8 Things You Absolutely Should Not Put in a Dishwasher (And Why)

5. Insulated Travel Mugs and Tumblers

Your favorite vacuum-insulated travel mug keeps your coffee piping hot for hours, but a trip through the dishwasher can permanently ruin its thermal performance.

• The Why: Most insulated tumblers rely on a vacuum seal—a double-walled space between the inner and outer layers where air has been completely sucked out. The high heat and pressure inside a dishwasher can compromise the delicate seal that holds this vacuum together. Once the seal is broken, water enters the inner chamber, destroying the mug's insulation capabilities and creating a breeding ground for mold.

• The Fix: Handwash the body of the mug. The lids are often top-rack dishwasher safe, but always check the manufacturer's guidelines first.

6. Vintage China, Crystal, and Hand-Painted Glass

If an item belonged to your grandmother or features a delicate gold rim, keep it far away from the appliance.

• The Why: Antique china and crystal are fragile and susceptible to chipping if they rattle against other dishes. Additionally, the aggressive detergents will chemically etch glass over time, turning beautiful clear crystal cloudy and permanently dull. Gold leafing, hand-painted details, and delicate glazes will easily chip or fade away under the pressure.

• The Fix: Wash these items gently in a sink lined with a towel (to prevent accidental clinking against the porcelain) using a mild soap.

7. Soft Plastics and Reusable Containers

While many modern plastic storage containers are labeled "top-rack safe," older or thinner plastics should be kept out.

• The Why: The heating element at the bottom of most dishwashers gets incredibly hot. Thinner plastics (like takeout containers or older water bottles) will easily melt, warp, or distort under this heat, making it impossible to fit their lids back on. Furthermore, there is a risk that warped plastic can release unwanted chemicals when exposed to extreme degradation.

• The Fix: If you do wash sturdy plastics in the dishwasher, strictly stick to the top rack, away from the heating element, or opt to wash them quickly by hand.

8. Aluminum and Copper Cookware

Copper pots add a striking look to a kitchen, and aluminum baking sheets are a baker's staple, but both metals react poorly to dishwasher chemistry.

• The Why: Dishwasher detergents cause a chemical reaction called oxidation on aluminum, turning shiny silver baking sheets into a dull, dark, and chalky gray. Copper, on the other hand, will quickly lose its brilliant luster, becoming tarnished and discolored after just one wash.

• The Fix: Handwash copper and aluminum to preserve their bright, metallic finishes and prevent unappealing surface discoloration.

The Golden Rule of Cleanup

When in doubt, a quick rule of thumb is to look at the composition of the item. If it’s an everyday item made of high-quality stainless steel, heavy porcelain, or thick glass, it’s usually safe for the machine. But if it involves specialized coatings, natural materials, precision edges, or delicate finishes, taking an extra minute to wash it by hand will save you money and keep your kitchen tools performing beautifully for years to come