“I have seen pics of the powder room, completely sheathed in copper so no sparks can ignite the magazine.”
From recent reading about the War of 1812 and those marvelously effective fighting ships and their crews, I believe I learned that wooden hulls were largely sheathed externally in copper as a defence against barnacles.
The powder rooms, of course, would have required inboard sheathing as well. I’d like to have seen the procedures for producing huge tonnages of sheet copper at that time. Did they have roller mills?
On one of my wooden sailing ships the Cutty Sark I think I picked up a roll of copper sheath material, its very thin, comes on a roll, bought it at a fabric store.
I cut out strips with a paper cutter for the width of my planks and then embossed it with scribe lines to simulate sheets of copper nailed to the hull, if the scale was bigger I would have used real nails, the copper is adhesive backed, looks OK except it is obviously brand new with no aged patina or paint over it.
Perhaps you are right about the copper and barnicles. My father was a chemist and a businessman, and many years ago he worked on copper sheathing to fight/retard barnicles (can’t remember a lot of the details as I was a child, and my brother destroyed or discarded my father’s records).