How to clean a 100 year old piano
At 100 years old, there's a good chance that the finish on that piano is Shellac. The stuff can get sticky over time. As the linked article points out, the stuff is soluble in alcohol. I've use a cloth soaked in denatured ethanol to swab away sticky spots on a few of my hundred year old oak doors. Doing so gently, not scrubbing so hard so as to remove all the finish, left me with no obvious discolored spots on the wood and no stickiness, but of course, your mileage may vary. I'd suggest trying it on a small, discrete bit of the piano before going whole hog.