Most likely a copper arsenic derivative or related "icky."
I would design a all wood joinery door with either protected and/or self draining joints and finish in a good old slathering of Pine Tar Oil and perhaps a few others.
Do they want it natural or a color finish?
Regards, j
Satamax Antone
gardener
Posts: 3471
Location: Southern alps, on the French side of the french /italian border 5000ft elevation