Restoring my original hardwood floors.

Do not sand that adhesive. It most likely contains asbestos. So did the tile you removed.

How well does a razorblade work at scraping it off?

As long as the stuff stays in that tar form, it's safe to handle...(use gloves)...but like Rusty said, don't sand it...that is when you take the risk of putting the fibers into the air.

I'm going to think that it's going to be a very manual scraping task....once you have it all scraped off, then maybe some solvent will remove the rest....trouble with solvent is that it might effect the wood. This is where getting an expert in might be in the cards...

Or, just put down new wood.

I would test for asbestos and if it is found to not contain it, then just sand it starting with a 16 grit. It will take a lot of time and se a lot of sand paper, but it will still be much cheaper than putting in a new wood floor.

I have already tried sanding it i didn't think about it containing asbestos until later. Sanding doesn't work. I wouldn't even know where to have it tested. Scraping it will take a long long time. I have tried everything but the buffer.

Sanding will work. You just have to use lots of sand paper. You literally might need to change the paper every few feet. It will take $100's of dollars in sand paper. I believe you can buy test kits online, but I am not sure how accurate they are. The best bet would be to have a company out to your house that specializes in asbestos removal and testing.

I found this same stuff in my 1940's vintage house, and was pulling up the vinyl flooring sheets to redo. I found this black adhesive stuff was actually water soluble. I *Soaked* the stuff till it was dripping wet and let it sit for an hour or two, then came back and was able to get it up by wiping it with a bunch of old rags and a bucket of water to wash out the rags in. It took a lot of scrubbing, which I think was much easier than sanding it alone, and once the lion's share of it was up like this, I was then able to then sand the floors with minimal effort and get a great finish.