Powder Houses were purpose built structures.
Here are a few examples, showing different designs.
I was surprised to find steel drill rods used
for re-enforcement!

Click on the thumbnails for a larger view and explanation.

             
These 4 pictures are of the S. S. & G. Powder House, west of the Central Mill.

                                              Bilharz- Bluebird              Woodchuck                    Mystery?                   Mystery?
 

Skelton