Making yogurt

By the way, here is my method for making yogurt:

I take 3 boxes of UHT milk and put them in a clean pot, lid in. I heat the milk until it is hot enough that, when I stick my little finger in it for a second, I have to pull it out and say to myself, “Ouch! That hurt!” I remove the milk from the heat and ladle it into a large measuring cup. I use the cup to pour the hot milk into the clean glass jars, which I then close the lid on and leave on the counter until cooler. I leave an equal amount of space at the top of each jar. For 3 boxes of milk, I use 4 large Black Cat peanut butter jars and one Postum jar.
The cooling usually takes more than 1 hour, less than 2. Once the jars of milk feel pleasant to touch, I add plain yogurt straight from the fridge. “Pleasant to touch” means not so hot that I can't just sit there holding the jar but not so cold that it's heat cannot be felt to be significantly warmer than my warm hands. I add enough yogurt to make the jars full to just below the threads on the mouth of the jar. Once I have added yogurt to all the jars, I stir the yogurt into the milk and seal the jars.
I place the sealed jars in a clean black ash bucket and put the cover on the bucket. I place the bucket in a sunny spot near the clothesline (so it doesn't get run over by parking cars or attract undue attention from curious neighbors). I read somewhere that it is important not to move the yogurt around during the bacteria-growth process as that slows the process, making the yogurt take longer to become firm. I like it firm so I only move the bucket once during the day, to keep it in direct sunlight as the shade moves. When the sun goes down and things begin to cool off, I take the bucket in and set it in the kitchen, waiting as long as I can before refrigerating it. I find that the heat will stay in the bucket for at least an hour after taking it inside the flat.
I usually heat and bottle the milk sometime before 8AM. That puts the yogurt mix in the yard by 10AM, giving it a good 8 hours or so before the sun is going down. It can sit in the kitchen after being taken in until maybe 8PM. I will check the bottles throughout the evening to see that they are still warm. When they get closer to that “pleasant to touch” stage again, that's when I refrigerate them. I also like to tilt them just before I refrigerate them, to see how firm they are.

They say to start a new batch within 5 days, so the bacteria are still active. I've just been starting new batches from store bought tubs whenever I'm outside the 5 day range, just in case, though I would like to be able to keep a steady stream of home-made yogurt going, reducing the amount of store-bought yogurt and its contaminants in each batch. That'll be easier later in the summer, I hope.