Thermal shock is when a material is damaged due to a sudden temperature change.
When cast iron heats up, the metal expands, and when it cools down, it contracts, according to cast-iron expert Chris Wing. Known as Cast Iron Chris on social media, Wing is also the host of a YouTube series called Basics of Cast Iron.
“If this process happens too rapidly, it creates uneven heating and cooling, and since cast iron isn’t as flexible or malleable as other metals, the uneven expansion and contraction can create internal stress on the material,” Wing explained. He said this reaction can cause the cast iron to warp, crack, or, in extreme cases, split in half.
If a cast-iron pan or skillet comes from a cold environment and is heated quickly — whether on an electric stove, an open fire, a grill, or in an oven — the risk of thermal shock increases. Senior staff writer Michael Sullivan, who co-wrote our cast-iron skillet guide, said you can also shock a hot cast-iron skillet by running it under cold water (or transferring it to another equally cold environment, like a refrigerator or freezer) without allowing it to cool first.
Enameled cast iron (cast-iron cookware coated with a layer of enameled glass) is also at risk of thermal shock. Wing pointed out that vintage or antique cast-iron pieces (which are very thin and light) are more susceptible to damage than modern pieces (which tend to be thicker and more durable).
Wing assured us that the risk of causing thermal shock to cast-iron cookware while grilling is relatively low. But if cast-iron cookware is handled incorrectly, thermal shock can happen during other routine activities. For example, it can occur if you wash a cast-iron skillet without allowing it to cool first. Thermal shock can also happen if you place your pan in or on a piping-hot appliance — like an oven, a stovetop burner, or a grill — without gradually warming it up first.
In some cases, damage caused by thermal shock is highly noticeable. For example, a cold cast-iron pan might split in half when it’s exposed to a piping-hot heat source. But sometimes the damage isn’t as dramatic. “Some older pans may experience ‘microcracking,’ small surface cracks that aren’t as noticeable to the naked eye and make a pan more susceptible to breakage over time,” Michael said.
Drastic temperature changes can exacerbate these fissures, big or small, so it’s important to avoid thermal shock whenever possible.
