What, are you crazy? Don't put that in your dishwasher!

As wonderfully convenient as dishwashers are, they are not all-purpose miracles of technology that you can entrust with cleaning anything you can get your mitts on. It’s surprising how many people have to learn this the hard way. While some things may survive a run or two, certain items should never be put through a dishwasher cycle. Here’s a rundown of what not to do.

Never put these in your dishwasher

Cast Iron

The inside of an active dishwasher is hot and wet—two conditions that are disastrous for certain kinds of cookware. Not only will dishwasher cleaning cause cast iron skillets to rust, it will also wash away the seasoning—that protective, nonstick, and delicious layer of oil and lard. Unless you want your cast iron cookware to end up looking like some sort of prehistoric anvil, don't put it in your dishwasher.

Wood

Once again, dishwashers get hot—really hot. Heat causes wood to warp, so avoid putting wooden objects in your dishwasher. That includes utensils, bowls, cutting boards, and even pots and pans with wooden handles. Furthermore, the dry cycle may cause the wood to crack, opening up a breeding ground for bacteria.

China and Delicate Glassware

This one should be obvious: Don’t put delicate wine glasses, china, or other formal wares in your dishwasher. What are you thinking? The chemicals in dishwasher detergents are abrasive, so they’ll wear away at delicate materials and cause them to crack. The intense heat of a dishwasher can also cause glasses and china to shatter, which is sure to put you at odds with whoever owns those precious family heirlooms you were too lazy to wash by hand. Not to mention, you’ll have to manually clean shattered glass out of the wash tub, which sounds like a blast but is probably hazardous, to say the least.

Gold-Trim Dishes

They're almost always delicate (see above), and even if they don't wreck the entire plate, the water jets can spray away flakes or chunks of gold.

Insulated Travel Mugs

Travel mugs have a vacuum seal between the inner and outer shell, which can be breached when it's put through the dishwasher. Aside from filling the vacuum with water for an annoying sloshing sound, it can hinder the mug’s ability to retain heat, thereby defeating the whole purpose of a travel mug.

You probably shouldn't put these in your dishwasher

Teflon

There’s a lot of debate over this one, but we’ll go ahead and say you probably don’t want to put Teflon in your dishwasher. Recent research has suggested that polytetrafluoroethylene—the chemical name for Teflon—is carcinogenic, particularly when it begins to break down. That reason alone should be enough to scare you away from the idea of agitating it in a high-temperature alkaline box. Manufacturers like Dupont say it’s fine, and dishwashing it a few times over several years is probably okay. But frequent washing is likely to hasten the breakdown process, coating other dishes and utensils with undesirable chemicals. And honestly, how easy is it to wash nonstick pans by hand anyway? Don’t be lazy.

Stainless Steel

Most experts say it’s okay to put stainless steel products in the dishwasher—but with a few caveats. The general fear is that detergent and excessive humidity will corrode the metal, and that acidic food soils will tarnish its polish. It’s also recommended that, if you do choose to put stainless steel in the dishwasher, it should be placed in the upper rack or the cutlery basket and spaced far enough away from other materials so as to ensure proper washing and drying. However, we feel there are far too many precautions, and that you’re better off just washing stainless steel by hand.

Aluminum

Some aluminum products are specifically designated to be “dishwasher-safe.” These materials are probably fine, but others may be prone to scratching. They may also develop a dull finish over time. We’d recommend taking this category on a case-by-case basis.

Knives

Knives turn into sporks when put through the dishwasher. OK, not really, but they do dull over time in the dishwasher. Then you have little more than a blunt object for cutting vegetables, and you’re no troglodyte, so wash your precious blades by hand. Just be careful.

Plastic

If a plastic item is not clearly guaranteed “dishwasher-safe” by the manufacturer, then wash it by hand. The heat from the water and dry cycle can warp the material and possibly even melt it. Even if the item is deeded safe for the dishwasher it’s a good idea to place it in the upper rack—or as far away from the heating element as possible. But anyone who’s ever had to deal with melted plastic in their dishwasher might recommend staying on the safe side and avoiding it altogether.

More on dishwashers

• The Best Affordable Dishwashers We've Tested
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• It's easy to buy a dishwasher online—here's why
• The Best Dishwashers We've Tested That Dry Your Dishes
• The Best Stainless Steel Dishwashers We've Tested
• The Best Quiet Dishwashers We've Tested
• The Best Countertop Dishwashers We've Tested
• Here's how dishwashers dry dishes, from zeolite to air dry
• What actually happens inside your dishwasher?