Hi
I've been wondering it one could use the method used to repair cracked automobile windshields to repair or at least stabilize a cracked mineral glass watch crystal ?
I've seen methods for repairing cracked porcelain dials that were very effective.

If such a repair were done properly would it help the watch retain value compared to a modern replacement ?

Ran across an interesting very old Cyma Trench watch with badly cracked but complete crystal, otherwise serviceable and running, and reasonably priced.

 

Try it and give a report.

 

In my book value in a cracked crystal doesn’t exist

 

“If such a repair were done properly would it help the watch retain value compared to a modern replacement ?”

A very old Cyma would have to have either a glass crystal, or a replacement plastic crystal. Mineral crystals would not likely be found on an old Cyma. Better to source a replacement crystal, in my view. It is hard to visualize just what kind of damage to any watch crystal might be repairable. If the damage was repairable, and you found someone to repair it, the cost of the repair would likely greater than the value of the watch. Value of an old Cyma watch in whose view? Your’s, or someone else’s?

 

I was going by this definition of "mineral glass"
"Mineral glass is a common glass and used in the manufacturing of windows and watch faces. Mineral glass is used in place of sapphire in watchmaking because it is more affordable. It can be very hard to tell the difference between mineral glass and sapphire; one sure way is to perform a scratch test."

I would expect that very old crystals had little if any additives to make the glass stronger, but chemically altering glass to make it less brittle has been practiced in one form or another since the 15th century.

When repairing a windshield a transparent glue, such as a cynoacrylic is forced into cracks under pressure. The glue has the same refractive index as the glass so when cured and polished the crack disappears.
Main problem with the crystal of the watch i'm interested in is that it might begin to disintegrate before I could find a proper replacement.
It looks to have been dropped, impacting one side with many small intersecting cracks.

I may as well put in a medium low bid and see what happens.

 

That’s the spirit!

 

There is a limit to how much damage that a vehicle windshield might suffer, beyond which no windshield repair service would attempt a repair. Major damage to a watch glass would likely be considered minor on a windshield. If you end up winning the watch, send a picture and dimensions of the bezel opening, and somewhere in my stash of 30,000 vintage glass watch crystals, I might have one to fit.

 

Any good restoration watchmaker can replace this.

 

The OP is perhaps optimistically over analysing this IMO and forgetting perhaps that a car windscreen is usually laminated with a clear plastic core, meaning that for small chips glue can be injected under pressure and won’t burst through to the other side. No such technique exists for watch crystals. Why would you bother anyhow. Mineral glass is a cheap material and crystals can be cut to fit any case. Even if successful any repair would be detectable since the optical properties of the crystal would be compromised. A replacement would be much cheaper and more effective.

 

Good point.
That's why I was asking around.
I suspect that only in the case of a very rare large diameter clock face would attempting such a repair be worthwhile.
Cynoacrylate glues were originally developed for casting of tiny optical lenses for military weapons sights of some kind. The lenses needed were tiny and almost impossible to mass produce by regular means.

If I get this watch I'll probably just put it aside till I can find a proper replacement crystal.

 

i would assume that a watch that old has had its crystal replaced, possibly more than once.

It's almost a wear item and so wouldn't bother me if it had been replaced. Kind of like spark plugs on a vintage bike. Yeah, my Triumph didn't come with NGKs, but they're so damn much better than the original stuff and who really cares about them.

 

I think if it was repaired like windscreen repairs it would be very noticeable to the naked eye. Everytime you would look at the watch the first and probably the last thing you would notice would be the repair. Could you live with that.