Silver Test Kits

I haven't used one of these specifically, so do not know exactly what is involved. As a retired chemist, however, I can tell you that there are LOTS of test kits out there and most of them answer a yes / no kind of question rather than a "how much?" question.

Unless you perform a destructive test wherein part of the coin to be tested is removed for testing, I don't see how a test kit can differentiate a 40% silver coin from a 90% silver coin unless one of them contains something that the other does not and there is a test kit available to detect the different substance.

Silver vs. not silver should be a lot easier to determine. Most test kits use color development to indicate the presence of a particular atom or molecule. This is usually done by having ingredients in the test kit solutions that produce a highly colored dye only if the species for which the test is run is present. In this case, no silver = no color.

Other test kits use the presence of a very finely divided precipitate to make the test solution cloudy and less transparent to light. Once a sample is prepared, the test sample and a blank solution are placed in vials and then put into an optical comparator. Differences in light vs. dark can then be used to roughly determine how much of something is present but it is a crude measurement.

Using an acid solution on a valuable coin may not be in the best interest of the person who invested in that coin. Acids do etch and dissolve metals, so it is certainly possible to damage at least the surface of the coin and make changes to its appearance that could reduce the coin's value.

Although there have been some fake coins seen in the market lately, they probably are only seen in coins or bars of high value. It's not worth the effort to make a fake coin unless it has numismatic value, is a coin or bar of decent weight, or is a gold coin of some sort. In many cases, fake coins and bars contain at least some iron or nickel. If they do, they will be attracted to an ordinary magnet. Testing a coin with a magnet is a cheap, reliable, and non-destructive test that any coin buyer should do as a precaution.

Excellent point. Any fake silver coin is bound to be silver plated to look like the real thing and any test of that plated surface would show it as being silver.