Those tiny cracks might not be as harmless as you think.
I have a few mugs I absolutely love. There’s my “I Heart My Library” mug, my Ravenclaw mug (because, of course), and the Les Misérables one I bought after seeing my favorite show for the first time more than 25 years ago. That one has been permanently reassigned to holding the most sacred of kitchen ingredients in my fridge: bacon grease.
But there are other mugs in my cabinet that have just … been around. They’re the dependable ones that have made it through multiple kitchen purges, several moves, and the general chaos of a house where dishes are, at any given moment, either in use, in the sink or being “repurposed” for something questionable by one of my kids.
Lately, I’ve started to notice something about a few of them: faint, web-like cracks across the surface. Not chips, but rather a subtle pattern in the glaze that you don’t see until suddenly you do. Once I noticed it, I couldn’t unsee it. And then I was left wondering whether this is just what happens when mugs live a long life—or if it’s a sign that they’ve aged out of daily use.
What is crazing, and why does it happen to mugs?
That network of fine, hairline cracks has a name: crazing. It may look like a structural issue, but it’s actually happening in the glaze (the glossy outer coating that seals the ceramic underneath).
That glaze is supposed to be smooth and nonporous; it acts as a barrier between whatever you’re drinking and the clay body beneath it. But ceramics aren’t perfectly static materials. The glaze and the clay expand and contract at slightly different rates when exposed to heating and cooling over time, and eventually that tension shows up as a series of tiny cracks.
Sometimes crazing shows up early, especially if the glaze and clay weren’t perfectly matched during manufacturing. Other times, it develops slowly—helped along by years of hot coffee, cold rinses, cycles through the dishwasher, and the general wear-and-tear of daily use.
In other words, this isn’t necessarily damage in the way a chip or crack in the mug itself would be. It’s more like a surface-level aging process. One that just happens to be very visible once you know to look for it.
Is it safe to drink from a crazed mug?
Unfortunately, those little cracks aren’t as harmless as they look.
An intact glaze is what makes ceramic drinkware food-safe. It’s a smooth, sealed surface that doesn’t absorb liquid or trap anything you don’t want lingering in your cup. Once that glaze is crazed, that seal isn’t quite so reliable.
Those fine cracks can allow liquid to seep into the more porous ceramic underneath. Over time, this seepage can lead to staining, lingering odors and—least appealingly—bacteria settling into spaces that can’t be cleaned. Most food safety guidance considers crazed dishware to no longer be food-safe for regular use, especially when it’s repeatedly exposed to heat and moisture (which is exactly what happens in a coffee mug).
With older mugs, this can be even more important. Vintage pieces may have been made with glazes that contain lead or other heavy metals, and crazing can increase the likelihood that those materials will leach over time.
That said, these aren’t situations where one cup of coffee suddenly becomes dangerous. Light crazing on a mug you occasionally use is unlikely to cause immediate harm. But for the ones in steady rotation, the risk does build over time.
My mugs that show those telltale cracks aren’t necessarily headed for the trash, but they’re no longer my first choice for my morning latte. They’ve started to drift into second jobs—holding pens, corralling odds and ends, and maybe even housing a plant on the windowsill.
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