How can I keep my jam from overcooking while I process it? As we've rolled into fruit season this year I've started canning up some jams using a traditional boiling process in my big stock pot (though I'm getting pretty close to investing in a pressure canner at this point) and I'm running into a problem: I wind up with too much jam to process in a single batch (which comfortably handles about 7 4-ounce jars or 4 8-ounce jars), so I'm forced to do one batch, then sterilize a new set of jars before I can process another batch &mdash; and in the meantime, my jam is continuing to cook down, pushing past 220F/105C and taking on a much thicker, taffy-like consistency.  Is there any way of keeping my texture consistent across batches, or should I just be making smaller loads or accepting the difference in results?
If it's getting cooked too much, well, stop cooking it. You can cover it to prevent water loss. If it's too hot sitting on the same burner (on an electric stove), move the pot. It sounds like your cast-iron pot might be part of the problem, too - if it's still staying too hot, you could pour it into something else. Finally, if you do still have significant water loss, I believe you should be able to add a little water (careful not to add too much) and stir and reheat.

P.S. you could also just get a lighter pot. (I know that's not ideal.)