The seasoning layer on a cast iron pan is a blend of the oil or grease used to season and ferric oxide
. It is very black. Badly rusted pans will appear red; this is ferrous oxide
and -- for our purposes -- bad.
I disagree about the cooking of tomato sauce in cast iron pans. Tomatoes are acidic, and perhaps if you have a very well-seasoned pan, you should keep it only for frying. But the acid helps to form the early seasoning on a pan. In any case, tomato sauce cooked in cast iron tastes better.
One way to start a pan is to simmer beans or peas in it. Like tomatoes, these will tend to blacken the pan. Use too much beans and not enough water; you'll be throwing these out.
It is very hard to find a new cast iron pan these days. I have gone to expensive gourmet cooking utensil stores, without success. Here in San Francisco, there is a very large such store down in the Ferry Building. They have a selection of what they imagine to be cast iron pans. They resemble the genuine article, but are cast inside and out. That is, they have not been machined flat on the inside. They are worthless for serious cooking. I suspect they are intended for blackening fish.
Long ago, I wasted much time on a pan bought at Safeway. It was machined, but not well, so the surface was uneven. No amount of work -- short of machine shop work -- will correct such a deficiency. Well, I was young and foolish.
The best place to find cast iron pans is at a junk or thrift shop. These pans may be in terrible shape, but they can always be reclaimed. Steel wool is sometimes not enough. Get a block of pumice at a beauty supply shop; use plenty of water and rinse often. Then use steel wool to eliminate any high spots that are left, and proceed to season.
Earlier writeups in this node discuss the normal seasoning process fairly well. Missing are two key ideas. First, work is involved. Soap is a popular cleaning tool because it helps to loosen dirt and crud. Without it, you will have to spend more time and energy scrubbing, at least until the pan is well seasoned. Second, the pan is not all done merely because you have completed the initial seasoning process. You must cook with it and care for it properly. The seasoning layer is built up gradually and this take time -- years, some say.
If you need to scrub a cast iron pan in the midst of the seasoning process, or when food has become too thoroughly stuck to be wiped out with a paper towel, the tool of choice is a green plastic scratchy pad. There is nothing at all wrong with using plain water, but be sure to heat the pan well and oil it while hot. Before you resort to scrubbing, though, always try the paper towel. There's nothing wrong with just wiping out bits of food and hanging the pan up.
There is no agreement on the proper oil for seasoning and use in a cast iron pan. The traditional agent is lard; my family has always used Crisco. My mother, always a maverick, oscillated between butter and margarine. I cook exclusively with olive oil, but this smokes at too low a temperature for initial seasoning.
Finally, use care when cooking. Although the seasoning layer is more robust than worthless Teflon, it is still soft compared to iron itself. If you scratch a groove in the seasoning, everything you cook will stick there until it fills in again.