Synthetic sapphire watch crystals
Sapphire on watch: the reason why,
some physical datas. Cheap watches ususally have a plastic crystal:
acrylic material having an interesting elasticity, permitting the
realisation of waterproof cases easily enough. Mineral crystals
have a good shock resistance, and a rather good scratch resistance
when then are properly tempered ( up to 900 Vickers ). They may
be facetted, shapped, and bended by heath: they are of common use
on medium quality level watches. Synthetic sapphire crystals, usually
called " Saphir" are set today on almost all watch cases of precious
or hard metals, like platinum, gold, ceramic, carbide or titanum,
owing to their durability that comes from their unscratchable chararteristics.
Diamond cutted in the synthetic sapphire material, an aluminum oxyde
Al2 03, their hardness is 9 on the Mohs scale, or 1.800 Vickers,
or 2.200 Knoop ( when oriented in C axis ). Thsi makes it a material
undoubtly recognised as unscratchable, according to the terms of
the Laboratoire Suisse de Recherche Horlogère. A specially interesting
feature when it is consedered that the material itself, not just
the surface, is unscratchable. Some tests ( and even production
) have been made recently, covering ordinary glass with a layer
of carbide, so that the surface may be said unscratchable, but the
layer is so thin, that scratches still happen as the lower material
is crushed.
Some facts about the sapphire story, and
the material. The first saphirre crystal came on the market in 1929
on a small Jaeger Lecoultre, the Duoplan watch, after a kind of
joke between M. Lebet, manager of Jaeger at that time, and M. Alfred
Dalloz, a french jura lapidary, who was cutting synthetic stones
for the jewellery market. That small industry was using the rough
material produced by the Verneuil process, named from hos inventor.
He was a french scientist, who had succeded, as far as 1892, to
cristallise a ruby in an oxhydric furnace, starting from a genuine
ruby germ. His invention had been made public in 1903. The invention
had been first used to produce rubies for the watch movements, the
color of which is obtaned by adding chrome oxyd to alun. Other colors
of genuine stones are obtained by adding other metallic oxyds, like
iron and titanum for the blue sapphire. By cristallising ultra pure
alun a perfectly transparent corundun is obtained, used for the
saphirre crystals. 
This is the schematic view of a Verneuil
furnace, and photograph of the raw material. The comments of the
"Societé des Sciences" , delivered in 1904 is still valid " From
the chemical, physical and cristallographic point of view, there
is a total identity of properties and structure, between fusion
flamed ruby and natural ruby."
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When taken out from the furnaces, the
rough material, called "boules" have strong internal tensions that
would make the pieces split in two parts at the lightest shock,
so that they have to be anhealed, which means heated again to a
very specific temperature.
Rough material is produced mainly in Europe by two swiss companies,
Djevahirdjian, and a subsidiary of the Swatch Group, and a french
one, Le Rubis SA, a subsidiary of Dalloz. Since the years 60, japanese
also produce rough material, and some more limited capacities are
existing in the East countries. A few different processes have developped
in Japan, United States and Russia, since the 80s, to make large
size boules or flat sheets. They have a limited importance in the
watch industry, as the Verneuil process is more rational for diameters
smaller than an approximate 34,00 mm.

The saphir crystals processing is typical of the lapidary processing:
sawing, size and shape adjustments, grinding and polissing are made
with diamond tools or powders.
From 1930 to the 70s, the crystals were exclusively used for precious
metal cases, and were usually small round or shapped facetted ones.
The production reached 3 to 4.000 pieces per day, by a few manufacturers,
the main ones remaining leaders in Europe, Dalloz in France, represented
by Saphir Product in Switzerland, Stettler in Lyss and Seitz, which
became Comadur, in Les Brenets.
From the 70s, the sapphire became progressively used for sport watches,
boosted by the success of the Rado Diastar, the unscratchable case
of which could not be equipped but by a sapphire. All the main swiss
brand names follewed, even for main steel cases, and also the two
big japanese. This leads to a global consumtion ( although there
is no official figures and it is rather difficult to have a precise
estimation ) coming from around 1,5 million pieces in 1970 to some
5 millions in 1985 and around 15 at the end of the century. One
of the open question today is if or how far, this production will
be delocated, as a n important part of the steel cases is made in
the Far East, althougf the prestigious swiss brand names keep a
high fidelity towards their their swiss suppliers.
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