Removing hardened grout from floor tiles.?

Question:I have recently had the floor of my bathroom laid with slate effect tiles (not real slate). The grout was not washed off properly when still wet and has left a "bloom" on the textured surfaces of the tiles and got into every little crenulation.I have tried cleaning them with water to very little effect. What is the most effective way of removing this without damaging the tile surface or the grout between each tile?

Answers:
There are acid based tile cleaning solutions that should help. Be careful to protect other surfaces and follow the instructions.