I have begun a flooring job in a small bathroom (5x5ish). This involved first taking up the ceramic tile already present. Removing the tile was pretty easy with demo hammer and wide chisel bit, but to remove the thinset, I had to use a floor scraper. When I did this, i found a thin black film under the thinset that also came up as I was scraping. I am now concerned that this black stuff may contain asbestos. I was wearing a mask and had wet down the floor because I had read that thinset comes off easier when wet.
The reason I am concerned about asbestos is that the house was built in the 50s and one of the closets on the same floor as the bathroom has vinyl asbestos containing tile that has been since covered up with sheet laminate flooring.
So my question is, what could the black stuff be? I have read that vinyl flooring used to be put down with some black mastic adhesive. It seems slightly greasy and there is a thin layer still stuck to the concrete flooring. How should I proceed with this floor?
The black stuff is most likely old "cutback" adhesive; I believe the name derives from it being asphalt cut back with turpentine.
It's good sticky stuff that usually contains asbestos.
How you handle it depends on what you're putting down over it. Generally, skim-coating the floor with a 1/16" layer of good patch (like Ardex Feather Finish or Armstrong S-184 or similar) prior to installing the new flooring is the way to handle it.
Keeping it wet when you're working with it is also the recommended way to keep asbestos from releasing into the air where it becomes dangerous to humans. Make sure any debris is bagged airtight in sturdy plastic bags.
Basically don't remove more than you need to and cover it up with patch.