Thoroughly clean the orange peels to remove any wax or residues, then cut the oranges into thin ⅛-inch (3mm) round slices, removing the ends.
Boil a large pot of water and blanch the orange slices in it for 10 minutes to soften and reduce bitterness, then immediately transfer them to an ice water bath to cool.
Empty the pot and combine 4 cups of sugar with 4 cups of fresh water, heating until the sugar dissolves and the syrup boils.
Add the drained orange slices to the simmering syrup, lowering the heat and cooking gently for 45 to 60 minutes until the peels turn translucent, stirring every 15 minutes to avoid sticking.
Place the cooked slices on wire racks set over baking sheets and dry them in a 175°F (79°C) oven for 30 minutes to start drying.
Sprinkle each slice with granulated sugar, return to the oven, and continue drying for another 1 to 1½ hours until mostly dry yet still a little flexible.
Once dried, toss the slices in granulated sugar again for a sparkling finish, let them cool fully, and store in an airtight container for later enjoyment.
