Explanation about the materials of watch glassAuthored by Tokunaga
Question:
I understand that Hardlex is used in the Marinemaster due to its shatter resistance. My question is why doesn't Seiko use an acrylic crystal in this case? Acrylic crystals are shatter resistant and unlike Hardlex can be buffed if scratched. Could a Sapphlex crystal be used in this application?
Reply:
The materials currently used as watch glass and its main characteristics are shown in the following table.
ince the acrylic resin (1) has soft hardness and a heat-resistant temperature is very low, concerning the glass of PROSPEX watches which have high durability performance, such as a MARINEMASTER of this question, we couldn't use acrylic glass in which a lot of scratches and deformation are occurred on the surface.
Although (2) and (3) glass are used by SEIKO with the trademark of "HARDLEX" (glass which raised mechanical intensity by special chemical strengthening processing), they are used for standard watches.
On the other hand, sapphire glass (4) is used abundantly for the dress watches and high-class water-resistant watches and also diver's watches, such as CREDOR and Grand SEIKO and also some PROSPEX watches, such as LANDMASTER watches, SCUBAMASTER diver's watches of SEIKO brand, and hardness and theoretical intensity are excellent, there is also a problem of being unable to make as theoretical strength by the mechanism in destruction of the glass with which a cut angle control is important and a micro crack being detected in microscopic examination in case of large and thick glass.
In SEIKO watches, in addition to the above-mentioned general materials, "High- quality HARDLEX" (the high intensity and highly efficient glass strengthened on (3) Borosilicate glass) and original "SAPPHLEX"(the high cost performance combined glass which adhered thin plate of (4) on (2) or (3) glass) are put in practical use, and those are used according to the aim and specification of the watches.
"High- quality HARDLEX" which is the high intensity and the highly efficient article of the glass are used for the PROSPEX watches and ordinary sports watches. Also "SAPPHLEX" glass is used for the sports watches or the sporty watches,
In case of diver's watch which are required so many characteristics totally, such as resistance to pressure, water resistance, shock resistance, wear resistance, heat resistance, and durability etc., we usually use "High- quality HARDLEX" glass having the most excellent cost performance and sometimes use sapphire glass as a glass of some PROSPEX diver's watches.
Ikuo Tokunaga---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
In watchmaking terms, the protective covering over the dial is known as the 'watch crystal'. It can be made of any hard, transparent material, but is usually made of ordinary plastic, acrylic (a glossy thermoplastic), or glass.
A plastic/acrylic watch crystal is cheap to make, very resistant to shattering but scratches easily. However, light scratches can be readily polished out. The older type of plastic crystals were prone to yellowing after prolonged exposure to UV light, but newer ones don't have this problem. Plastics also deform badly at relatively low temperatures. Despite these shortcomings, plastic crystals were successfully fitted to some classic watches such as the Breitling Navitimer, the Omega Speedmaster & Seamaster, and the Rolex/Tudor Submariner. And of course, many Seiko and Citizen models as well.
Glass watch crystals come in various forms, but by common usage they are all lumped into the collective term of 'mineral crystal'. However, the term 'crystal' here is a bit of a misnomer, as by itself glass is not crystalline in nature but is amorphous with very high viscosity. To all intents and purposes the viscosity is so high that glass can be considered as a solid; however, examination of a piece of window glass in a mediaeval building will reveal evidence of the bottom of the pane being thicker than the top, because the glass has flowed -- ever so slowly -- downwards under the influence of gravity. In fact, it never stops moving. (There's no need to worry about this having an effect on your watch crystal, however. )
Seiko developed various chemical treatments for their glass crystals to make them harder and thus more resistant to scratching.
Hardlex is one step up from plain mineral glass in terms of hardness and comes in two flavours: ordinary Hardlex and high-quality Hardlex. They come in different thicknesses, depending on the pressure-resisting performance required. So yes, a Hardlex crystal in a 200+m diver's watch will be thicker than that in a Seiko 5.
Sapphlex has higher scratch resistance again. If you look at the table below (source - Mr.Tokunaga, Seiko watch designer), items 2 and 3 are Hardlex and high-quality Hardlex respectively. Item 4 is Sapphire glass, which is much harder and more scratch-resistant than Hardlex, but has the drawbacks of being a lot more expensive to make in greater thicknesses (3-4mm) for high-performance dive watches, and also more susceptible to shattering from micro-cracks in the thick material. Sapphlex is Hardlex with a thin plate of Sapphire adhered to it.
Much has been said about Seiko's decision to use high-quality Hardlex in the MarineMaster rather than sapphire. The usual reason given is that because of the thickness of crystal needed for 300m WR, shatter-resistance won out over scratch-resistance. If the truth be told, Seiko's decision was probably more influenced by cost than anything else. Sapphire crystals have been standard fare for years on the high-end Credors, Grand Seikos and Prospex models without problems but the customer is forking out lots of dosh for these models, so probably doesn't notice a bit extra for the sapphire crystal. In fact, the customer is probably demanding sapphire at this price point and is prepared to pay for it.
By way of a footnote, a sapphire crystal is standard on the 1000m diver (where high-performance is really needed). No worries about the crystal shattering at depth, then!
