Black Spots On Yorkstone Paving
I have seen more and more black spots, though blotches may be a more accurate description, in the last few years and have wondered what exactly is it and why it has started to grow on yorkstone paving.
I first started to supply reclaimed yorkstone paving in 1991 and since then have bought and sold thousands of square metres without black blotches, which is why I think it is quite a new problem.
In the last few months I have looked at the paving I supplied to friends and aquaintence who live nearby. Most have black spots on their paving but I don't.
I have been wondering why this is? am I doing something they are not which discourages black spot or are they doing something I'm not which encourages the growth of the black spots.
Though my "investigation" is very far from scientific, I have come to think that pressure washing is a major cause, I have never pressure washed my paving but all my friends who pressure wash their paving have black spot. I have also noticed that pressure washing can remove the pointing between the slabs.
The surface of yorkstone paving is the hardest part of it. Before it is quarried yorkstone is full of moisture which evaporates after it is cut depositing the minerals held in solution and suspension on the surface making it hard enough to protect the stone while creating the patina that marks out weathered yorkstone paving.
If that surface is removed by pressure washing you are left with a softer more porous surface allowing and possibly encouraging the growth of organisms on and into the stone that cause the black spots to spread all over the surface.