We tend to develop a special relationship with our cast iron pans. We spend years building up the proper seasoning, cleaning them ever so gently, and drying out any bit of moisture to maintain that perfect crust that delivers the most scrumptious bacon and eggs on Sunday morning. And with proper care, cast iron pans can last for decades (many people believe the oldest pans are the best pans).
Alas, they can also be mistreated over the years, and sometimes the only option for an old cast iron pan is to let it go. Here's when you know it's time.
1. It's cracked.
This is one of the few times an issue with a cast iron pan becomes an actual liability. If a cast iron pan is cracked, that crack is a hazard: It will expand and contract as you use the pan, which, one day, will cause it to crack all the way through. If you cast iron pan is cracked, it's toast.
2. It has a hole in it.
Okay, it's really hard to make a hole in cast iron. It either means it has rusted all the way through, so obviously it won't function anyway (a little surface rust, however, is no problem), or that someone deliberately put a hole through it to prevent someone else from using it (say, if it had been previously used with materials that aren't food-safe, like lead or motor oil). So if you inherited something with a hole, I'm sorry, but it's no good to you.
3. It's warped or wobbly.
Cast iron pans can get literally bent out of shape under super-high heat, leading to irregularities along the bottom, which can prevent it from sitting flat on a surface. Technically it should still work fine, but if it's so wobbly that your food might spill out of it or it could easily be knocked off-balance on the stove, you might consider getting a new one. It's not worth the risk of spilling a scalding-hot meal.
4. It's covered in dust.
No, dust doesn't harm a cast iron pan. But it's probably a sign that you just don't use it. If you find a cast iron pan too unwieldy to use, or it's just not part of your cooking routine, consider passing it along to someone else who can really use it. There's no point storing something just because you feel like you should be "a cast iron person" if you just never use it.