Blackberry Jam, page 280
Two gallons of blackberries thoroughly cleaned and washed in cold water. Place in granite preserving kettle with 1 quart of water, cook till soft, stirring with wooden spoon and break fruit; press all through a wire sieve. Mix thoroughly. Take one quart of this when it boils, add one quart of granulated sugar previously heated in the oven. Cook rapidly for 15 minutes or till it jellies, shaking now and then to prevent burning. Test and turn into small dishes, seal tightly and put in a dry place. It is much better to cook only 1 quart at a time.
Mt. Vernon Cook Book, Second Edition, 1908, Thompson Company Printers, Carthage, Mo.