Conserving an Iron Find

Confirm the object's legal status and ownership before anything else. Excavated finds are frequently subject to reporting law, and a significant object belongs with a conservator and a museum, not on a workshop bench.

Keep a freshly excavated iron object in the condition it came out in until you can treat it. An object that dries fast on a windowsill can shed its surface before you have even looked at it.

Record the object from all sides with a scale, and weigh it. Everything after this alters it, and the record you take now is the only evidence of what it was.

Have the object radiographed. A shapeless lump of corrosion routinely turns out to contain a knife with a pattern-welded blade or inlaid decoration — and the surviving surface is usually inside the corrosion, not on top of it. Cleaning without an X-ray is cleaning blind.

Do not put archaeological iron in acid, rust remover or electrolysis without training. These strip corrosion — including the corrosion layer that is the original surface. The object comes out shiny and its shape is gone permanently.

Remove soil and loose corrosion with hand tools under a microscope or strong lens, working toward the original surface identified on the X-ray. Slow, reversible, and it stops the moment you reach the surface.

Make up an alkaline sulphite solution — the published treatment uses roughly 0.1 M sodium hydroxide with 0.05 M sodium sulphite — or a deoxygenated sodium hydroxide solution. Wear gloves and eye protection: sodium hydroxide is caustic and burns skin and eyes.

Submerge the object fully and keep the vessel closed. Oxygen in the bath lets corrosion continue during the treatment that is supposed to stop it.

Change the solution regularly and test each spent bath for chloride. Plot the chloride released against time — the curve falls steeply and then flattens, and the flattening is what tells you the treatment is finished.

Treatments typically run for months. Published results extract most of the chloride — commonly in the region of 60 to 99 % — because chloride adsorbed on surfaces comes away readily while chloride held inside the akaganeite structure comes out slowly and incompletely. You are reducing the problem, not abolishing it.

Rinse in several changes of deionised water until the rinse runs neutral. Alkali left in the object is a new salt problem replacing the old one.

Dry slowly and completely, finishing with solvent dehydration or a desiccator. Water left inside is what the remaining chloride needs in order to act.

Apply a reversible consolidant and a microcrystalline wax coating. Reversible matters: a future conservator with better methods must be able to undo what you did.

Store below about 15 % relative humidity with silica gel in a sealed box, and inspect regularly for orange weeping droplets or fresh flaking. Those are the signs that active corrosion has restarted, and they mean the treatment has to be repeated.

Record every solution, concentration, duration, chloride reading and material applied, and keep it with the object forever. An object whose treatment history is unknown is one a future conservator has to guess about.

Why chloride is the enemy. Chloride ions concentrate at the metal-corrosion interface and support a self-sustaining electrochemical cycle. In humid air with chloride present, akaganeite (β-FeOOH) forms, and its growth is associated with the corrosion layer exfoliating — spalling off in flakes. Each flake exposes fresh metal, more chloride migrates, more akaganeite forms. This is why a stable-looking find can disintegrate on a shelf.

Why the corrosion is the object. On badly corroded archaeological iron the original shaped surface is often preserved as a layer within the corrosion crust, marked by the boundary where the metal used to be. Strip the corrosion and you strip the artefact, leaving a smaller lump of sound metal with no form. This is the single most important thing separating archaeological conservation from restoring a rusty tool, and it is why acid and aggressive electrolysis are wrong here even though they work beautifully on a garden spade.

Treatment is a reduction, not a cure. Desalination removes most of the chloride and slows the cycle drastically. It does not remove all of it — the chloride trapped in the akaganeite structure resists extraction. Long-term survival therefore depends on dry storage as much as on the treatment, and any claim that an iron find has been "stabilised permanently" is overstated.

Reversibility as a principle. Conservation ethics require, wherever possible, that a treatment can be undone. Methods improve; today's best practice is tomorrow's regrettable intervention. An irreversible coating or a hard consolidant forecloses options for everyone who comes after — which is why a plain wax over a reversible consolidant beats anything permanent, however good it looks now.