Try, if possible, to avoid the tools whose use is fairly obvious but its uniqueness lies in the very specific application in which it is used. A good example of what I mean is the wrench used for the pentagonal valve nut on the top of a fire hydrant. Its use is obvious - it's a wrench - but it's only used by firemen. There are literally thousands of special pupose tools that fall into this category, especially automotive and firearm tools.

The powder tester was an excellent choice. An intricate antique with good clues to its era (a flintlock mechanism and the pistol handle style) but obscure enough to demand some thought. Plus, it had an exposed mechanism so it was possible to analyze its operation from the photograph.

When I was a kid (in the later Jurassic) television still had some shows with content. One was a program put on by the Philadelphia museum of natural history. Objects would be brought up from the museum's storage cellars and offered to a panel of experts whose job it was to determine the age, location and use of the object. I loved that show and learned a lot from it. My own grandchilder, raised in cotton wool by their yuppie parents, can't recognize or describe the use of a chisel. It makes me glad that I'm not going to be around to see how it all turns out.