How to choose a watch crystal
The glass protects the dial and gives the watch its look. It is also the part that scratches, cracks or yellows first, and the one that is most often replaced in workshops.
Three questions are almost always enough to find the right reference: what material, what diameter, and what shape. Here's how to answer them.
The three materials
Acrylic (plexiglass)
The most flexible and forgiving. It scratches easily but repolit It adheres to the paste in minutes, making it ideal for vintage watches where you want to preserve the original appearance. It withstands shocks without shattering. This is the type of crystal used in watches made before the 1970s and in most shaped crystals.
Mineral
Tempered glass, significantly harder than acrylic, resists everyday scratches well but cannot be polished: a scratch is permanent. In the event of a violent impact, it cracks rather than deforms. This is the most common compromise found on watches from the 1980s to the present day.
Sapphire
The hardest of the three, virtually scratch-proof under normal use. More brittle on an impact at an angle, and more expensive. It is reserved for valuable pieces or watches that are worn frequently.
Measure the diameter
This is the point that causes half of the ordering errors. The diameter to be measured is that of the glass itself, not that of the bezel or the case.
Remove the old glass, lay it flat, and measure with calipers, edge to edge, to the nearest tenth of a millimeter. Take two measurements at 90 degrees: an old glass may be slightly oval.
If the glass is broken or missing, measure the accommodation, That is to say, the inner diameter of the case's mounting surface. An acrylic crystal is generally fitted with a few hundredths of a millimeter of tightening, while a mineral or sapphire crystal is fitted with a gasket or press, depending on the case.
Also consider the’thickness and to the height of the curve : two glasses of the same diameter are not interchangeable if one is flat and the other highly curved.
Shaped glasses
Not all cases are round. Rectangles, barrels, ovals, squares, curved or domed: these glasses follow the shape of the case and cannot be replaced by a round glass.
They compete on two sides, Length and width, measured at the ends, plus the height of the curvature if there is one. Antique shaped lenses are numbered by series and by manufacturer: the number on the original packaging remains the most reliable reference.