If you have slate floors or countertops in your bathroom or kitchen, you will probably have to deal with lime deposits at some point. These deposits, which come from hard water, form cloudy swirls that detract from the appearance of the stone. Cleaners containing sulfamic acid, a relatively weak acid, can dissolve these stains, but acidic cleaners generally aren't recommended because they can etch slate. You should try alternatives before using this type of cleaner.
Slate Flooring Care
Slate is a metamorphic rock formed by layers of mostly mica and quartz, with a smattering of other minerals, including chlorite and hematite. It has a hardness of 6 on the Moh scale, which is a little harder than glass, and most varieties aren't very porous. None of these characteristics seem to contraindicate acidic cleaners, but tile experts nevertheless recommend avoiding them, because they can dull the smooth, glass-like finish, which is part of slate's appeal. The best cleaners are pH-neutral or those that are mildly alkaline, but neutral cleaners may not remove salt deposits.
Acidic De-Scaling Cleaners
The unsightly white deposits that discolor slate tiles are natural salts, such as calcium carbonate; vinegar quickly dissolves these from other bathroom surfaces, such as glass and chrome. Vinegar has a pH of 2.4 -- the lower the pH, the stronger the acid -- and that of sulfamic acid is in the range of 1.2. The latter is commonly used for descaling, but since many tile professionals recommend avoiding vinegar for cleaning slate, they are unlikely to recommend sulfamic acid. The cleaners most often recommended for slate are pH-neutral, which means they have a pH value close to 7.
Safe Cleaning Strategies
The key to cleaning lime deposits from slate tiles with minimal impact to the finish is to get them early. The safest cleaner is warm, clear water, but adding a gentle detergent, such as dish soap, won't lower the pH and may emulsify the scale. If that doesn't work, you may have success by making a paste with peroxide and baking soda and leaving the mixture on the scale deposits until it dries. Avoid this treatment on colored grout, because peroxide is a bleach. Another alternative is to use a commercial, non-acidic tile cleaning product.
Cleaning Stubborn Scale
If the lime stains have set into the tile, they may not come off with a pH-neutral or mildly alkaline cleaner. Before resorting to an acidic cleaner, make a strongly alkaline solution by mixing 2 tablespoons ammonia and a tablespoon of borax in a gallon of warm water and use this to rub off the stains. Test this solution first in an inconspicuous location, because alkaline cleaners can also dull slate. If this solution doesn't remove the stains, acidic cleaners are your last alternative. Try a one-to-one solution of vinegar and water before cleaning with a sulfamic acid-based cleaner.
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