The textbook covers copper, silver and iron. The field hands you bi-metal, a coin with a stone fused to it, a mount with the remains of a leather strap, and a dozen other things where the standard protocol isn't merely unsuitable but actively harmful.
Before any method: three questions
- Is the material genuinely identified? Colour and shine prove nothing about the alloy, about plating, or about the absence of a composite. 2. Is the original surface still there? Photographs, hallmarks, stable patina, organics, enamel, glass and a thin plated layer can matter far more than clean metal. 3. Is there a stop signal? Wet organics, layered glass, plating or gilding, bi-metal, powdery active corrosion, an inscription, a dangerous object or a significant context all mean pause and a conservation route.
If even one of the three has no confident answer, do not move on to active cleaning, soaking, reagents or electrolysis. Start with the first 24 hours after a find and fill in a condition card.
The general framing is in principles, chemistry and preservation; water and salts have an article of their own, water, salts and hydrolysis. Below are the exceptions.
There is no single process here
This article used to try to give one order of operations for everything unusual. That does not work: lead, zinc, gilding, bi-metal and a composite object fail for different reasons and are treated in different places. What follows is five risk cards. One card is one group of materials, and every card carries the same set of fields:
- Sign — how you recognise that the object belongs to this group at all.
- Risk — what is actually being destroyed, and why.
- What you can do — actions that take no information away.
- What you must not do — the things that cannot be done “a little” or “carefully”.
- Where to go next — the next article, or a specialist.
Cards are not blended. If an object falls into two of them at once, the stricter one applies.
| Card | Stop signal in one line |
|---|---|
| Lead and tin | Soft, heavy, toxic; the white powder is not dirt |
| Zinc and aluminium | They dissolve in acid and in alkali alike |
| Plating, gilding, silvering | A valuable layer microns thick |
| Bi-metal | Two metals in one solution make a battery |
| Composites: organics, glass, enamel | A fragile complex where the “dirt” may be the object |
Card 1. Lead and tin
| Field | Content |
|---|---|
| Sign | Heavy and soft: a brass needle leaves a mark straight away. Lead is grey with a white or yellowish loose crust, often powdery. Tin is lighter, sometimes with grey patches and brittle areas. Seals, weights, cast figures, vessels, mounts. |
| Risk | Lead is toxic: the dust, the solutions and what transfers to your hands all matter. Both metals are soft, so mechanical work leaves grooves you cannot take back. White products on lead are fed by organic acids — oak, chipboard, cardboard, glue, the vinegary air of a closed cabinet. Acid dissolves the metal, not the crust. |
| What you can do | Gloves, a separate tray, washing your hands after every contact. A scaled photograph, a soft dry brush, isolation from wood, cardboard and PVC, storage in inert containers at steady humidity, an entry in the conservation log. |
| What you must not do | Do not carry the iron or copper protocol over to this card. No acids, no vinegar-and-salt, no electrolysis, no abrasives, no polishing. Do not store the piece in an oak or home-made wooden box. Do not hand it to children. |
| Where to go next | Water, salts and hydrolysis, then monitoring after conservation. Growing white powder means a conservator. |
Card 2. Zinc and aluminium
| Field | Content |
|---|---|
| Sign | Aluminium is noticeably lighter than the hand expects, its oxide is matt white and the metal is non-magnetic. Zinc is brittle with a sugary fracture, common in cheap castings, fittings and twentieth-century casings. Both typically show a white powdery bloom and pitting. |
| Risk | Both metals are amphoteric: they dissolve in acid and in alkali. Washing soda, caustic solutions, a home-made electrolyte and electrolysis eat the metal faster than the crust. Pitting runs inward and is irreversible — there may be no metal left under the white spot. The matt oxide on aluminium is protection, not dirt. |
| What you can do | Dry dusting with a soft brush, before-and-after photographs, dry storage without humidity swings, separate packaging. |
| What you must not do | No alkali (washing soda, caustic soda, a soak in laundry powder), no acid, no electrolysis, no polishing to a shine. |
| Where to go next | Chemistry and its limits, monitoring after conservation. |
Card 3. Plating, gilding and silvering
| Field | Content |
|---|---|
| Sign | A different metal in the chip than on the surface. A thin yellow layer that survives in the recesses of the relief and is worn off the high points. A “white” object that pulls to a magnet is usually nickel-plated steel. A sharp edge at a chip points to electroplating; a dense layer that merges softly into the base points to fire gilding. |
| Risk | The layer is microns thick: polishing, paste, abrasives, fiberglass and electrolysis remove precisely that layer. A chip turns plating and base metal into a galvanic pair, and the base corrodes faster along the edge. An even colour after cleaning almost always means the plating has gone. |
