During Amnesia: Rebirth you will encounter many puzzles, and one of them will require nitrate.
Selitra – one of the three ingredients needed to create a shell for the cannon stuck in the fortress tank. And if sulfur with coal can be found in the arsenal where the projectile itself is made, then to get the saltpeter will have to leave it.
Get out of the arsenal, go to the green door and follow a little to the right, in the aisle, leading to the warehouse apartment master. Through this door you passed, when you fled from the monster. So go back to the same place, go up the steps to the left and go through a couple of doors. In the end, you will see in front of the opened storage door. There will be a shelving with a can of nitrate! Do not worry, the monster is no longer there.
After taking the saltpeter, go back to the arsenal and sprinkle it into the shell. Add the powder of sulfur with crushed coal to it too!