The odds are you are dealing with black (cut back) mastic and it more than likely contains asbestos. The Sentinel Company based in Monneapolis has products to remove this type mastic (soimetimes it is available in HOME DEPOT). Also DUMOND CHEMICALS has a black mastic remover. Both products work well.
Hy also recommends products from a company called Peelaway. He seems in several posts to be quite knowledgeable. Type Peelaway into the above search utility under the "Body" option to look for more on this. Luck, Kizmet