We faced this problem when a neighbour with identical bathroom tiles decided to renovate their bathroom and remove them. We wanted some spares as we're thinking of converting our en suite with bath to a shower room, and it's fully tiled so would need extra matching tiles. They were carefully removed with a chisel & hammer but every single tile has so much solid tile cement on it that it's proved impossible to clean most of them. I've soaked them in hot soapy water overnight to try and soften the tile cement so it can be easily scraped off but at least 70% of the tiles have not responded. I've also tried soaking them in water with some Limelight added in the hope that this would dissolve the cement, to no avail. Am afraid it's been a time-consuming and largely futile exercise.