A few days later, Jeff hit me back; he had gathered some info from a couple places on the internet. He said, Well, I tried the water test like you suggested, and water doesn't seem to discolor the floor . . . . But . . . I learned some other things too.
Repeating what he found, he said, I learned that rubbing alcohol (actually denatured) can be used to test for shellac, acetone for varnish, and lacquer thinner for lacquer. Mineral spirits can be used to clean "unsounded" floors, and I also learned that polyurethane only really became popular in the last fifteen (editor's hmmm) years. Strippers can be used to test for that. [ . . . ]