OK, the white at the edges could be one of 3 things:

Salt efflorescence – mineral deposits from the grout and or subfloor, grout is porous, the slate is not very porous, so any moisture trying to get out of the system, will come through the grout, not the slate, as moiture comes out of the grout, any disolved minerals that may be in the moisture will be brought to the surface where they will re form as crystals. If this is the case, it would temprarily disapear, or face at least when wet, only to become visible again when dry- a simple test, if you think you have this, a very mild phosphoric based acid cleaner, diluted with water, (nothing stronger like brick acid) will help.

Grut residue – ot could be the remnants of uncleaned grout residue from the installation, if it appears to have got worse recently, then the above is more likely, if it has be there unchanged for years then it could be grout residue. – Again same traement as above, mild acid clean.

It could also, as per your suggeetion, be the last traces of a topical sealer, but I would expect to see traces in other areas, like in crevices in the stone face, not just at the edges, even a non-riven slate may have some unduations and so some low spots that would protect the sealer from wear. It it was a sealer residue you would need a stripper to remove it.

Hope this helps

Ian

I am hoping you can help. We have a smooth (not riven) slate in our kitchen and hall. It has not been looked after very well and I am pretty sure the majority of the seal has come off, we have certainly never done anything with it in 8 years so you can imagine it is a bit of a state.

So far I have scrubbed it with a nylon scourer using a slate cleaner. It is looking much better but there are parts that look like they have a sort of white haze on them which I can’t get off it seems to be at the edges, is this the old sealant do you think? I have no idea what was used to originally seal the tiles, I am loath to reseal without getting the floor totally clean. Many thanks.

You are going to have to go back to the surface of the slate. If it has been sealed with a topical or coating sealer (which is now over the grout residue) then you will first need to strip that back with something like Sealer and coating sealer or equivalent.

Then, if there is still some grout residie (it may come off as a result of the stripping opperation if you are lucky) you will need a phosphoric acid based cleaner to remove the grout residue, and this may take several goes. Do not be tempted to try to speed this up by going for a brick acid – that can and often does, create more problems than it creates.

Hope this helps

Ian