It doesn't work well for circulated, gray color coins, but it does for BU material. It's all about how silver captures light better than the clad coins. My LCS showed me that a couple years ago. Very cool.
One of the oldest test there is for telling silver from non silver. Been a long time since I've even seen it mentioned here.
It hasn't been that long: http://www.cointalk.com/threads/1976-kennedy-half-dollar-is-it-silver.238970/#post-1819241 http://www.cointalk.com/threads/ike-dollar-dansco-complete-with-all-proof-issues.223412/#post-1650228 I think you are the one that originally brought the tissue paper test to everyone's attention. Perhaps you invented the test?
This is an example of one of the earliest silver coin tests. This may have been one of GMJMSP's earliest attempts (before the invention of tissue paper).
Other way around. Silver is the most reflective of metal, something like 95% of all the light that hits silver is reflected. The copper nickel clad reflects much less so the silver coins show white through the tissue and the coppernickel are much darker.
Maybe here on this forum, but I first heard of the idea from Conder many years ago on the old Coin World forum. And green - gonna getcha for that
Thanks for the photo! I'm curious about what someone else mentioned. Have you done the test with silver plated coins or tried it with genuine gold coins vs 24 K gold plated? I'm guessing if a coin is plated, it's going to appear just as bright, but it would be nice if that wasn't the case.
Although this simple trick may work well with certain light and tissue, one should consider the above mentioned coin surface problem. Toning ( interference layers ) and random roughing of surface from wear or corrosion can cause bad results. Below is graph showing 'perfect' reflectivity of some metals in reference to the light spectrum. I doubt the human eye has the ability to detect new tin from new silver. Luckily, pure tin corrodes relative rapidly and isn't commonly used for coins. The tissue test measures reflectivity and is not accurate below the surface of the coin , so coins plated with pure silver will pass the test. It can not detect counterfeits from the correct metal mixture.
Eventually there would be a level (%) of 'dirtiness' which would reduce the reflectivity of silver to close to a new Cu-Ni surface, and then the test would be inconclusive. But how much would be an interesting experiment if done with some accuracy, although it wouldn't help separating metals.