For the orange slices: Using a mandoline or a sharp knife, cut oranges into thin slices and lay on a baking tray. Bake on low heat, 110C (230F) for one hour and then turn over and bake for another hour. The slices need to have zero moisture and be dry and crispy. If they need longer in the oven, then return and check on them every 15 minutes. The thicker the orange slice, the longer it will need to de-hydrate.