Asbestos was in piano and organ felts. Article (although it's mainly just a list.)

In addition to lacquer, varnish, glues, solvents, metals, what was/is in stains? Cadmium is used in yellow and red paint.

There are also possible hazards just because it was an old factory of any sort. Did the building itself have lead paint in/on it? Did it have lead solder in the plumbing? Lead flashing on the roof? Was there asbestos insulation or wall/ceiling material? If the place wasn't torn down and the materials disposed of properly? A lot of junk has been 'disposed of' by plowing it under - not good if they're planning on digging anywhere.