On 8 Dec 2019 at 18:56, Kurt Feltenberger wrote:

> I'm wondering how someone in the late 16th to early 18th century would
> seal a container to make it waterproof and also protect it from the
> ravages of being submerged in seawater for several hundred years. 
> I'm thinking it could be a regular wooden chest, but then wrapped in
> lead sheets with all the edges sealed with molten lead.

Well given that it's not intended to deal with long submersion (ie
the sender didn't expect it to sink) the lead sealing is a bit much.

"Normal" practice in Napoleonic times was wrapping things in several
layers of oilcloth, and then maybe using tarred canvas for the final
layer.

Wax might be involved as well, but in cold water it tends to get
brittle. and in warm water it might get rather soft.

For the latter part of your time range, I could see it being "tinned"
Basically having the oilcloth and canas assembly described above
being put into a box made of shhet metal (iron or steel, coated with
tin) and the lid being soldered on. That was fairly common practice
for some crackers and the like meant for sea voyages. Sort of a
high-class alternative to hard tack.

Have it inside a chest that doesn't rot easily underwater (and is
unpalatable to the local equivalent of ship worms) and it might last
quite a while.

If it was in tropical or semitropical waters on Earth, I'd expect it
to be covered thickly in coral.
--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com