I'm repairing a damaged bathroom due to a flood and I want to reuse the backsplash tile. I have the grout removed from the tile, but how do I get the old mastic off or is that necessary?
For porcelain and ceramic tiles, a weak vinegar solution will often loosen mastic (and/or grout) to the point that it can be scraped off pretty easily. Do not use this on a limestone tile. Other natural materials might necessitate a test with a broken tile, just to check. If you see bubbles from anything other than the mastic, don't use vinegar.
We usually soak in really warm water (use tub sometimes) and scrape with putty knife. You don't have to have it perfect to reinstall. Also if there is too much you can reinstall using thinset.