Your favorite mug might be lying to you. Those fine, web-like cracks spreading across the glaze are called crazing, and they could mean it’s time to retire your morning coffee companion.

Crazing is easy to miss at first. You use a mug for years without noticing anything, and then one day the light hits it at just the right angle and suddenly you can see the whole network of hairline fractures spreading across the surface. Once you see it, you can’t unsee it.

Here’s what’s actually happening. Ceramic mugs have two distinct layers: the clay body underneath and the glaze on top, that smooth, glossy coating that makes drinkware food-safe. The problem is that these two materials expand and contract at slightly different rates every time you pour in hot coffee, run cold water over the mug, or cycle it through the dishwasher. Over months and years, that repeated tension creates tiny cracks in the glaze. It’s not the clay itself breaking down. It’s the outer seal.

Sometimes crazing happens early if the glaze and clay weren’t well-matched during manufacturing. More often, it builds up slowly over time, helped along by the kind of daily punishment that a well-loved mug takes.

It looks like ordinary wear. It isn’t.

An intact glaze is what keeps ceramic drinkware actually safe to drink from. That smooth, sealed surface doesn’t absorb liquid and doesn’t trap bacteria. Once the glaze crazes, that seal breaks down, and those fine cracks can let liquid seep into the porous clay underneath. From there, you’re looking at staining, lingering odors, and bacteria settling into spaces that a sponge and dish soap simply can’t reach. Food safety guidance generally considers crazed dishware no longer safe for regular use, especially when it’s holding hot liquids, which can push the seepage deeper.

The full breakdown of what crazing means for your cabinet, covered in detail by Taste of Home, puts it plainly: those cracks aren’t just cosmetic. The mug is no longer doing its job the way it should.

So what do you do with a crazed mug?

Retire it from the drink rotation, but don’t just throw it out. A crazed mug is perfectly fine for dry use. Set it on your desk for pens and scissors. Put it on the windowsill for small plants or herb cuttings. Use it in the garage for loose screws or craft supplies. The crazing only becomes a problem when the mug is holding something you’re going to drink, especially hot beverages that accelerate the seepage. For dry storage purposes, it’s still a perfectly functional object.

Checking your cabinet doesn’t require any tools. Hold each mug up to a bright light, or run your fingernail lightly across the surface. You’ll feel crazing before you see it sometimes, a faint texture where there should be smoothness. Mugs that have been around for a decade or more, especially ones that live in the dishwasher regularly, are the most likely candidates.

If you want to keep mugs from crazing quickly, a few habits help. Hand-washing extends glaze life better than the dishwasher, since the high heat and harsh detergents speed up the expansion-contraction cycle. Letting mugs cool down before running cold water over them cuts down on the thermal shock. Neither approach prevents crazing forever, but both slow it down.

The Ceramic Arts Network has solid background on how glaze chemistry affects durability if you want to go deeper on why some mugs last decades without a single crack while others show crazing after a few years. Clay body composition, firing temperature, and glaze thickness all play a role, which is why even mugs from the same set can age differently.

Your kids probably have a mug or two in the rotation that’s been around since they started drinking hot cocoa in elementary school. Worth pulling those out and checking them this week. Crazed mugs that get daily use by younger kids deserve particular attention, since they tend to get the most dishwasher cycles and the roughest handling.

The good news is that swapping out one or two mugs isn’t a big project. Most mugs are inexpensive to replace, and knowing what to look for means your whole family can feel confident about what’s sitting on the shelf above the coffee maker every morning.