1. Scald a granite-ware or heavy enamel coffee pot by rinsing it with boiling water, then discard the rinse water. (Scalding: pouring boiling water over a vessel to sanitize and preheat it.)
2. Wash the egg, then crack it and beat it lightly in a small bowl. Add half the cold water (1/2 cup / 120 ml) to the beaten egg, add the crushed egg shell, and stir to combine. Mix this egg-and-shell mixture with the ground coffee.
3. Transfer the coffee-egg mixture to the preheated coffee pot. Pour the 6 cups (1.4 L) of boiling water over the grounds and stir thoroughly.
4. Place the pot on the front burner over medium-high heat and bring to a boil. Boil for exactly 3 minutes, stirring once or twice. Note: fewer than 3 minutes leaves the coffee cloudy; boiling longer than 3 minutes develops excessive tannic acid and bitterness.
5. Cover the spout with a small piece of soft paper to retain the aromatic volatile compounds. Stir the pot, then pour a small amount into a cup to verify the spout is clear of grounds. Return the test pour to the pot.
6. Add the remaining cold water (1/2 cup / 120 ml). The cold water, being denser than hot water, sinks and carries the grounds to the bottom — this is the clearing step.
7. Move the pot to the back of the stove over the lowest possible heat (or off direct heat entirely) for 10 minutes. The coffee must not boil during this resting phase; it should be still and just barely steaming.
8. Serve immediately by pouring carefully to avoid disturbing the settled grounds. The coffee is ready when it runs clear and amber-brown into the cup with no visible suspended particles.
9. If any coffee remains, drain it off the grounds at once and reserve for use in jellies or desserts.
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