Question: My slate patio stones are covered with black spots. Power-washing does not remove them. What causes them, and do you have suggestions for removing them without damaging the stone?
Answer: The spots are mildew, which doesn't damage the stone but does look unsightly. The website for the Natural Stone Institute (naturalstone institute.org) recommends using ammonia, chlorine bleach or hydrogen peroxide to remove mildew stains from stone – with the huge caution to never mix ammonia and bleach, or even use one right after the other, as they combine to produce a highly toxic gas. But hydrogen peroxide costs too much to be practical for as large an area as you have, and ammonia stinks even more than bleach does.
Charles Muehlbauer, technical director for the Natural Stone Institute, suggests starting with bleach diluted in water. He recommends first testing a small area in the most out-of-the-way place. "But slates are chemically very inert," he said in an email. "About the only residual effect that I would expect is that it would in fact 'bleach' the slate slightly, so the color might be a bit lighter after cleaning."
Rather than getting down on your hands and knees to scrub, mix bleach and water in a bucket and use an old broom to spread the solution over the stone. Wear rubber boots and gloves and protect your eyes. Wait 15 to 20 minutes, then hose off the stone.
If the stains persist, Muehlbauer suggests using a commercial cleaner formulated to remove mildew. But because bleach is cheaper, he'd start with that.