Scrub tongue with a stiff brush; rinse well with cold water. Place tongue in a large Dutch oven. Add celery, onion, salt, and water to cover. Bring to a boil; reduce heat. Cover; simmer 3 1/2 hours or until tender.
Remove tongue from broth, and plunge into cold water. Strain broth, reserving 1 1/2 cups. Remove and discard skin from tongue. Cut tongue into 1/4- inch-thick slices.
Melt butter in a heavy saucepan over low heat; add flour, and stir until smooth. Cook 1 minute, stirring constantly. Gradually add reserved broth; cook over medium heat, stirring constantly, until thickened and bubbly. Add vinegar, sugar, and raisins; cook 5 minutes.
Add sliced tongue; cook an additional 10 minutes or until sauce is thickened. Transfer tongue to a warm serving dish. Pour raisin sauce over tongue to serve.