Turn over a mug, a plastic container, or a baking dish, and you'll often find a little picture stamped on the bottom. Usually it shows dishes or glasses with water spraying over them. Sometimes it adds the words top rack only. But what is that symbol really telling you?
A dishwasher-safe label means the maker says the item can handle what happens inside a dishwasher: hot water, strong detergent, and lots of splashing and bumping. That's a tougher test than it sounds. Hot cycles can reach about 167 degrees Fahrenheit, and dishwasher detergent is strongly alkaline. Plenty of kitchen items can't take it.
Why Dishwashers Are So Rough
Dishwashers clean with a combination of heat, chemistry, and force. The detergents are strong, the spray is powerful, and items knock into each other during the cycle. That's great for baked-on lasagna, not so great for delicate things.
Aluminum, brass, and copper can discolor. Nonstick coatings wear down. Glossy, gold-trimmed, and hand-painted dishes can dull or fade. Sterling silver and pewter can tarnish, and pewter melts at such a low temperature that it may even warp.
Wood suffers too. High heat and moisture damage wooden cutting boards and spoons, and glued items like hollow-handled knives can soften or come apart. Cast iron pans lose the seasoning that makes them naturally nonstick.
Home machines are actually designed to go a bit easy. Most use about 167 degrees rather than the 181 degrees of some commercial machines, to reduce burn risk, save energy and water, shorten cycles, and avoid damaging plastic. Modern enzyme detergents work best even cooler.
The Secret of Top Rack Only
If you've seen top rack only on plastic items, there's a good reason. Many dishwashers, especially in North America, have an exposed heating element at the bottom of the machine. High temperatures damage many plastics, and items in the bottom rack sit closest to that heat.
Squeezing plastic into tight spaces can also warp it. Plastic also tends to come out wet, because water forms drops on it that don't dry as well as they do on china and glass.
Cloudy Glasses and a Different Symbol
Even glass isn't completely safe. Over time, glasses washed in a dishwasher can develop a white, cloudy haze. It begins as a rainbow-like sheen and turns milky, and it can't be scrubbed off, because the alkaline detergent slowly dissolves the glass surface itself.
Don't confuse the dishwasher icon with another common one: a wine glass and a fork. That's the international symbol for food safe, meaning the material is considered safe to touch food. It's mandatory for food-contact products sold in Europe, but it says nothing about whether the item can survive a wash cycle.
The Woman Who Made It All Possible
We have Josephine Cochrane to thank for the dishwasher itself. A wealthy Illinois woman, she was frustrated when her good china got chipped during hand washing. In 1886, she and mechanic George Butters built a successful hand-powered dishwasher in her tool shed and patented it that December.
Her machine was shown at the 1893 World's Fair in Chicago. Hotels and restaurants were her main customers, and home dishwashers didn't really catch on until the 1950s. Her company later became part of KitchenAid.
So before you load Grandma's gold-rimmed china, take a peek at the bottom. That tiny symbol could save you a whole lot of heartbreak.
