Acrylic vs. Mineral vs. Sapphire watch crystals
This question comes up from time-to-time, and thought a brief explanation of what they are and how to tell what they are may be useful. Most collectors, WISa€™ already know, but for those that dona€™t know, here it is;
First off, acrylic is a fancy name for plastic, or Plexiglas. Plexi, of course, is the least expensive. It is also the least likely to shatter and the most likely to become scratched. Mineral glass, even though it has been hardened by a tempering process, is more likely to break than Plexiglas. But it is also more scratch-resistant than that material. Synthetic sapphire is the most expensive glass crystal material and the most scratch resistant. Because it is so hard, it is also brittle, and shatters more easily than mineral glass. There is one other out there, which is kinda a laminate. It uses a base of mineral glass, with a layer of sapphire laminated on the outside. This hybrid offers the shatter resistance of mineral, with the scratch-resistance of sapphire. This stuff is proprietary, and Seiko uses it quite a bit. This is whata€™s called a€?synthetic sapphire.a€?
Cheap-o watches usually use Plexi, but some very expensive vintage watches also use it. It was found on high-end divera€™s watches due to ita€™s shatter-resistance under extreme pressure (like diving in the Marianna Trench). Advantages in glass technology and gas-release systems have pretty much relegated Plexi to Wal-Mart watches. Remember, if you have a Plexi-crystal on your watch, it will scratch like hell. Dona€™t even wipe it off with a paper towel. One advantage, however, is that you can quite easily polish out very minor scratching easily with any commercial (very fine) polishing compound.
Ok, leta€™s write off Plexi. The big question is how do you tell if you have a mineral or sapphire crystal? Well, there are three ways (ok, actually four, but you probably dona€™t want to do one of a€?em!)
The firsta€¦..and best is to use a hardness tester. A lot of people have heard of the a€?Rockwell Hardnessa€? of stuff, but Rockwell is primarily used to determine the hardness of metal. Wea€™re gonna talk about the Mohsa€™ Scale (I guess it was invented by Moe?) Anyway, ita€™s a non-destructive test performed on the surface of glass. Mineral glass has a hardness of 5, while sapphire rates a 9. The scale is 1-10. This test is DEFINITIVE.
Next is the stainless steel knife test. Crude at best, but it does kinda work. A stainless steel knife will scratch mineral, but not sapphire. Stainless steel is harder than mineral, but softer than sapphire, thus how this test works (duh!)
Number 3, the color test. Look at the edge of the crystal (not straight down like youa€™re checking if ita€™s time for another beer), but from the side (like if youa€™re laying on the floor looking at the side of your watch because you didna€™t check to see if it was time for another beer!) A lamp or other light on the opposite side of the watch will help. A sapphire crystal will have a pinkish hue, a mineral crystal will have a greenish hue (usually, sometimes!). Not foolproof.
Last, the water test - place large drops of water on the crystal, then tilt the watch and watch the water slide off. On the sapphire crystal, water will easily slide off - leaving small well-formed round globules of water on the surface. On the mineral crystal - most of the water will “stick” to the crystal and the drops of water will spread across the surface of the crystal to form a non-uniform “puddle” instead of sliding off. Make sure the crystal is clean, and completely free of wax, oil, tears (if you bought it from Bestswiss), drool, poop (hey, none of my business, ok?) etc. This is simple physics a€“ harder material is more dense, hence less porous. If you can, use distilled water, a€?cause tap water and bottled a€?Bear Piss Natural Mineral Spring Watera€? has, you guessed it, MINERALS in it, which can affect the test.
If youa€™re gonna use the color or bear-piss test, use a€?em both. If you can find Moe and his magic hardness tester, youa€™ll know. And if you take a big a€?ol stainless steel knife and start carving on your crystal, you better hope you got sapphire.
REMEMBER, other than the Mohsa€™ test, the rest are pretty accurate, but not 100%.
Hope somebody learned something!
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