The milk left on the back of the stove is probably referring to clabber. I make that to use as a culture for cheeses, rather than purchased mesophilic cultures and it works great.
Clabber can only be made from raw milk. It is best to start with a small batch (about a cup of milk), then use a tablespoon or so of that to culture slightly more, until you have about 3-4 generations, as the earlier batches will be rather tangy because they take longer to thicken.
To make clabber, use a clean jar and pour about a cup of skimmed milk into it. Cover with a cloth - tea towel or butter muslin is good. Do not cover tightly, as the milk will go rather rancid. The first batch will take 3-5 days to thicken, longer if the room is below 70 degrees. It is "ready" when it is about the thickness of yogurt.
Get another clean jar and spoon about 2 tablespoons of the clabber into it, then whisk it until it is thin. Whisk in about 2 cups of milk, then cover and allow to set until thickened, covering it as above. Provided the environment is the same, it should thicken in less time than the first batch. For your next batch, you can use about a cup of the clabber to a quart of milk, if you are feeling adventurous. Taste the clabber to see if it is pleasing to you - it should be something between sour cream, cream cheese and sour cream. Not too tangy, but with a character all its own.
To use as sour cream, you can drain some of it through butter muslin or cheesecloth to thicken it a bit.
If you have cats or dogs, chickens or pigs, any and all of them will love the "extra" clabber that you make. In fact, all of my chickens and pigs are fed clabber on a daily basis - I have two 3-gallon pails of it rotating at all times. (one thickening while the other is being used)
