Choose very ripe blackcurrants, remove the stalks and place the blackcurrants in a pan with 1 dl (3½ fl oz or ½ U.S. cup) of water per 1 kg (2¼ lb) fruit. Place on a low heat so that the fruit swells and breaks and their juices are released into the pan. Turn out on to a sieve and allow the juice to run into a basin; this obviates the need to squeeze it out through a cloth.
Weigh out 850 g (1 lb 14 oz) of sugar per 1 litre (1¾ pt or 4½ U.S. cups) of blackcurrant juice, place it into a pan with a little water and allow to dissolve then cook to the hard-boil stage, skimming it well. Add the blackcurrant juice together with 2 dl (7 fl oz or ⅞ U.S. cup) whitecurrant juice per 1 litre of blackcurrant juice.
Keep on a low heat for a few minutes to dissolve the syrup then cook the jam rapidly until it reaches the setting stage, skimming it carefully.