Chill a small plate in the freezer for 30 minutes. Meanwhile, trim off and discard the ends from the oranges.

Using a sharp paring knife, carefully remove the peel and pith from all of the oranges.

Slice half of the peel (with pith attached) into very thin, bite-size strips and transfer to a large pot; discard remaining peel and pith.

Thickly slice all of the oranges, discard any seeds and cut flesh into small pieces; transfer to pot with peel and pith.

Add 4 cups water, sugar, lemon juice, ginger and salt and bring to a rolling boil.

Reduce heat to medium high and boil, skimming off and discarding any foam on the surface and stirring occasionally, until very thick, about 45 minutes.

To test the marmalade for doneness, drop a teaspoonful onto the chilled plate, wait five seconds, and then run your finger through the center of the marmalade.

When it's thick enough, the trace of your finger should remain clear.

If the marmalade runs back together, return the pot to the stove and the plate to the freezer and check again in a few minutes.

Ladle hot marmalade into jars, seal tightly, set aside until room temperature and then chill in the refrigerator for up to 2 weeks.