Hard-anodized cookware is made of aluminum that's been treated through an electro-chemical process to increase durability. Aluminum is put into an acid electrolyte bath, and the bath is charged to create a protective aluminum-oxide coating. It's considered twice as strong as stainless steel, is resistant to corrosion and abrasion, and because of the aluminum base, it offers even heating. Originally, hard-anodized cookware was used in the restaurant business in the United States, but after awhile, it became available for home use, too. Hard-anodized cookware tends to be a matte gray color.
One downside to hard-anodized cookware is that it's porous and isn't nonstick, so all kinds of food can get baked on if you don't oil the pan properly before use. Although there are now versions of hard-anodized cookware that have nonstick coatings, the traditional version is still prone to problems of encrusted food and stains. However, no matter what gets caked on to hard-anodized cookware, you should never clean it with steel wool or any other abrasive scouring tool, including chlorine bleach. In addition, unless otherwise noted by the manufacturer, hard-anodized cookware can't be washed in a dishwasher; the detergent discolors it and damages it.
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Instead, hard-anodized cookware should be cleaned by hand using a normal, mild dish soap and warm water. If you have to scrub, use a dish sponge, dish cloth or nylon pad. If you don't like water spots, you can dry your hard-anodized cookware after you wash it. It's important to clean your cookware after each use to prevent any greasy buildup, considering the porous nature of the cookware. If you end up with a stain, mix baking powder and water to form a paste and scrub your hard-anodized cookware with that.