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After the new Victorian tiles are layed they get sealed. I have found that after a period of time dirt can find its way into the tiles and sit below the sealer. So you will find that you are unable to clean it off as the water and detergent does not penetrate below the sealer. You can think of sealer like a type of silicone that sits inside the pores of the tiles.
(this is a bit of an overkill and is typically what I do to old floors when restoring them but the principles are the same for new tiles)
Get this product from Topps Tiles http://www.toppstiles.co.uk/tprod129/Fila-Tile-and-Stone-Stain-Remover-1ltr.html
If the dirt is not coming out then you need to remove some of the sealer that is trapping the dirt.
To do this you will need to get some white spirit and wipe it over the tiles. White spirit will take off sealer and allow you to get beneath it. I typically will use my scrubbing brush as in the image above. Work the solvent into a small patch at a time and use white kitchen towel/toilet paper to remove any brown/solvent material coming off the floor. Use clean towell as you dont want dirt to be worked back into the floor.
After the solvent stage you now can do the cleaning part again with the detergent from Topps Tiles in the link above. Use the brush and work it into the floor creating a wet foam on the surface. Detergents work better if the floor is kept wet so use a fair bit of water. If its not working make the mixture stronger. And any difficult patches just make it very strong..
Cover the floor with sheets a few hours later and wait till the next day till it is bone dry to re apply the sealer.
Get a sealer from Topps. SPECIFY THAT YOU WANT AN IMPREGNATING SEALER FOR QUARRY TILES the Limestone sealer they have is good for Victorian tiles, here is the link http://www.toppstiles.co.uk/tprod4467/section1162/Fila-Limestone-Protector-1ltr.html
Take off shoes and walk in your socks.
THE MOST IMPORTANT THING IS TO APPLY IT SPARINGLY. If you just threw it all over the floor it would saturate the tiles and sit on the surface. Apply a nice even thin coat working your way back to the door (wiping off any little pools and into the floor). Kinda like putting a thin coat of oil on a wooden kitchen worktop,,,easy peasy..Dont spend too long on one patch just wipe a little bit in and move on.
After short while you will see that it all absorbs into the tiles and there are no little pools/patches left on the surface. You can then apply a second thin coat again. So essentially 2 thin coats as opposed to overdoing it with one heavy coat. Just remember no shoes for a few hours however typically you can check to see if it is dry after 30 mins. The floor will dry but the sealer only hardens a few hours later. So walk carefully on it in your socks for a while and it will not get dirty.