The hardtack in the photo is a cracker made from flour, water, and salt.
Inexpensive and long-lasting, it is and was used for sustenance in the absence of perishable foods, commonly during long sea voyages and military campaigns… It is known by other names such as pilot bread (as rations for bush pilots), ship's biscuit, sea biscuit… or pejoratively "dog biscuits", "tooth dullers", "sheet iron" or "molar breakers"…Hard Tack was also a well-known thoroughbred racehorse (a son of Man O' War). He was famous not for any racing accomplishments, but because he sired the world-famous Seabiscuit. Now you know why Seabiscuit got his name.
To soften it, it was often dunked in brine, coffee, or some other liquid or cooked into a skillet meal. Baked hard, it would keep for years as long as it was kept dry. For long voyages, hardtack was baked four times, rather than the more common two, and prepared six months before sailing.
[These biscuits were also] used extensively as a source of food by the "gold diggers" emigration to the gold mines of California in 1849. Since the journey took months from the starting point, pilot bread was stored in the wagon trains...
During the American Civil War, 3-inch by 3-inch hardtack was shipped out from Union and Confederate storehouses. Some of this hardtack had been stored from the 1846–8 Mexican-American War. With insect infestation common in improperly stored provisions, soldiers would breakup the hardtack and drop it into their morning coffee. This would not only soften the hardtack but the insects, mostly weevil larvae, would float to the top and the soldiers could skim off the insects and resume consumption
Those who buy commercially-baked pilot bread in the continental United States are often those who stock up on long-lived foods for disaster survival rations. Hardtack can comprise the bulk of dry food storage for some campers… Hard tack is also a mainstay in parts of Canada.
The recipe for hardtack is here.
A tip-of-the-hat to Scribal Terror for suggesting this topic.