The black coating you observed on the inside of skillets used in restaurants is most likely due to a process called seasoning. Seasoning is a common practice in professional kitchens where oil is applied and heated onto the surface of the pan to create a natural non-stick coating. This coating is formed by polymerizing oil and creating a layer of carbonized oil on the surface of the pan, which appears black.
Seasoning offers several benefits for cooking in skillets:
Non-stick properties: Seasoning creates a smooth and slick surface on the pan, reducing the chances of food sticking to it. This is particularly useful for cooking delicate items like eggs or fish.
Enhanced flavor: Over time, the seasoned layer develops a complex flavor that can add depth and richness to the food being cooked. It acts as a natural seasoning that imparts subtle flavors to the dishes.
Protection against rust: The layer of carbonized oil helps protect the pan from rust and corrosion. It creates a barrier between the metal surface and moisture, preventing the pan from deteriorating.
Easy maintenance: Seasoned pans are generally easier to clean. The non-stick properties of the seasoned surface reduce the likelihood of food residue sticking to the pan, making it easier to wipe or wash off.
It's worth noting that the seasoning process is not exclusive to restaurant kitchens. Many home cooks also use this technique to maintain and improve the performance of their skillets. The black appearance is simply a visual indication of the seasoning process, and it signifies a well-used and well-maintained pan.