I will back up Soyabean. Acetone often works, although sometimes a toothbrush is needed and a LOT of time. I've had a couple of miserable experiences with riven porcelain and polished porcelain even after I'd asked advice from the suppliers PRIOR to fitting. I had one job where the manufacturers of some polished white porcelain explicitly explained that the tiles required stripping off a manufactured wax coat with a Fila stripper prior to fixing and then sealing (with Fila). Then fix and grout and seal again (with Fila). Understood, I thought, however, once the factory wax is removed, it's all up to the sealant, and although you may follow it to the book, I managed to stain a couple by rubbing a knee pad on them during fixing. Took me forever to get it out, and I still had to warn the customer to never walk on the tiles in shoes. Some tiles aren't fit for purpose methinks...
