An authentic recipe for Civil War hardtack comes from The American Table. This simple recipe calls for 2 cups flour, 1/2 tablespoon salt (optional), and 1/2 to 3/4 cup water. Add the ingredients together and mix with your hands until a firm dough forms.
Roll out the dough until it reaches a 1/3 inch thickness. Cut the dough into squares. The squares should be large enough to place sixteen small holes in each with a small dowel rod. This process is called docking, and these holes prevent the dough from rising as bread does when baking. If the dough isn't pliable enough to roll or mold, add small amounts of water until it becomes softer. However, do not add too much water or the hardtack softens to a cracker-like consistency.
This recipe needs to bake at a low temperature of 250 degrees Fahrenheit for at least four hours to ensure the tack dries out and hardens. Flip the squares once about halfway through baking them. Depending upon your environment, if the atmosphere isn't humid, you can allow the squares to sit out after baking to allow them to dry out further. This recipe produces fifteen 3-inch by 3-inch hardtack squares.