Clean the rhubarb and cut into large pieces. Set aside 400 g rhubarb. Bring the rest of the rhubarb to the boil with 200 ml water and cook for 10 minutes.
Place in a sieve lined with cheesecloth and drain overnight. Collect the juice.
Chop the rhubarb that was set aside. Mix with 700 ml of juice and preserving sugar. Bring to the boil while stirring and cook according to the instructions on the packet. Stir in the campari. Fill into jars and close.