Well, because of our orange peel textured walls, the adhesive wasn't long-lasting. Less than a year later, the tiles were starting to peel off. It may never have been a problem if we had lovely, smooth walls, but since the entire house is orange peel, longevity of stickiness was not in the cards. Or on the wall.
We liked the way that slate behind the firewood looked, and we were intent upon keeping it, so we went down to the hardware store and picked up some Devcon WeldIt. To re-adhere the tiles, (and not all of the tiles fell off the wall. Only a few decided to be a problem.) I first squirted a zigzag pattern on the back of the slate. Then I repeated the process on the wall, making sure to keep the tip of the tube against the wall. I didn't want to drip this stuff anywhere else! Once the adhesive becomes tacky, it's time to slap those slate pieces back up. I was working on dinner while this transformation occurred, so I checked it after ten minutes, and it was ready to go. Make sure everything is squared up before adhering because once those two sticky surfaces find each other, nothing's going to tear them apart.
So far, everything is behaving nicely. The slate is back on the wall, and the adhesive is doing what it's supposed to -- being sticky. The tiles seem really solid now, and I'm not too concerned with future slate sagging. This "quick" project may have ended up requiring more attention than I had hoped, but it's still a very easy way to add tiles -- and interest -- to a wall.