A ceramic mug may become too hot to handle, yet there are ways to pick ceramic mugs that stay cool in the microwave. Modern mugs often state on the base "Microwave safe" or "Not for use in microwave." Yet there's more to microwaving ceramic mugs than how they're labeled. Using the wrong ceramic mug in the microwave can be hazardous.

Setting a timer on the microwave oven
credit: humonia/iStock/Getty Images
A woman holds a yellow ceramic mug and adjusts a microwave timer.

Microwave-Safe Labels

All that "Microwave safe" label tells you is that you can use the mug in the microwave without damaging the mug. The labeling isn't standardized, and manufacturers aren't required to test their ceramicware to make sure it's safe to handle after heating, Good Housekeeping reports. The label also doesn't ensure that the mug is nontoxic, another issue with heating liquids to boiling in a ceramic mug in the microwave. Don't use a ceramic mug that isn't labeled "Microwave safe" in the microwave because the mug may break.

Hot Drinks and Cool Hands

Color provides one key to choosing a safe mug to use in your microwave. Dark glazes may have more metal in them, such as manganese, according to Argonne National Laboratory's Ask a Scientist website. Even when these metals are safe to use on dinnerware by U.S. standards, they tend to heat to a hotter temperature than glazes with little or no metal. Some mugs, even those labeled "Microwave safe," become excessively hot in the microwave and can cause burns. Light-colored ceramic mugs of recent manufacture are more likely to stay cool.

Ceramic Hazards

Certain colorful pre-1960s mugs, such as trademarked Fiestaware, got their color from uranium and other radionuclides, according to the U.S. Environmental Protection Agency. These glazes give off radiation, and the EPA advises against using them for food or drink, so don't nuke them. Glazes used on ceramics may contain lead or other heavy metals, including arsenic. These are toxic and may contaminate liquids microwaved in these mugs. Good Housekeeping's tests found that some contemporary mugs subjected to a month's worth of dishwasher cycles and microwaving were more prone to becoming too hot than newer mugs, and one of the aged mugs leached lead, although at a low level.

Safe Microwaving

Although the lead levels found in the tests were fairly low, it's best to avoid lead exposure. Lead can be particularly harmful during pregnancy. Avoid using chipped or cracked ceramics and lead-glazed pottery from cottage industries based in other countries, the Centers for Disease Control and Prevention advises. Don't put mugs with metal parts or metallic paint in the microwave. Even a delicate line of gold paint at the top of the rim isn't microwave safe because metal becomes superheated in the microwave.