The difference in surface texture between old and new cast-iron frying pans is primarily due to the seasoning and usage history of the pans.
Seasoning: Seasoning is the process of coating the cast iron pan with a layer of oil and then heating it to create a natural non-stick surface. Over time, with repeated use and proper seasoning, the surface of the pan becomes smoother as the oil polymerizes and forms a more uniform, non-stick coating. This process fills in the micro-pores and irregularities in the cast iron surface, resulting in a smoother cooking surface.
Usage and Cleaning: As you use the cast iron pan, cooking oils and fats get absorbed into the surface, contributing to the seasoning process. Additionally, the action of cooking and cleaning can help smoothen the surface as food particles and debris are scrubbed away, slowly leveling out any roughness.
Age and Wear: As a cast iron pan ages, it tends to become smoother due to the repeated heating and cooling cycles it undergoes during cooking. The heat causes the metal to expand and contract, which can contribute to filling in surface imperfections over time.
Manufacturing Techniques: In recent years, some manufacturers have started producing pre-seasoned cast iron pans that come with a smoother initial surface. These pans undergo specialized treatments during manufacturing, resulting in a smoother texture compared to traditional cast iron pans that require seasoning from scratch.
It's important to note that the roughness of the surface does not affect the performance of the cast iron pan. In fact, some people prefer slightly older, more seasoned pans as they tend to have better non-stick properties.
If you have an old cast iron pan with a rough surface that you'd like to improve, you can do so by re-seasoning it. To do this, you'll need to thoroughly clean the pan, apply a thin layer of cooking oil, and then bake it in the oven at a high temperature. This process will help build up a new layer of seasoning, contributing to a smoother surface over time.