Ask the Architect

Is this black stuff mold?

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Q. We have these black spots on our front yard and side yard windowsills and, against the white paint, it looks awful. We only started noticing them within the past month. Could this be mold?

A. Your photo looked like mold to me, too, but sometimes a picture just isn’t enough when it comes to understanding the problem. There can be so many contributing factors that need to be more defined. We spoke, and I was invited to have a look. At first I wanted to get up close and see the spots, which, indeed, were black and on your windowsills. They were tiny, and could have been mold, except that when I touched some of them, they were dry and were different three-dimensional shapes, like tiny specks of dirt or dust. I would have been able to smear mold into the white paint or wipe it away with an alcohol or bleach-based cleanser.

So we looked further for clues. I asked how often you dusted those sills, whether your vacuum cleaner filter had been changed regularly or whether the hose had a leak, and then focused on the kitchen vent fan above the stove, and we worked our way to the boiler. You have oil heat, and the idea that a “puff-back” could be the culprit was explored by looking at the chimney pipe connections, wiping off surfaces nearby and trying to identify the residue left from the backup of a chimney that hasn’t been cleaned or has animal blockage. Nothing out of the ordinary was found, and there were no matching black, dry, rounded pieces of dirt.

So we went back to the windowsills. I looked at the little dirt specks more carefully and thought about air movement. Every day, for at least two months, the winds have been stronger than usual. There have been very few still days. The new suspects were possibly coming from farther away. Crumbling one of the larger black specks between my fingers, I noticed that the crumbled dot wasn’t just black but a variety of colors, from light brown to dark. That was when I began to look through the screen and window to the outside.

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