Even the best bakeware can suffer cracks or chips from everyday use. This kind of damage can affect the look of the dish, but it doesn't always make it unsafe to use. Depending on the kind of dish and the variety of cracking, you might be able to safely continue baking. In other cases, you may have to keep the dish away from heat or discard it entirely. Examine your bakeware carefully to determine whether it's safe to keep using it once a crack develops.
Enameled Cast Iron
If you drop or strike an enameled cast iron pot, the enamel can chip or crack. Treat damage to the outside of the pot by rubbing a little oil on the surface and continuing to use it. If the chip is on the inside, however, it could pose a health risk. While the exposed iron does little more than contribute additional iron to your diet, the enamel could continue to chip over time. You might be able to contact the manufacturer for a replacement, but the original pot should be discarded.
Fine cracking on the surface of a ceramic dish's glaze is known as "crazing." If you have a new dish that you know was made with safe glazes, you can continue to use it. Some older dishes contain trace amounts of lead and other heavy metals, however. These can leach into food through the crazed surface. If you have crazed vintage ceramics, discard them or use them for decoration only.
Sometimes stoneware and porcelain dishes develop deeper cracks. They can affect the stability of the vessel, but not all cracks require you to stop using the dish. Many cooking liquids seal the surface of the dish once you use it a few more times. If you're worried about baking with a cracked ceramic dish, make sure you examine it carefully before and after use to make sure the cracks aren't getting worse. Never bake a dish that you have repaired with epoxy or super glue, however. These glues are not food-safe and will lose their sticking power at high temperatures.
Even very fine cracks could be a problem for a glass baking dish. They can cause the dish to heat up unevenly, leading to a major disaster in the oven. You can keep your glass dishes from developing cracks by handling them carefully, keeping them away from the broiler and avoiding stovetop use. It's also a good idea to heat frozen dishes in a cold oven, since placing them in a preheated one increases the risk of cracks. Once your glass dish does develop cracks, use it only for cold dishes or discard it in favor of a new one.