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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 Horlogčre. 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."
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.