starleen2 wrote:If what you said is true even after seasoning- then even, water will past the seasoning and cause the CI to rust - with or without soap. I agree Cast iron is porous. If you use soap on a cast iron pan that is not completely sealed by seasoning, the soap will get into the cast iron and impart that taste to your food. Once the pan has been thoroughly seasoned and has a hard, impervious coating, using a mild soap to wash it will do no harm.
True and false. If you have a nice thick seasoning built up over many years, then a little soap most likely won't penetrate into the pores, but...okay, science lesson time. Water is bipolar. It has a negative side and a positive side. Opposites attract so water clings to itself. Oil has no charge and so it doesn't stick to water. Water still sticks to itself, so it pushes the oil aside. Water will still stick to some of the carbon and other atoms present in the season, pulling them out. This is why if you let a dutch oven sit with water in it, it will rust no matter how good a season you have.
Now, soap. Soap is a mix of chemicals and oils. It has a side that likes water and a side that like oil, hugging both sides. Thus is pulls oil and disperses it in the water. This means that as soon as you add soap, it starts to pull oil out of the season and mix it in the water as the molecule hugs all sides. Some soap molecules won't have a strong enough connection to the oil and will let go and float back up into the water. Some will have a stronger connection to the oil in the season, forget about the water side, and stick. No matter how well you rinse, you will have some of these oil loving sides remain attached to the season, even if you can't see them. But, if your palate is like mine, you will taste them as the heat of cooking forces them to let go again in your food.
I hope that makes sense.