The initial heating step is to bring the milk up to 180 F, a sort of follow-up pasteurization step. The cooling period is to bring the milk back down to 110 F, where the bacterial strains of yogurt do their magic. Any hotter and you'll chance killing them, any cooler and they won't be as active so your yogurt probably won't get as cultured.
Since I bend towards science and numbers, I preferred to be a little more precise in finding the right temperatures. Also, since this was the first time I used the crockpot, I thought it would be prudent.
2 hours of heating on low netted me about 160 F; I switched to to high for the next 30 minutes in order to even reach 178 F. I figured that was okay. Good thing I was measuring the cooling process: with the lid off and periodic stirring, my milk cooled to 115 F in under 2 hours. Mixing in 1/3 C of plain yogurt as the culture starter dropped the temperature down to 111 F. That's where I stopped everything, placed a double-thickness hand towel over the lid, and draped a double-thickness fleece blanket over all. When I checked this morning, it was a nice consistency and tasted deeelicious. It's sitting with the lid on in the fridge today, to chill and thicken as far as it likes. Tomorrow I'm going to set up some cloth to make a little yogurt cheese, and get some whey for other purposes.
Milk started heating at 5:20, turned off at about 7:50, added culture around 9:40, checked results a little after 7 this morning. Really easy.
While I was telling my boyfriend about the process of making yogurt, he made a comment along the lines of it being disturbing to have milk hanging out at a high temperature. Isn't that was spoils milk? I find it interesting that we, in our modern lives, are so surrounded by carefully processed, refrigerated, and frozen food that the idea of the natural food preparation and preservation methods are so foreign and strange. In my mind, certainly, eggs and milk have to be refrigerated, otherwise they will explode or strangle you. :) It's been an interesting educational experience to play with food, out at room temperature for days to ferment, and be able to then move it into the fridge to consume over months. I'm certainly not prepared to go without electronically-powered cold storage yet, but I see how it could be an option under certain circumstances.