Scrub the clams under cool running water. Discard any clams that will not close when tapped firmly with a fingernail. Bring the beer to a boil in a four-quart pot with a tight-fitting lid. Add the clams all at once with the thyme sprig, cover the pot immediately, and cook over high heat until the pot recovers a boil. Lift the pot lid and stir well from time to time, until the clams have all opened.

Remove the clams to a baking sheet with a large slotted spoon or Asian spider, then strain the broth through a fine sieve to remove sand and grit, and reserve for use in the chowder.