Place clams in large pot. Cover with cold water. Simmer until clams open. Remove. Strain liquid reserve. Cool and chop clams.
Render salt pork. Remove. Add onions and celery. Sauté until softened. Dust with flour to form roux. Cook 2-3 minutes. Add clam broth and warm milk. Whisk to avoid lumping. Add thyme, potatoes, clam meat and salt pork. Simmer until potatoes are cooked. Finish with cream, salt and pepper.