Army BreadTooth GrindersIron Plate Bicuits
Hardtack was a ration you were sure to find in a Soldiers Haversack.
The Army would contract large orders of Hardtack out to bakers. Fresh from the Baker it was more like a large cracker. It had to be this way as so it would last
without molding, ect. A cracker is how I would describe it. It was then placed in shipping containers, barrels, etc. and shipped out to where the soldiers were, that were in need of the rations. Sometimes this could have been months before it would have reached the soldiers. By this time it was as hard as a rock.

The Solders would then eat it, any way they could. One way was to dunk it in their coffee. Another way was to soak it and fry it in pork fat.  Some soldiers may have smashed it up on a rock with their rifle butt into bite size pieces and sucked on them
until they were soft enough to chew.
    Be carefull of your Grinders or, you might be visiting your Dentist.
Small Batch, Just for a taste.
2 1/2 Cups Flour
1/2 Tablespoon Backing Powder
1/2 Tablespoon Salt
1/2- 3/4 Cup Water

Type of Flour, It seams that all I have seen is  the words add flour on the recipes I have found, so, I have went to several Baking Universities and Flour Companies that date to the mid 1800's with this question. The survey says, The flour would have been a wheat grian stone ground. The siftting  proscees of the flour would not of been as todays fine blends. all
said the same but, remember, it was just a survey.
In the reenacting communities, it seems , all flours are said to be used by someone.
Mix to a stiff dry dough. It should not stick to your hands. Add water slowly.
Add more floor if needed.
Cut to 3 "by 3"  squares 1/4" to 1/2" thick .
Now put 16 little holes in each one.  "Note the picture above."
Use a 10 d nail or other. toothpick is too small.
Bake on ungreesed cookie pan.
Preheat oven to 400 about 20 to 30 min. on each side. Or till dry.
Check, now and then, You don't want to send them off to your soldiers raw or burnt. 

Hardtack Recipe