Why Oil Is Still Necessary When Cooking In A Seasoned Cast Iron Pan
Cast iron pans are known to be well worth their weight (literally and figuratively) as they can last generations and are kitchen workhorses. They do, however, require quite a bit of upkeep. When you season a cast iron pan, you are essentially baking a layer of oil into the surface that serves as a protective coating. Without this layer, the iron would corrode and rust. And while a seasoned cast iron pan will offer some non-stick properties, it is still imperative to use a cooking fat every time you use it.
The surface of the metal of your pan is full of tiny, microscopic pores, crevices, and divots. In other words, it's not perfectly smooth despite it looking so. Seasoning the pan alone isn't enough to spackle over these holes, so adding additional oil or fat will fill them in and prevent food from sticking as you cook. The oil will also help transfer heat to cook whatever you add to the pan properly.
For this reason, you also want to sufficiently heat the pan before adding the cooking fat or oil. As the metal heats up, the surface will expand, widening those microscopic divots and cracks so they're not as deep and there's less space for food to get trapped in — think of a runny egg white just seeping right into every tiny nook and cranny.
How seasoning a cast iron pan works
When you coat a cast iron skillet with oil and heat it to a very high temperature, the oil's fatty acids reorganize and bond together, known as polymerization. This creates a layer that attaches to the metal and leaves a glossy, slick coating. The more you cook with sufficient oil in your seasoned cast iron pan, the thicker this protective layer will become and the more effectively non-stick your pan will become. Think of every time you cook as an opportunity to strengthen the bond and fill in any gaps in the seasoning of your beloved pan. Even if a pan comes pre-seasoned, as many do these days, it will still do well with an at-home seasoning with each use.
To maintain the seasoning and added layers you've built up with each use, never soak your cast iron in soapy water or (gasp!) pop it in the dishwasher. A simple wiping with a cloth and kosher salt as a scrub for tough bits of stuck-on food will suffice.