York stone, sandstone mould

[akist]

I have looked at houses which have "sandstone" type window cills, balcony ballustrades, and some times sandstone paving slabs on the patio and there is black markings like streaks of dark water running down or completely covered cills. Right next to other windows whose cills are perfectly clear. I thought that north facing may be an issue, but that does not explain how one cill is darkended and the other right next to it is fine.

I guess this very dark residue/discoloration is some sort of mould? Do these cills need to be treated maybe?

I am trying to decide if I want to buy resin/stone cills like that or maybe just use bricks and be done with it.

 

[foxhole]

Blacks is often lichen which likes cool locations .Easily cleaned .

 

[akist]

Thanks.

I spoke to one manufacturer she said there are sealing and cleaning products available although she did not mention which. I would start by using the karcher first and if that did not work I'd use chemicals.

So I think I will order the cills and hope for the best.