Question: I have a large ceramic soup cup which appears to be internally damaged after being rinsed with cold water while hot:

![Ceramic cup with visible cracks throughout](https://i.stack.imgur.com/kj0v8.jpg)

Is this cup still safe to use, or is it likely to shatter or otherwise fail during use?

Answer: 
That is called crazing. It is a crack or fissure in the enamel coating on the cup, not indicative of deep structural flaws.

Your cup is unlikely to fail in the sense of completely breaking due to the craze in the glaze.

On the other hand, they will stain over time, and be unsightly, and hard to wash out. 

If the piece is old enough, the glaze may contain lead, which can leach due to the crazing. In that case, the piece should definitely be discarded.

Technically, most health codes require crazed dishware to be discarded, although you can assess your tolerance for risk.
potter of 28 years experience here. What you are seeing is called crazing. It's caused by a slightly poor "fit" between glaze and clay body of the ceramics, usually because of too much silica in the glaze. With some glazes, it's intentional; [celedons craze](https://digitalfire.com/glossary/celadon+glaze), as do [white raku](https://ceramicartsnetwork.org/daily/ceramic-glaze-recipes/low-fire-glaze-recipes/fergusons-white-crackle-raku-glaze-recipe/) glazes.

What you're seeing there is unintentional crazing. It's fairly common for this to appear at the first time you use it with a hot liquid, or microwave it. What's happened is that the glaze is 1% smaller than the clay body, so the first time it heats and cools, it crackles.

Whether or not eating off of crazed glazes is a health hazard is [a matter of some debate](https://digitalfire.com/article/crazing+and+bacteria%3A+is+there+a+hazard%3F). It's prohibited for most restaurants in countries with strong sanitary regulations, although rarely enforced. For home use, I personally wouldn't worry about it; you pick up more bacteria off your kitchen counter than you will from the cracks. There's one exception to this though: if the dish is low-fire earthenware, or made from a clay that was not [fully vitrified](https://stoneleafpottery.com/vitrification-nation/2014/08/19/), then the cracks can allow the underlying clay to absorb liquid. In addition to being a bacteria hazard, such absorbed liquid could cause (for example) the dish to explode in the microwave.