Drain and chop clams, reserving liquid (may substitute 5-pound can chopped clams).
Fry salt pork in a heavy pan until all fat is rendered; add onions and celery and brown lightly.
Add butter; melt.
Blend in flour and stir constantly for 5 minutes.
Add clams, potatoes, milk, clam liquid, bay leaf, and thyme.
Cook until the potatoes are tender.
If desired, add fish and/or shellfish to make seafood chowder.
Serve in a bread bowl for an extra-hearty meal.