Use a diluted solution of muratic acid to clean the tile. ALWAYS add the acid to the water. Dilute 1/2 cup of muratic acid to 1 qt. of water. Add another cup or so if that doesn't do the job. The concentration should just slightly result in a bubbling action on the tile. Rince with a quart of water mixed with a couple of tablespoons of sodium bicarbonate to nuteralize the acid solution and let dry. Use just enough solution to get the job done. Don't soak the area. This stuff will etch the tile so be careful.