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. DO AHEAD: Clams and broth can he made 1 day ahead. Cover separately and chill.

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. Stir cornstarch and 2 Tbsp. water in a small bowl to form a slurry. Stir slurry into chowder base; return to a boil to thicken. DO AHEAD: Base can he made 1 day ahead. Let cool; cover and chill. Keep clams chilled. Bring base to a simmer before continuing.

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.