Maintaining External Terracotta Tiles

I'd guess that it's efflorescence caused by salts etc in moisture rising up through the unsealed tiles from the substrate. First try agua fuerte to see if that will remove it. Do a spot test in the middle of a tile and see if it works. If it does, then you can do the rest. Be careful and don't leave the agua fuerte on for long, because if the tiles have cement based grout between them, the acid will attack and weaken the grout.

It is possible to seal the tiles, but you'd need a breathable sealer to let moisture escape and not the salts.

Thanks Jim. I’ll try that .

With respect, I don’t see any point in sealing them, and it won’t last outside anyway. They would have been very unlikely ever to have been sealed when these kinds of raw/rough terracotta tiles were all that there was for flooring! Just do what my mother used to do with her terrace composed of genuine extremely antiqued terracotta tiles - scrub them intermittently and sweep them daily.

Once the efflorescence has gone - and it may keep reappearing for years - wash them down and leave them to get on with it. Of course, one of the original UK methods of sealing unglazed floors - in this case, quarry tiles, which although much harder are in principle ‘fired earth’! - was to wash them over with milk as this penetrates and forms a very effective sealant.

Hi Stephanie, I didn't mean the sort of sealer that sits on the surface and does wear off. I meant the sort of sealer that soaks into the body of tiles. 

5 litres fresh milk?!!!

Milk is what I used to use to age concrete, as it encourages the formation of mould and algae. I was thinking more of liquid silicone. How about we do a comparison test?!

Yoghourt also good!! Would play comparisons but currently no time. Silicone good, yes, but they’re natural, they’re outside, leave them be!!!

Trained teams of felines working backwards across designated areas of floor?