1. Bring 2 cups water to a boil in a large soup pot or Dutch oven over medium heat. Add clams; steam 7 to 8 minutes or until shells open. (Discard any that do not.) Remove clams from pot with a slotted spoon. Strain liquid through a fine wire-mesh strainer, reserving 4 cups. Remove clam meat from shells, and coarsely chop.
2. Cook bacon in soup pot over medium heat about 8 minutes or until crispy. Remove bacon from pot with a slotted spoon, and drain on paper towels. Discard pan drippings.
3. Melt 6 tablespoons butter in soup pot over medium heat. Add onion, celery, and thyme; cook, stirring occasionally, 10 minutes. Add flour; cook, stirring often, 2 to 3 minutes. Add reserved clam liquid; cook, stirring often, 5 minutes. Add potatoes, reduce heat to low, and cook 20 minutes or until potatoes are tender.
4. Add cream and milk; bring to a simmer. Cook, stirring often, until slightly thickened. Add bacon and chopped clams; cook 3 minutes or until heated through. Ladle into bowls, and top each with a pat of butter.