1/2 - 1 gallon buttermilk
1 large Pyrex bowl with lid or any other fireproof form with lid
1 large colander
1 cheesecloth, or plain white cotton or linen cloth
Pour the buttermilk in an oven proof form, cover with lid and put into oven overnight at lowest temperature (150°F). If your oven is too hot you may put the covered bowl on a double boiler with barely simmering water for 10 minutes, then turn off heat and let stand overnight.
Next morning line the colander with cloth and pour the lumpy buttermilk into it. Tie a knot with the ends of the cloth and let drain in large bowl for about 6 hours.
The watery liquid is called whey and makes a delicious healthy drink. You may also save the whey (in fridge) for use in other recipes, such as making Sauerkraut or to bake bread.
This recipe will make 1/4-1/2 lbs of quark.