What is it?

[svp07 14]

I cleaned this blue stone walk a couple days ago. This black stuff will not come out and is only present where there is a higher amount of sun. I put down a bleach wash be for I cleaned with my surface cleaner and then went back with the wand at about 2000 psi.

Anyone got some info?

[Aplus 525]

You might look for a website from the manufacturer of the stone and ask them. Or, ask the distributor who provided/installed them.

I would be concerned about stronger chems possibly affecting the color.

[Beth n Rod 1,279]

Flagstone is a porous surface and home to the most wonderful microbial infestations

that can be found in a residential yard.

I have used bleach and outdoor bleach in order to get this condition to improve but never been able to get it totally clean.

Sodium hydroxide can be risky but a possibilty as long as you rinse well after neutralizing.

Pressure is the biggest enemy, since it is a shale stone. ie: it will flake and shale off if hit too hard with pressure.

Dwell time!

Hope this helps.

Rod~

[svp07 14]

Thanks Rod,

I'll try some SH when I do the deck.