Battling Grout Grime

Q The grout around my tub is turning black, and a bleach solution doesn't seem to help. What's going on, and is there a way to make it white again?

AIf bleach doesn't help, it's probably not mildew. Black stains could be caused by manganese, a mineral, in your water. Because shower walls are usually left to dry on their own, the manganese stays behind when the water evaporates. Over time, it builds up and becomes especially noticeable on grout, which absorbs more water (and therefore more minerals) than the surrounding tile.

There are a few ways to investigate whether this might be the cause. Lift the lid on your toilet's tank. Water laden with manganese usually makes tanks black on the interior. You can also contact your water supplier and ask for a copy of the annual testing report, which is known as a "consumer confidence report." It may also be available on the water supplier's Web site. Or, if you have a private well, you can arrange to have the water tested.

Water with more than 0.05 milligrams of manganese per liter is capable of staining, unless your house has a treatment system designed to remove the mineral before the water flows out of the taps and showerhead. A high iron concentration, over 0.5 milligrams per liter, can also stain, but the color is red or reddish brown. Having manganese or iron, or both, in your water doesn't make it unsafe to drink. But dealing with the visual effects of these minerals is definitely a hassle. You may have noticed stains in your dishwasher or on your laundry as well.

Acidic cleaners help remove manganese and iron stains. White vinegar (dilute acetic acid) and lemon juice (citric acid) are commonly suggested because they are relatively benign and because you may already have them on hand. They do indeed work well for many cleaning jobs around the house, but they aren't very effective on shower walls because of the simple fact that they are liquids and run off before they have time to work.

Instead, try making a paste of cream of tartar (the acidic potassium salt of tartaric acid) mixed with hydrogen peroxide, which is also slightly acidic. Use hydrogen peroxide as it is sold at drugstores, which means it is already diluted in water to a safe level and contains just 3 percent actual hydrogen peroxide. With an old toothbrush, apply the paste to the grout and wait 20 minutes or so, then rinse with clear water. If you see some improvement, repeat the procedure until the stains have faded. Don't expect the quick results you get when you douse mildew with a bleach solution. But cream of tartar plus hydrogen peroxide does often work if you give it time and use it several times.

If you're in a rush, you may conclude that you need a more powerful cleaner. Commercial grout cleaners often contain oxalic acid, phosphoric acid, hydrochloric acid or sulfuric acid, which are all quite dangerous in high concentrations (as is hydrogen peroxide). Although you might be able to buy these acids straight, using a packaged cleaner is a lot safer. The acid or acids are already diluted, and there are instructions tailored to that concentration, which affects the length of time that the product can be left on a surface without etching it. Plus, there may be additional ingredients such as detergents or chelating agents, which help tie up minerals so they don't stain. Bar Keepers Friend and Zud are two powdered cleaners that contain oxalic acid in concentrations that are generally considered safe to use. Products sold specifically as grout cleaners are often more potent.

If you buy something more powerful, read the full label before you leave the store so you're sure the product is one you feel comfortable using. Many of the hazardous chemicals found in a typical home are products that were purchased but never used, presumably because the buyers decided too late that the risks weren't worth it. If ingredients aren't listed, choose another product or ask to see the manufacturer's safety data sheet, which the store should be able to supply. Products without any hazard warning are considered safest, followed by those with a label that reads "warning" or "caution." Products with "danger" or "poison" labels are most hazardous. "Nontoxic" means nothing on a label. Follow the instructions exactly, including donning any safety gear (such as goggles and rubber gloves) that the label recommends.

You may decide that removing the stains and restoring white grout is just not worth the bother. There is still something you can do. Consider staining the grout with a product known as a grout colorant. Black stains on white look a lot worse than black along with tan or gray. Colorants are easy to apply, and they don't scrub off.

All of this assumes that your grout, while stained, is still intact and that the tiles are all fully stuck to the wall. If the grout is crumbling, it needs to be removed and replaced. You can buy a little tool called a grout saw to make the process easier. If any tiles are loose, there is probably also an underlying moisture issue that needs to be addressed.

But if you're lucky, stains are your only issue. Once you have removed the mineral deposits or tinted the grout to make the stains less noticeable, apply a grout sealer, which will reduce the amount of water the grout absorbs and therefore help keep it from staining again.