On a work surface, slice and discard both ends of the oranges. Cut each orange into -inch (5 mm) thick slices. Remove any seeds. Set aside.
In a large pot over medium heat, bring the water and 3 cups (630 g) of the sugar to a boil. Add the orange slices and simmer over low heat for 10 minutes. Remove from the heat. Let cool in the pot for 1 hour and 30 minutes.
Bring the oranges and syrup back to a boil, adding cup (70 g) of the sugar. Simmer over low heat for 5 minutes. Let cool for 1 hour and 30 minutes off the heat. Repeat the
cooking and cooling process twice more, stirring gently a few times so the oranges cook evenly. Let the candied slices cool for 1 hour and 30 minutes in the syrup in the pot. The slices are candied when the peel turns translucent.
Place a wire rack over a baking sheet. Drain the oranges
and lay each slice flat on the rack. Let dry for 24 hours, turning after 12 hours (see note).
The candied orange slices will keep for several weeks
in an airtight container in a cool, dry place. To prevent the
slices from sticking together, place them between squares
of parchment paper.
If you're short on time, you can press the slices in granulated sugar about 12 hours into the drying process. This prevents them from sticking together so they can be eaten right away.