You're probably more likely to get some type of iron poisoning from using cast iron to bake your bread in than from any process to actually make the bread alright, so what you did is kept a piece of one dough to use as a "pre-ferment" in your next one. The french do this all the time for baguettes. they'll keep a piece from a previous batch, add it to the next to create flavor, texture, hole structure etc. They call it a pate Fermentee when it came from a previous dough. The biggest difference between this and other pre-ferments is a pate fermentee, having come from another dough, has salt in it. I worked at a bakery that did that. We called it "scrap". Yours must've been pretty old, or kept out of refrigeration to get that sour. Eventually, bacteria will take over the yeast and you'll end up with a sourdough starter. It probably wasn't quite there yet, but you could've refreshed it over a few days and it likely would've become one. That's how you got the sour flavor. The no knead bread isn't as revolutionary as the articles would lead you to believe. Mixing flour and water together will develop into a bread dough with little to no help as many are seeing. It is considered a "straight dough", so many would argue its inferior to one that uses a preferment, yeasted or sourdough, but it isn't because you basically ferment the whole batch in the same manner one would when using a preferment. Make sense? The no knead method can be used just as well in 3-4 hrs, with 2-4 folds, as in the 12-18hrs of that article. The difference is that you would do what you just did and use a preferment to jump start the fermentation, thereby eliminating the need to have a full batch of dough sitting for 12 hrs. Bread takes time. There is no way around that. However, there are 100's of ways to create that time. The long fermentation of a straight dough as in the no knead recipe, or by using pre-ferments. You wouldn't want to perpetuate the dough. ie: keep pulling a piece to add to the next since that would become a sourdough, or maybe you would? It depends on how you want your bread to taste. In your case, it also might have been so sour because I'm assuming you still did that long fermentation from the no knead recipe. If you used more yeast to shorten the process, it might have been less sour, but more complex. It sounds like your bread when straight to bacteria. As for the pans, you're looking for thermal mass. Cast iron pans have a ton of thermal mass. Probably more than most pizza stone. When you put bread into your cast iron pan (great that you're preheating-required), pizza stone, my new brick oven or my commercial deck oven, the bottom heat radiates bottom up creating lift and eventually (with enough steam) a nice crust. The reason you can't make really great bread in a convection oven which pummels your dough from all sides. You want a more even, gradual bottom up heat. the thermal mass creates that, and helps maintain a better baking environment.