I was installing a new crystal on a beautiful Hamilton pocket watch today. I’ll write more about the watch in the coming days. I didn’t get the crystal installed yet. I had encountered this before but it has been quite some time. Old glass crystals for pocket watches were marked with a sticker on them to indicate the size. The sizing system is archaic, I like millimeters but the problem isn’t the system, it’s the sticker. These crystals go back quite some time and the stickers have been on the glass for 50 years probably. Unfortunately the adhesive they used has an acidic component to them and sometimes after removing the sticker you will find that the glass underneath is actually etched because of it. That was the case today. Ultrasonic, steamer, alcohol, acetone, even spit (the miracle solvent) couldn’t remove the “shadow” of the sticker. I’ll have to get another one. I don’t know how this could be prevented other than removing the stickers now before they etch other crystals, but than how would you sort them? Anyway, be aware, sometimes these crystals with stickers aren’t all you bargain for.
Regarding the numbering system: The crystals are marked with several numbers. Sometimes one of the numbers represents millimeter diameter, other times that measurement is absent. There is often a number to indicate how high of a dome the crystal has. This is usually a number between 1 and 10 where 1 is the highest dome and 10 is flat. The most important number is the diameter as measured in 16ths of a ligne. A ligne is 1/12 of a french inch. A crystal size 12 would measure 27.07 millimeters and a crystal size 24 would measure 54.13 mm. The crystal I needed today was an 18 9/8ths or approximately 41.9 millimeters.
As I searched for information about crystals and this old sizing system, I discovered a great blog. You should visit Montres/Watches. If you search for crystals it will turn up a nice conversion table.