Sources:
Ball Blue Book Home Canning and Freezing Recipes and Methods 1956

Dig For Victory - Ministry of Agriculture and fisheries - March 1941
Victory Garden Recipes


JAM

A GOOD jam must keep well, and should be clear and bright in colour, with a pleasant, fruity flavour, well set but not too stiff.

Fruit

The fruit must be fresh, and is best just ripe. Jam made from over-ripe fruit may not set well. Most fruits, and some vegetables, can be made into jam, but some contain more pectin and acid than others, and jams made from these set more readily.

Fruits of high setting quality include cooking apples, black and red currants, damsons and gooseberries.

Those of medium setting quality include apricots, blackberries, loganberries, plums and greengages, and raspberries.

Fruits of poor setting quality include cherries, pears, marrows and strawberries. The addition to these of lemon, red currant or gooseberry juice, or of citric or tartaric acid is often necessary.

Typical Recipes

Fruits of high setting quality, e.g., Black Currant:

• 4 lb. fruit
• 3 pints water
• 5 Ib. sugar
  

Remove the stems and simmer the fruit with the water for 45 minutes. Add the sugar, stir until it is dissolved and boil rapidly until setting-point is reached.

Fruits of medium setting quality, e.g., Raspberry:

• 6 lb. fruit
• 6 lb. Sugar
  

Heat the fruit in the pan until the juice begins to flow. Add the sugar, stir until it is dissolved and boil rapidly until setting-point is reached.

Fruits of poor setting quality, e.g., Strawberry:

• 4 lb. fruit
• 3½ lb. Sugar
• plus either 2 tablespoonfuls of lemon juice, I teaspoonful citric or tartaric acid, or ¼ pint of red currant or gooseberry juice.
  

Plug the strawberries (remove the stem end)  and simmer them with the fruit juice or acid until the juice begins to flow. Add the sugar, stir until it has dissolved and boil rapidly until setting-point is reached.