 came this restriction when he hit on the extremely 

 ingenious idea of introducing the raw material thr : 

 the blow-pipe, and thus placing it on the aupport .1 ■ 

 matically. The blow-pip<; is arranged vertically ov< r .< 

 small insulated chamber containing the support on u., i 



the mass is to be built up. The oxy:* ■> >"-!ii;:; 



cates at its upper extremity with a 1; ;!' '• 



in which is suspended a small sieve : r.iw 



material, which is rhythmically shaken by means of a 

 small hammer actuated by an electromagnet or cam. Each 

 time the hammer taps the support of the sieve, causin;^ i; 

 to vibrate, a small quantity of the powder falls through 

 into the tube bek>w, and, carried along by the gas, p;iss-» 

 out at its lower extremity into the zone of flame, wh<?re 

 it is immediately raised to the fusion point, and falls as 

 a melted globule on to the support below. 



This support is arranged with a screw adju§tment, to 

 that as the mass of corundum is gradually built up by the 

 constant addition of fresh globules the surface can be kept 

 at a constant level, and the portion already formed re- 

 moved from the zone of heating so as to allow it to 



stiffen. \Vh»>n rlio 



'HIS is first started the blow- 



Fig. 2.—" Boules " of ArtificUl Corundum. 



pipe is adjusted so as to give a comparatively cool flame, 

 and the powder is admitted slowly. By this means a small 

 " stalk " is formed, which insulates the mass from the 

 support and prevents the fusion of the latter. When this 

 has been formed, the full pressure of the blow-pipe is put 

 on, and the rate of admission increased, with the con- 

 sequent formation of a " boule," as it is termed, having 

 the shape of a pear, as illustrated in Fig. 2. 



With this apparatus a boule weighing some twenty to 

 thirty carats, and capable of yielding two cut stones of 

 about six carats each, can be jwepared in about half an 

 hour almost automatically, a single operator being able to 

 control several machines. 



The " synthetic " corundum produced in this way, if 

 pure ammonium alum is used^ is, of course, colourless, and 

 can be used as artificial white sapphire. If a small pro- 

 portion of chrome alum is added, the resulting stones are 

 rubies, and other colours may be produced in the same 

 way. For a long time all attempts to reproduce the fine 

 blue of the sapphire failed. A year or so ago, however, 

 the problem of producing synthetic sapphire was finally 

 solved by the use of titanium oxide, a very unexpected 

 result considering the chemical position of this element. 

 The artificial production of the corundum gem-stone may 

 be considered to be completely solved, and cut stones can 

 now be obtained in every variety of colour, from pure 



J " M^moire sur la reoroductinn art'fic'elle da nibu par fusion," M. A 

 Verneuil, Annales de Chimie et <ie Phvsigite, September, 1904. 



