Combine 1/2 cup stock or soup, egg white, and crushed shell in a small mixing bowl; beat well to combine. Place remaining stock or soup in a Dutch oven; add egg shell mixture, stirring well.
Bring to a boil; boil 3 minutes or until a scum forms on surface of soup. Remove from heat; set aside until scum starts to settle. Strain through several layers of damp cheesecloth.
Note: Larger quantities of stock or soup may be clarified by increasing egg white and shell proportionately.