8 pounds cherrystone clams, scrubbed
1 tablespoon unsalted butter
4 slices if thick sliced bacon, cut into 1/4" pieces
½ cup celery, minced
1 large onion, minced
4 garlic cloves, minced
2 1/2 pound potatoes (Russet or Yukon Gold), peeled, cut into 1/2" pieces
1 tablespoon chopped fresh thyme
1 bay leaf
2 cups heavy cream
Kosher salt, freshly ground pepper
1 freshly ground pepper
Chopped fresh chives
Bring clams and 4 cups water to a boil in a large pot over high heat. Cook until clams just open, 8-10 minutes (discard any that do not open). Using a large slotted spoon, transfer clams to a large rimmed baking sheet; set broth aside. Let clams cool slightly, then pull meat from shells; discard shells.
Chop clams into bite-size pieces. Strain broth through a fine-mesh sieve set over a large bowl. Add water if needed to measure 6 cups.
Melt butter in a large heavy pot over medium heat. Add bacon and cook, stirring occasionally, until fat is rendered and bacon begins to brown, about 8 minutes. Add celery, onion, and garlic and cook, stirring often, until onion is translucent, about 10 minutes. Add reserved broth (or 6 cups bottled clam juice), potatoes, thyme, and bay leaf. Bring chowder base to a simmer; cook until potatoes are tender, 20-25 minutes.
Remove base from heat. Discard bay leaf. Stir in reserved clams (or two 10-ounce cans baby clams) and cream. Season with salt, if needed (clams' brininess varies), and pepper.
Divide chowder among bowls. Garnish with chives and oyster crackers.