This was a weekend for family bar-b-ques. I attended two. They were both exceptionally good but the real news is that I made yogurt successfully for the second time. I've tried twice before, with a 50% success rate. I think this latest success marks a no-return point in terms of favorable outcome.
I have Margaret to thank for the encouragement - she earlier gave me some of her home-brew yougurt culture (with which I failed) and some commercial culture that I've now used successfully. I think that my earlier failure was not intrinsic to a more difficult to handle culture, but rather to improper heating and cooling.
I've had success with heating the milk to 185 F (the surface of the milk is foamy) and then cooling on the stove top to 105 F at which point I spoon off some milk and add it to some culture. That gets stirred all around and added back to the pot on the stove. I incubate in my gas oven with a towel wrapped around the pot and the oven light on for a little extra warmth.
I haven't yet made yogurt when it's been cold out, but I've used incubation times of 12 to 24 hours thus far.