Kota black/black limestone/lime black will fade in col because the surface is attacked by acidity in the atmoshere not uv rays etc . The reaction causes calcium blooming which is the whiteness you see on the paving. To restore the col you need to remove this but to do it properly is impossible, sand blasting is the only real answer but as soon as it is cleaned it will begin to turn white and the process will start again.
If you can get it clean the best sealer on the market is Romex but even after a few months it will turn white.Marshalls have had huge claims under their 10 year gtee scheme.
Any form of acidity will make the stone go white , patio cleaner, brick acid, jiff, surface cleaners, even lemon/ orange juice.
Many people are trying to develop sealers but have yet found an answer for this stone.
Cleaning dark limestone patio - How to clean a dark limestone patio?
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London Stone Paving
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Jackie
The general consensus, not only in this particular thread but on black limestone in general is that it does lighten over time and that while sealing will slow down that process you are in effect fighting a losing battle.
However from all your replies in this thread and the email that you sent me privately you seem convinced that the layer of white is actually caused by the builders not cleaning the job properly after installation.
Something else which further complicates the situation is that black limestone can be difficult to clean once any cement type residue has been allowed to dry on it. So it is also possibble that the problem has been caused by builders mess.
My feeling is that the process of acclimatization has been accelerated by the mess that the builders have left on the surface. I have witnesssed this before.
We are currently employing a stone specialist to have a look at a project in Hampstead where black limestone was used. While the problems they are facing are not identical to the ones you are facing there may be a common solution.
If you can leave it with me for a few days I will see what the specialist says and come back to you. Also please stop using jif and stop scrubbing it with a wire brush in the short term.
The general consensus, not only in this particular thread but on black limestone in general is that it does lighten over time and that while sealing will slow down that process you are in effect fighting a losing battle.
However from all your replies in this thread and the email that you sent me privately you seem convinced that the layer of white is actually caused by the builders not cleaning the job properly after installation.
Something else which further complicates the situation is that black limestone can be difficult to clean once any cement type residue has been allowed to dry on it. So it is also possibble that the problem has been caused by builders mess.
My feeling is that the process of acclimatization has been accelerated by the mess that the builders have left on the surface. I have witnesssed this before.
We are currently employing a stone specialist to have a look at a project in Hampstead where black limestone was used. While the problems they are facing are not identical to the ones you are facing there may be a common solution.
If you can leave it with me for a few days I will see what the specialist says and come back to you. Also please stop using jif and stop scrubbing it with a wire brush in the short term.