Thinset is the mortar that holds ceramic and stone tiles to the substrate, and even though it isn't supposed to get on exposed tile faces, it often does. It's fairly easy to wipe off before it sets, using soapy water, but if it hardens on the tile, you can't simply wash it off. You can use an acid wash to dissolve it, but that isn't safe for glazed ceramic and natural stone because it can etch the surface. In most cases, you shouldn't need acid -- you can usually knock off thinset with a stick and a hammer.
Knock It Off
Thinset is exceptionally hard, and it's a strong adhesive, but it's brittle, and you can usually dislodge it from the surface of ceramic or stone tiles with a sharp blow from a hammer. Of course, you don't want to strike the thinset directly or you'll damage the tile. Similarly, digging the metal blade of a putty knife into the surface can also damage the tile. A handy solution is to use a flat-ended wooden stick; it's rigid enough to dislodge the thinset while being too soft to damage the tile.
Make a 6-inch battering ram from a 1-by-2-inch piece of hardwood, such as oak or maple. Cut one end square, using a chop saw, and cut the other end at a 30-degree angle.
Wedge the angled end under thinset deposits and strike the hardwood ram with a mallet. The thinset should pop off -- if it doesn't, reposition the ram and strike again. In some cases, simply pushing the wedged end of the ram across the surface of the tile will remove the thinset.
Clean off the residue with a solution consisting of at least 1 ounce of dish detergent per gallon of water. Scrub with a fiber abrasive sponge, such as a kitchen scrubber. Do not use steel wool -- it can scratch the tile.
Recycling Used Tiles
Old tiles have a charm all their own and can often be recycled. After chipping out a quantity of old tiles, you're faced with the daunting task of removing the thinset from the backs before you can reuse the tiles. Mechanical removal is a better option than immersion in an acid, which could damage the tiles. Use a 4-inch angle grinder and a diamond masonry wheel or a rotary tool and diamond-grinding accessory. Wear goggles and a dust mask -- the procedure raises clouds of dust.