MartinRyan has the elements of the answer.Nails are considered dead when unable to spark(in the context of a lamproom)Lamprooms are provided any where on board a ship or ashore where flamable or explosive materials were stored. The lamp is kept behind a glass so it can light the storage area, without giving rise to setting light to or exploding the materials stored should it fall or be knocked over. As a further precaution no iron ware or fittings were used wthin or adjacent to the storage area, any sheathing or nails were made of copper, hinges of brass or bronze as any iron could if struck create a spark. Hence as dead as a nail in a lamproom door. If you visit any of the Napoleonic forts constructed for the defense of Portsmouth UK you will find such lamprooms in the powder stores along with explanations of their use. The extension of this facility to ships was natural in the days when they were almost entirely constructed of wood.