THINGS YOU SHOULD NEVER PUT IN THE DISHWASHER
The dishwasher is one of those “best things since sliced bread” inventions, saving you countless hours over the sink. Unfortunately, some of your most-valuable kitchen tools just can't stand up to the appliance.
We're talking cutting boards, wooden utensils (like spoons) and knives with wooden handles. Why? Dishwasher detergent is abrasive and will scratch wood over time. Plus, the heat of the drying cycle can cause the wood to warp or even crack. Rinse and hand-wash cutting boards soon after you use them, but don't submerge them in water. A little baking soda can help scrub away stains, too.
It took a lot of effort to get your cast-iron cookware perfectly seasoned! Don't ruin it (or potentially cause your pan to rust) by using the dishwasher as a cleaning shortcut. Instead, rinse your cast iron in hot water, scrubbing with a soft sponge to lift stuck-on food. Don't use dish soap (it could break down the seasoning), and dry thoroughly with a towel.
Aluminum pots and pans can technically go in the dishwasher, but they'll oxidize and fade from shiny to dull after a single cycle. Aluminum cookware's doppelganger, stainless steel cookware, can often go in the machine - but always heed manufacturer guidelines.
Copper or Other Precious Metals
Those pretty copper pots could dull or discolor in the dishwasher. Keep silver, gold and bronze heirlooms out, too; they'll tarnish and will eventually lose their gleaming finish.
Generally, the nonstick coating just doesn't hold up well in the turbulent washer. You can likely make an exception for items that are labeled "dishwasher-safe" by the manufacturer, but give them a second look after they come out of the machine, to note any damage.
Certain Plastic Items
You should take the manufacturer's lead on this one - some softer plastics can't handle the dishwasher's heat. Hardier items like food processor bowls, plastic cutting boards and mixing bowls can often go in the dishwasher, but know that most clear plastic will get scratched and dulled over time if you wash it this way. Also, it's often wise to place plastic items in the top rack of the washer, where they're farther away from the heat.
Placing your good knives in the dishwasher is a big no-no. The detergent will dull a knife's blade - frustrating as you slice and dice, and also unsafe, as you're more likely to lose control of a dull knife. Plus, the hot water and heat in the drying cycle will loosen knives' handles. Wash your kitchen knives carefully by hand in hot soapy water instead.
Fine China, Crystal or Painted Plates
Don't ruin the heirloom hand-painted dishware your grandmother passed down to you (or the pretty plates you bought at Anthropologie, for that matter). These delicate items can chip, fade or lose their finish in the dishwasher. Any silverware with carved antler or bone handles should stay out of the machine as well.