Soak them first with plain water then clean them using PSRP XTreme Heavy Duty Tile & Grout Cleaner and a mono rotary machine fitted with a soft to medium brush. Allow to dwell then scrub again. Extract with a wet vac, rinse scrub and extract again. Once dry apply Miracle Economical Impregnating Sealer and then the following day apply either Miracle Mira Matte or LTP Antique Wax to give the sheen