Steam the clams in 1 cup of water over medium-high heat, covered, until they open, about 8–10 minutes. Discard any that don't open. Remove clams, strain the broth through a fine sieve or cheesecloth (this broth is your flavor base — don't skip it), and set aside. Chop clam meat once cool enough to handle.:
In a heavy-bottomed pot, render the salt pork over medium heat until crisp, about 5 minutes. Remove and set aside, leaving the fat in the pot.:
Add onion and celery to the pot, cook until soft, about 5 minutes.:
Sprinkle in flour, stir constantly for 1 minute to cook out the raw taste.:
Slowly whisk in the reserved clam broth, scraping up any browned bits. Add potatoes. Simmer until potatoes are just tender, about 15 minutes.:
Reduce heat to low. Stir in milk and cream. Do not let it boil — this will break the dairy.:
Add chopped clams and butter, stir until butter melts and clams are just warmed through, about 2–3 minutes.:
Season with salt and pepper. Serve hot with oyster crackers and the reserved salt pork crumbled on top.:
Notes No fresh clams? Use 2 (6.5 oz) cans chopped clams plus 2 cups bottled clam juice — it won't be quite as good, but it'll work. Chowder thickens overnight — thin with a splash of milk when reheating.: