I received the following excellent comment from a blog reader and thought I would take this post to elaborate:
"I think you're wrong about clabber being the same as buttermilk. Buttermilk is the liquid left over from making butter out of cultured cream. Clabber is the curds formed when whole milk is left out for 24 or more hours. Curds and whey - the curds are the clabber."
You are absolutely right. Cultured buttermilk is the liquid left when one makes butter from cultured cream. Ironically, most recipes I have found for making cultured butter require adding a bit of cultured buttermilk to the cream to get the required bacteria started. (Personally, I don't like butter made from cultured cream and make my butter from sweet cream.)
To differentiate between the two types of buttermilk, the milk left over after making butter from sweet cream is called buttermilk. The liquid left after making cultured butter is cultured buttermilk. The buttermilk that one buys in the grocery store is cultured buttermilk.
To address the second part of this.........yes, clabber is made when raw (unpasteurized) milk is left at room temperature and forms a solid mass as good bacteria begins to do it's job. The curd will seperate from the whey and you have curds and whey!
Now, here is the fun part! It took me a while to put all of this together but when I did, it was such a revelation for me. From Faunkhauser's Cheese page I have taken the following recipe for making home-made buttermilk:
MAKING CULTURED BUTTERMILK FROM SCRATCH
Allow a cup of filtered fresh raw milk to sit covered at room temperature until it has clabbered (usually several days).
Place 1/4 cup of the clabbered milk in a pint mason jar, add a cupof fresh milk (does not have to be raw at this point), cover, shake to mix, allow to sit at room temperature until clabbered.
Repeat this transfer of sub-culturing several more times until the milk dependably clabbers in 24 hours. Taste a small amount to confirm that it is tart, thickened, and has no off flavors. It should taste tart not bitter, for instance.
To then make a quart of buttermilk with this culture, add 6 ounces of the buttermilk to a quart jar, fill with fresh milk, cover, shake to mix, allow to sit at room temperature until clabbered.
I seem to have ideal conditions in my house for making clabber and can clabber raw milk in 24-48 hours. The clabber I make has good taste with no off flavors. Should one experiment and find that their beginning clabber has a bitter or "off" taste or if it takes longer than 24-48 hours to clabber, then I would suggest using Faunkhauser's method above. What Faunkhauser describes is simply taking a bit of your clabber and using it to start another batch until you get the desired taste.
Once you have the taste you are looking for in your clabber/buttermilk then you can use that in the place of mesophilic culture to make cheese.
I will say that cheese making is first of all a science and then an art. If you are looking to duplicate your cheese exactly each time you make it, then you are better off to go with a commercial mesophilic culture.
For myself, I enjoy knowing that I can make my own starter and not have to rely on a cheese making supply store.