This question comes up a lot in my line of work and I think it's time to bring clarity to this commonly used/misused term.
There are no glass manufacturers on the planet that produce a specific glass formulation called "mineral glass". It is simply a term used after the glass has been processed into watch crystals primarily. Mineral glass is a general but accurate description for all types of glass made using the basic silica/soda/lime formula, whatever the glass type may be. The only glass-like materials that are probably not associated with the mineral designation are sapphire and fused quartz but in reality, they are mineral too! The type of glass that has most commonly been referred to as 'mineral' over the years is the post 1940's US and European made low-iron clear drawn thin sheet glass used for microscopy, photography and watch crystals. In my work I use fine optical crown glass, low iron float glass and borosilicate and even though they are technically "mineral", I don't call them that and neither do the giant factories that make them.
So today this really isn't an issue of deliberate misinformation as much as it is an issue of antiquated marketing lore and legend .... especially in the watch world. Glass is mineral ...and mineral is glass by a different name.
William, why is safety glass apparently not used in say sheet glass that would be used in the manufacture of watch crystals ? Is it to do with cost prohibitive or ? I realize personal injuries would be minimal, but what about damage a shattered crystal sometimes does to a $100+ dial ?
regards, bb
Posts: 6376 | Location: Texas in the USA | Registered: July 27, 2009
Once glass is tempered, either by heat or chemical ion exchange process, it can not be further processed without the temper being lost. For example if you take a typical heat tempered glass shower door and try to cut a watch crystal from it, you will end up with about 1,500,000 tiny shower doors on the bathroom floor. Tempering of watch crystals is done chemically and only after the crystals are finished. From my understanding, crystals that have been chemically hardened do not break in the same way as the shower door, they are just physically harder, stronger and more resistant to scratching or breaking than the same glass untreated. As far as I know, it is not possible to heat temper something as thin and small as a watch crystal. The result of tempering is that the outer layers of the glass are under great compressive strain in relation to the center and this results in a denser, harder and less penetrable surface.
I have seen some eBay sellers call non-plastic watch crystals as "Carbon Glass" and wondered if that, too is a way to differentiate a type of glass from another.
Clark Reed
Posts: 531 | Location: Anderson, South Carolina in the USA | Registered: April 09, 2013
