Wash the prunes thoroughly and remove the pits.
Put the clean prunes in boiled milk for about two hours. Wash them well with water several times over. Separately, add the sugar to the water and boil the syrup until thickening.
Remove from the heat and put the fruit in. Allow the fruit to take in the syrup. Then, return to the fire. Boil to thicken the syrup.
Five minutes before removing from the heat, add the citric acid. Pour into jars and close while still hot. Turn with the caps down to create a vacuum.