Things you should never put in the dishwasher

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I have a different relationship with each of my kitchen appliances. With the fridge, I feel a friendly familiarity, and with my microwave, a sort of unspoken camaraderie. I interact with both frequently, sometimes several times a day.

With dishwashers, though, there is a reverence. My family didn’t own one until I was much older, and it has always seemed like a mysterious invention to me. Every time I use one now, I’m worried about doing something wrong, specifically whether I’m putting in an item that I shouldn’t. If you’ve ever had similar worries, here’s what you should know.

Things you should not put in your dishwasher

Acrylic dishware or accessories: Crazing (small cracks through the acrylic) might occur.

Brass, bronze, copper, and pewter items: High-temperature water and detergent can cause discoloration, says Larry Ciufo, who has tested hundreds of dishwashers at CR over the past 20 years.

Cast-iron cookware: You risk removing the seasoning—the baked-on oil in your pan that helps prevent food from sticking—which can cause the pan to rust.

Insulated mugs: Unless they’re labeled as dishwasher-safe, water can get inside the seal between the inner and outer layers of these mugs, which might affect their ability to retain heat or keep drinks cold.

Nonstick pans: If the manufacturer doesn’t say it’s dishwasher-safe, avoid putting them in the dishwasher because the washing process could wear away the coating, Ciufo says.

Sharp knives: Even if they’re labeled dishwasher-safe, you should wash them by hand because they can get dull after rubbing against other cutlery in the basket.

Tin items: Tin will rust if washed in the dishwasher, so you should hand-wash and dry them immediately.

Wood: Water and heat can result in your wooden salad bowls or cutting boards warping and cracking.

Things that are generally safe, but with caveats

Aluminum is usually dishwasher-safe, but minerals in the water might cause spots or darkening. To remove those, scour with a soap-filled steel wool pad.

China, crystal, and stoneware are mostly safe, but if the pieces are hand-painted or fragile, wash them by hand. It’s also better to hand-wash glasses or dishes with gold leaf.

Milk-glass items might yellow after repeated washings in the dishwasher.

Plastic items vary in their ability to withstand heat and detergents, so check whether they’re dishwasher-safe and make sure to place them on the top rack—away from the heating element—and don’t select cycles that use higher wash or dry temperatures.

Stainless steel and silver are generally OK to put in the dishwasher, but make sure you run your rinse cycle if you’re not running the wash cycle immediately, because prolonged contact with food can damage their finish.

By Pang-Chieh Ho, Consumer Reports Senior Home & Appliances Writer