Bring water and rock salt to a boil; cool. Place eggs in a crock or glass jar.
Pour salt-water mixture over eggs to cover. Cover crock and let stand in a cool place (not refrigerator) for three weeks.
Remove eggs from salt bath and store them in the refrigerator if not ready to use immediately. Yolks should be a bright yellow-orange color and quite firm. The white should be slightly cloudy and still runny. Eggs without a firm yolk should be discarded. To hard cook, cover with fresh cold water and simmer for 20 minutes. Shell an quarter. Serve with hot rice or congee.
NOTE: Salted duck eggs may be purchased in mud-pack form or in brine in Oriental markets. If in mud pack, scrape off mud, wash well and proceed with recipe.