1. Bring water to a boil in a large pot. Add clams, cover, and cook until shells open, about 10 minutes. Transfer clams to a large bowl, reserving cooking liquid. Discard any clams that do not open. Remove meat from shells, and return to bowl. Discard shells. Pour reserved liquid through a fine sieve lined with cheesecloth into a large bowl (you should have 2 1/2 cups). Sprinkle a few tablespoons liquid over clams to keep them moist.

2. Melt butter in a large pot over medium heat. Add bacon, and cook, stirring, until golden brown, about 3 minutes. Pour off excess drippings, leaving just enough to coat bottom of pot. Add onion and celery, and cook until softened, 3 to 4 minutes.

3. Wrap thyme, bay leaf, and peppercorns in a piece of cheesecloth, and tie with kitchen string. Place cheesecloth in pot, and add potatoes and 2 1/2 cups reserved clam broth. Simmer over medium heat until potatoes are tender, 5 to 7 minutes.

4. Chop clams. Add clams and half-and-half to pot, and heat until just warmed through, about 1 minute. Discard cheesecloth, and season with salt. Serve immediately.