Mortar is a mixture of sand and cement - with water that makes it pliable to work with and gives it strength. If the mortar is "very old" I'm guessing it is hard ... The only way to remove it is by chipping it away. You can normally use a chisel and a hammer - but you're going to have to work incredibly gently to avoid chipping the tiles. A flat scraper might be better - with no more than the force of your hand. I'm wondering though if this isn't grout that has dried on the surface of the tiles. It should come off more easily than hard, dry mortar. The moral of the story is to gently clean all surfaces to get rid of mortar or grout BEFORE it dries and hardens on the surface.
Every time I clean my mirrors, they end up having tons of streaks and almost look worse than before I started. What could I use to clean them that won't leave streaks...
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Muriatic acid should do the trick
Gently
Mortar is a mixture of sand and cement - with water that makes it pliable to work with and gives it strength. If the mortar is "very old" I'm guessing it is hard ... The only way to remove it is by chipping it away. You can normally use a chisel and a hammer - but you're going to have to work incredibly gently to avoid chipping the tiles. A flat scraper might be better - with no more than the force of your hand. I'm wondering though if this isn't grout that has dried on the surface of the tiles. It should come off more easily than hard, dry mortar. The moral of the story is to gently clean all surfaces to get rid of mortar or grout BEFORE it dries and hardens on the surface.