Any idea what this might be?

[pxlkid]

Evening all!

I have a customer I’ve been cleaning windows for for a while now and they have asked what this stain might be above the ground floor window (see picture)?

I’ve never done any soft washing what do you think would be the best way to get rid of it?

 

[Pjj]

Looks like black mould to me often appears above window vents

 

[scottish cleaning service]

Looks like smoke or coal dust. Funny how its in a set pattern.

 

[pxlkid]

Looks like smoke or coal dust. Funny how its in a set pattern.

I was thinking that, any advice on how to remove it?

 

[K in Kent]

I was thinking that, any advice on how to remove it?

where would it have come from?
If you haven't got any hypo and don't want the extra expense just brush on some bleach to a small area. Brush it well in and perhaps top it up a few days later then go back a week later and see if it's had any effect whatsoever? If it has it's likely to be mould/algae etc, if not start considering alternatives - UBIK perhaps? ....or I'm sure some would say save yourself the hassle and don't get involved.
But if you're being paid for it get stuck in

 

[scottish cleaning service]

I was thinking that, any advice on how to remove it?

You would need a chemical like sodium hydrochloride. Try some neat bleach on a corner to see if there is any reaction. Come back with result and we will go from there.

 

[Pjj]

You would need a chemical like sodium hydrochloride. Try some neat bleach on a corner to see if there is any reaction. Come back with result and we will go from there.

If it is soot or burn marks that won’t remove it that will only remove organic material