A reader’s slate patio stones are covered with black spots. (Reader photo)

Q: My Tennessee 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?

Bethesda

A: The spots are undoubtedly mildew, which doesn’t damage the stone but does look unsightly. The website for the Natural Stone Institute (naturalstoneinstitute.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. They combine to produce a highly toxic gas. However, 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 testing a small area in the most out-of-the-way place first. “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, dip in an old broom and spread the solution over the stone with the broom. Wear rubber boots and gloves and protect your eyes. Wait 15 to 20 minutes, then hose off the stone.

Power-washing the stones hasn’t helped. (Reader photo)

If the stains persist, Muehlbauer suggests using a commercial cleaner formulated to remove mildew, such as D/2 Biological Solution Stone Cleaner ($40 a gallon from granitecitytool.com ). But because bleach is cheaper, he’d start with that.

Q: I foolishly used a sheet of aluminum foil on the bottom of my oven while baking pizza at 400 degrees for eight minutes. The aluminum melted. I managed to scrape three-quarters of the foil off with a single-edge razor. I also used oven cleaner for a short time — about 10 minutes — and I tried the oven’s short automatic cleaning function. Neither of these had any effect. Now I’m left with six slightly whitish spots and two spots with flecks of white. I called the oven manufacturer, the aluminum foil manufacturer and the oven spray manufacturer but got no information other than that I should not have used the foil. I was also told that the bottom of this particular oven is not removable or replaceable. I searched Google and found that this is not an uncommon problem. I found recommendations for using Naval Jelly (phosphoric acid) and a toilet bowl cleaner called the Works, which contains hydrochloric acid. I am reluctant to use either product for fear of further damaging the oven. What do you recommend?

Chevy Chase

A: Unbeknownst to many people who buy newer ovens, the heating element is sometimes under the bottom panel . So although it might have been possible to line an oven bottom with foil in the past without a problem, these ovens produce intense heat there that can melt aluminum even when the oven is on briefly. And then the aluminum coating is very difficult to remove.

Because you have already scraped off as much as possible, the safest, cheapest solution is to just live with what remains. It is purely a cosmetic issue.

If you can’t bring yourself to accept that, chemicals are your only option. With these, though, your prime concern isn’t just the oven finish. It’s how to protect your eyes, skin and lungs. Before you buy one of these products, look it up online. Stores such as Home Depot and Amazon usually provide a link for the SDS (safety data sheet, which is sometimes called the MSDS). Click on that and read about the hazards and what you need to do to protect yourself. You’ll also find a rating of 1 to 4 for various hazards, with 1 being the severest. (Just to make it more confusing, if you ever look up fire hazards, the rating system used by the National Fire Protection Association uses a 1-to-4 rating that goes in the opposite direction, with 1 meaning the least-severe hazard.)

Anything that dissolves aluminum is powerful stuff, so if you do use one of these products, wear goggles and rubber gloves. And because you cannot remove the bottom panel to clean it outside, open up all the doors and windows so you have plenty of ventilation.

As you discovered, some people report wonderful results with the Works toilet bowl cleaner, which costs $2 for a 32-ounce jug at Home Depot. It is 9.5 percent hydrochloric acid. Others are fans of Naval Jelly, which is 10 to 30 percent phosphoric acid (a 16-ounce bottle of Loctite Naval Jelly Rust Dissolver is $7 at Home Depot). And there are those who recommend oven or drain cleaners with sodium hydroxide (lye).

You mention that you already tried oven cleaner, but what type was it? Some kinds depend on ingredients other than lye. One of these is Easy-Off Fume-Free Oven Cleaner ($6 for a 24-ounce can at Home Depot), which is advertised as being safe to use even without rubber gloves. Yet the SDS warns to wear rubber gloves — with a notation that the SDS applies to people who use the cleaner in a workplace, where federal Occupational Safety and Health Administration rules apply, but not to consumers, who will presumably have shorter contact with the chemicals. You decide whether that distinction matters to you.