ARTIFICIAL PRECIOUS STONES—-HEATON. 227 
With this apparatus a boule weighing some 20 to 30 carats, and 
capable of yielding two cut stones of about 6 carats each, can be 
prepared in about half an hour almost automatically, a single operator 
being able to control several machines. The boules, on cooling, very 
often split in half in the direction of their growth, as in the lower 
example seen in figure 2, and this is a convenience rather than other- 
wise, as the resulting shape can be cut to greater advantage. 
In the first instance reconstructed rubies were made in this way 
after the manner introduced by Gaudin, the material fed into the 
blowpipe being pulverized rubies and chips, and this method is still 
employed by some workers. But more commonly nowadays the 
corundum is produced direct from amorphous alumina by using 
pure ammonium alum as the raw material. On reaching the flame this 
decomposes, the ammonia and sulphuric acid volatilizing, leaving the 
alumina. Stones made by this process are generally known as ‘‘syn- 
thetic,” as distinct from ‘‘reconstructed,” although, of course, to be 
pedantic, the process is one of decomposition rather than synthesis. 
The ‘“‘synthetic”’ corundum produced in this way, if pure ammo- 
nium alum is used, is of course colorless, and can be used as artificial 
white sapphire. If a small proportion of chrome alum is added, the 
resulting stones are rubies, and other colors may be produced in the 
same way. For a long time all attempts to reproduce the fine blue 
of the sapphire failed, because following the apparent analogy of 
silicates, cobalt was invariably employed as the coloring agent. This, 
however, does not readily form an aluminate in the same way that it 
does a silicate, and, in consequence, it is impossible to produce a satis- 
factory coloration in the corundum by its means; it is possible to get 
the cobalt in a state of combination by adding a large proportion of 
magnesia to the alumina, but then the product formed isnot acrystalline 
alumina but magnesium aluminate, and its properties are fundamen- 
tally different. Its refractive index is lower, its refraction single, and its 
hardness lower. In fact, the result is blue spinel instead of sapphire. 
Moreover, such blue stones have the characteristic absorption of 
cobalt, and appear purple in a light that does not contain a large 
proportion of blue rays. 
In 1908 Paris attempted to avoid this latter difficulty by preparing 
a calcium aluminate colored with cobalt, as it is found that in this 
case the transmission of the red rays is less pronounced. But the 
calcium aluminate so formed is not crystalline at all, but amorphous. 
A year or so ago, however, the problem of producing synthetic sap- 
phire was finally solved by the use of titanium oxide, a very unex- 
pected result, considering the chemical position of this element. With 
this last advance 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 color, from pure white to ruby and sap- 
