Wash the cranberries and put them into a nonreactive saucepan with the water, a pinch of salt and if you wish, the orange slices.
Cook over medium heat until all the berries burst -- about 10 minutes.
Pour the fruit and liquid into a cheesecloth-lined sieve.
Strain the juice to the saucepan, add the sugar and boil for two or three minutes.
Taste and add more sugar if it is needed.
Cool and chill the juice before serving.