Cleaning pots with eggshells

Last year I learned a method of cleaning the blackened bottom of pots and pans, using eggshells. The way is to flatten an eggshell and use it to scrub the bottom of the pot. (I use a hard sponge to "hold" the eggshell.) The blackened bits will come off due to the abrasive edges of the eggshells, while the pot remains unscratched.

I recently cleaned my pan again, and decided to take a photo of "before" and "after" surfaces.

Needless to say, the cleaned part is on the left. (This is in the middle of cleaning, so you can see bits of eggshells around)