Jam simply consists of fresh fruit boiled with a half to two-thirds its weight of white cane sugar until the mixture jellies.
Nearly every housekeeper has her own recipe for jam. One that I know of uses a whole pound of sugar to a pound of fruit and boils it for nearly two hours. The result is a very stiff, sweet jam, much more like shop jam than home-made jam. Its only recommendation is that it will keep for an unlimited time. Some recipes include water. But unless distilled water can be procured, it is better not to dilute the fruit. The only advantage gained is an increase of bulk. The jam may be made just as liquid by using rather less sugar in proportion to the fruit. A delicious jam is made by allowing 1/2 pound sugar to every pound of fruit and cooking for half an hour from the time it first begins to boil. But unless this is poured immediately into clean, hot, dry jars, and tied down very tightly with parchment covers, it will not keep. Nevertheless, too much sugar spoils the flavour of the fruit, and too long boiling spoils the quality of the sugar. A copper or thick enamelled iron pan is needed.
The best recipe for ordinary use allows 3/4 pound sugar to each pound fruit. Put the fruit in the pan with a little of the sugar, and when this boils, add the rest. Boil rather quickly for an hour. Keep well skimmed. Pour into hot, dry jars, and cover.