Iron from the ground

Archaeological iron comes out of the ground as a shell of magnetite and goethite around a diminished metal core, with chloride ions trapped at the interface. Exposed to ordinary room air the chlorides draw moisture, the object weeps orange droplets, the crust cracks and flakes off in sheets, and within a few years a sound relic is a heap of rust. Civil War and colonial metal detected in the eastern states, where soils are acidic and wet, is the worst case.

The safe options are two. Keep it dry: a sealed box with indicating silica gel at under 20 % relative humidity, checked monthly, stops the reaction outright. Or desalinate: alkaline sulphite or repeated deionised-water soaks over weeks to months, monitored by chloride tests, followed by drying and a tannic-acid or microcrystalline-wax coat. Electrolysis is the metal-detecting community's shortcut; it works on solid objects but can strip surface detail and blow apart a thin one, and it is irreversible.

Do not oil, do not paint, do not spray with clear lacquer over an undesalinated surface: all three seal the chlorides in and hide the damage until the crust lifts. Mechanical cleaning with a scalpel or air abrasive under magnification is how the original surface (the profile the object had when it was lost) is found in the crust; the surface is often a few millimetres out from the surviving metal.

Record what was done and where it was found. A dug relic's land status is the same question as a point's — federal and state land require a permit, private land the owner's permission — and the Preparation card should say “desalinated, tannic acid, waxed, 2024” so a buyer knows why it is stable.