Fill a large bowl with ice water and set it aside.

Add 4 cups (960 ml) of the regular water to a large saucepan and bring to a boil over high heat. Add the orange slices. Boil for 2 minutes. Transfer the orange slices to the bowl of ice water and discard the boiled orange water.

In the same saucepan, add the remaining 4 cups (960 ml) of water and the sugar. Bring to a boil over medium heat, stirring occasionally until the sugar has completely dissolved.

Turn the heat to medium-low and add the orange slices. Simmer for 45-60 minutes or until the rinds are slightly translucent. Gently swirl the oranges every 15 minutes to make sure they are evenly coated with the sugar water.

Transfer the slices to a cooling rack set over a large baking sheet. Let them sit for up to 24 hours or until dry.

Dip the candied orange slices in granulated sugar, if desired. Use immediately or store them in an airtight container in the refrigerator for up to 1 month.