Cut meat off soup bones. Trim off excess fat and discard. Place meat in a large heavy saucepan with soup bones, shin bones and marrow bones. Add cold water enough to cover bones and meat and bring to a boil. Skim foam from surface as it rises. Reduce heat and simmer continuing to skim if necessary. Add carrots, celery, onions, parsley, bay leaf and thyme. Partially cover and simmer very slowly for 5 hours. Skim as foam rises. Strain the stock through a colander lined with a double thickness of cheesecloth. Allow to cool. Cover and refrigerate overnight. The next day, remove fat that has hardened on surface.
TO CLARIFY STOCK: Bring stock to boil. Beat egg whites and tbsp. of cold water until foamy. Beat egg shells into small pieces. Stir egg shells into egg whites. Slowly add this mixture to boiling stock stirring constantly. Reduce heat, cover, and simmer 15 minutes without stirring. Strain through a fine mash sieve lined with several thicknesses of cheesecloth. Allow to cool thoroughly uncovered. Stock may be stored, covered in the refrigerator for a few days or frozen for long term use. Yields: 2 quarts.
