Boil 15 cups water. Wrap salt, tea leaves and spice powder in a small cloth bag and add to water. Cook over low heat for 20 minutes, let cool. When soup becomes cool, add ginger, green onions and 1 teaspoon saltpeter. Soak duck in the soup for 10 hours (overnight).
Scald duck in boiling water, hang outside to dry in the air (for about 1 or 2 hours).