Wash and clean the rhubarb and cut into small pieces. Wash and dry the orange and rub the peel and squeeze out the juice. Put everything together with the preserving sugar in a large saucepan and let it steep for 30 minutes. Bring to the boil and cook for 4 minutes while stirring. Clean the glasses, place them on a damp kitchen towel and fill them to the brim with the jam. Close the lid immediately.