Does anyone know why my cast iron has developed pits or craters, whether I should be concerned, and if there's anything to be done?
Pits are generally caused by rust or acids. If they aren't too bad, seasoning can eventually smooth them out some.
There was a time before the internet, when I didn't know much about cooking, especially about caring for cast iron. The world was young and reckless, and so was I. Back then, when I washed my favourite cast iron skillet, and yes, I washed it in soapy water at least once a day. When I washed my pan, I put it upside down to dry. Little droplets of water gathered on the bottom, and slowly over the years, it pitted, then the pits became craters, then... well, what's bigger than craters? It's now at the point where I am totally ashamed to bring the pan out in public.
I know better now, and I still keep my pockmarked pan as a reminder of what not to do. Even though I still wash my cast iron in soapy water (gasp and horror) I stand it up so that water doesn't gather and inflict it's malice upon my pans. Since this change, no more pitting on my old or any other cast iron pans.
Back to your pan. I can't see in the picture, but is the pitting on the inside or the outside?
If it's on the inside, another cause of pitting in pots (this goes for almost all metal pots by the way, not just cast iron) could be salt. Adding salt to a cool pan, without stirring it enough to dissolve, can cause pitting. It sometimes settles on the bottom and can make a right mess of a pan. That's why a lot of old cookbooks advise salting the water after it's warm.
Should you be concerned? It is definitely something to keep an eye on. The surface is uneven now, which will attract whatever it was to start the pitting to gather in the rough spot...well, you get the idea. If you can discover the cause, then it will be easier to prevent it getting worse.
Like Greenstuff says, seasoning and using the pan will help repair it over time. Though without discovering the initial cause, it may be a race between the seasoning and the pitting.
I can't wait to hear what others can tell us about this. There is some absolutely fantastic cast iron knowledge lurking about this forum.
Thanks! I suspect the cratering started because I used the cast iron to hold ice cubes to steam 4-hour baguettes (see the Saveur / Food52 Genius Recipe).I finally re-seasoned my cast iron using the Cook's Illustrated method here: https://www.cooksillustrated...It looks brand new! I really recommend it for your cast iron.