I have a lot of boulders in my garden and the one nearest my house used to be a beige coloured granite. It is now covered in something black, that's almost like a matte paint. It doesn't look like a fungus or moss or lichen. It didn't arrive gradually. It arrived overnight. Black spots also appeared all over a nearby cedar deck (and they can't be removed by soaking in a mix of TSP & bleach, scrubbing, then power-washing, so my mould theory is out). Any ideas what this tenacious black menace might be?
No, I don't think so. The deck and boulder are in a fenced backyard and the spotting pattern over a large deck would be difficult to achieve with paint. The rock is about 25 feet away from the deck, in the middle of an herb garden. The rock is almost completely covered in the mysterious black stuff. Nothing in the surrounding areas (herbs, lawn, hot tub, chairs) has a trace of black anywhere. I'm baffled.
Yeah, me too! Could you give us a photo? Or several? The suddenness of this stuff's appearance is mystifying. Does it smell? What does it look like under magnification?
The black stuff doesn't have an odor and it doesn't have any kind of a texture I can see with a zoom lens. Photos are attached. The black on the boulder seems to be fading a bit lately (I don't know if that's related to the hot dry weather we've been having lately). The photo of the cedar deck board is after TSP & bleach soaking/scrubbing & power-washing. It looked a bit worse before (but that might have just been dirt).
I don't think so. I don't know of any possible source of coal dust in the area and I've lived in the same house for 17 years without encountering this problem. Also, I would think that coal dust would scrub/bleach/power-wash away. And if it was coal dust, why didn't it land on the surrounding gardens and furniture?
