A newly seasoned cast iron pan has an oily film on the surface because of the seasoning process itself. Seasoning is the process of applying a thin layer of oil to the surface of the cast iron and then heating it to create a natural, non-stick coating. This coating is formed by polymerizing the oil, turning it into a durable layer of carbonized oil or polymerized fat.
When you season a cast iron pan, you typically apply a thin coat of oil or fat to the pan's surface and then heat it in the oven. During this heating process, the oil undergoes a chemical transformation, breaking down into simpler compounds and releasing volatile molecules. These molecules then bond to the surface of the cast iron, forming the protective coating.
The oily film that you notice on a newly seasoned cast iron pan is likely residual oil that hasn't fully polymerized or carbonized during the seasoning process. This is normal and can be easily addressed by wiping off the excess oil with a clean cloth or paper towel.
Over time and with regular use, the seasoning on a cast iron pan will continue to improve, becoming even more non-stick and developing a rich, dark patina. The seasoned surface also helps to prevent rusting and enhances the flavor of the foods cooked in the pan. It's important to note that the seasoning layer is not the same as a layer of oil; it has undergone a chemical transformation that makes it more durable and resistant to sticking. So, even if you wipe off the initial oily film, the seasoned coating will remain on the pan's surface.