Drain clams, reserve and measure the clam liquid, add water if amount is less than 4 cups.
Over medium heat melt 2 tablespoons butter, add onion, celery and garlic, sauté for 5 minutes.
Add potatoes, celery salt, thyme, pepper and clam liquid, simmer until potatoes are tender.
Make a roux by melting remaining butter and mix in flour, cook 1 minute.
Stir roux into potato mixture, cook 1 minute more.
Add salt, as necessary to taste.
Add clams and half-and-half and heat through, but do not boil.
NOTE: Use three 10 1/2-ounce cans or five 6 1/2-ounce cans.