Discard cracked clams and any open clams that do not close when tapped. Bring steaming water to a boil in a wide covered pot, add clams, cover and steam for 5 to 8 minutes, removing them as they open; discard any that remain closed.
Lift clam meat from shells, chop coarsely and refrigerate briefly. Strain the cooking liquid through a fine sieve lined with damp cheesecloth, then measure 2 cups, adding water within the stated allowance if needed. Discard shells and any sediment.
Cook bacon in a soup pot over medium heat for 8 minutes until crisp. Retain 1 tablespoon fat with the bacon, discard excess and cook onion and celery for 5 minutes until soft.
Add potatoes, strained clam stock, bay leaf and pepper. Simmer gently for 12 to 15 minutes until the potatoes are tender.
Meanwhile, heat the oven to 375°F. Cut lids from the bread boules and hollow the centers, leaving walls about 3/4 inch thick; reserve the removed bread for another use. Toast bowls and lids on a sheet for 8 to 10 minutes.
Stir milk and cream into the chowder. Whisk cornstarch with slurry water, add it and simmer very gently for 2 to 3 minutes until lightly thickened, then fold in clam meat for 1 to 2 minutes until the chowder is hot and clams reach 145°F.
Remove the bay leaf. Set each bread bowl on a plate and ladle in about 1 cup chowder to start, serving the remaining hot chowder for topping up as the bread is eaten.