| What you can do | Raking light, macro photography, a record of where the surviving layer still runs, made before any decision. Dry dusting with a soft brush. A protective coating only if it is reversible and written into the log. |
| What you must not do | Do not polish, do not electrolyse, do not attack it with chemistry, do not even out the colour, do not spot-test a reagent on it. |
| Where to go next | Silver and silvering, the first 24 hours; a readable hallmark or painted decoration means a conservator. |
Card 4. Bi-metal
| Field | Content |
|---|---|
| Sign | Two colours in one disc, a visible boundary between ring and centre, later circulating coins. Sometimes riveted or clad parts where one metal is mechanically seated in another. Different zones may answer a magnet differently. |
| Risk | Two metals in one solution work as a battery: the less noble one corrodes faster, and it does so along the seam — exactly where the insert is held. A long bath, an ultrasonic tank and electrolysis all loosen that seat. Acid-and-salt mixtures and rust converters give a fast fresh look and an equally fast bleached one; for a collection that is a loss. |
| What you can do | A soft dry brush, a wooden stick against soil, local work under a loupe. If contact with water is genuinely justified, only once both metals are identified: briefly, without soaking, with immediate complete drying and a log entry. |
| What you must not do | Do not soak it, do not put it in an ultrasonic bath (least of all together with plain copper or silver), do not use vinegar and salt or rust converters, do not electrolyse it. |
| Where to go next | Electrolysis and its exclusions, water, salts and hydrolysis. |
Card 5. Composites: organics, glass and enamel on metal
| Field | Content |
|---|---|
| Sign | Leather, textile, wood, bone or thread grown into the corrosion. A stone or glass inset in a setting. Enamel with a conchoidal chip. Iridescence, bloom and lamination on archaeological glass. Lacquer, paint, gilding over enamel. |
| Risk | Organics and corrosion form a single fragile complex: the “dirt” on a mount may be the remains of the strap, and the dating goes with it. Sudden drying causes shrinkage, cracking and delamination — that is mechanics, not hydrolysis. Water alters the surface layers of archaeological glass. Any bath washes the setting out: you will lift the object out of the solution and the stone separately. |
| What you can do | Scaled photographs in raking light, an AI preview instead of a test clean, separate packaging supported from below, steady humidity without swings, isolation from other finds. |
| What you must not do | With insets you do not use chemistry at all. Do not separate the “dirt”, do not dry the piece on a radiator or with a hairdryer, do not soak the whole object, do not wash glass until it is clear, do not open up enamel, hallmarks or painted decoration with a tool or an abrasive. |
| Where to go next | The first 24 hours after a find, especially while organics are still wet; after that a conservator, not the next method. |
Thick crust is not a sixth card
A crust dense enough to hide the shape tells you nothing about the material — it is precisely what stops you choosing a card. So the first step is not to take it off but to work out what is underneath: weight, magnet, size, the character of a chip, raking light and, where possible, a specialist's eye or an X-ray.
If the material is identified, the object is stable, and there is no plating, no inset and no organics, then the order from the article on that metal applies:
- Mechanics with a scalpel or needle, wet, spot by spot, under a loupe.
- Then mild chemistry for that specific metal — never a general one.
- Finish with fiberglass in water: wet, long soft fibres, no pressure, and well away from plating and insets. Sandpaper down to metal, never.
If you do not know the alloy, do not start. An unidentified material is not a candidate for chemical treatment: a condition card, a photograph and a question to somebody who knows are cheaper than a universal acid.
Cast iron, cannonballs and shot belong to another card
Heavy iron lives by the rules of iron, not of special cases: electrolysis only for an identified, non-archaeological, non-composite object, with documentation and proper disposal of the solutions — see electrolysis. After any wet work, complete drying and a barrier layer are not optional: cast iron picks rust back up considerably faster than you'd expect.
Heating to red heat stays a last resort with a high crack risk; for thin hollow castings it is not suitable at all, and on an archaeological object it destroys the traces of how the thing was made.
Separately: anything resembling ordnance is not a restoration object under any card. Do not touch it, wash it, heat it or take it home.
Five processes that must not share one word
Hydrolysis has become the amateur's blanket explanation for any kind of decay. When it comes to choosing an action, that habit is harmful: different processes carry different risks and different limits.
| Process | Where the word belongs | What it changes for you |
|---|---|---|
| Crystallisation of soluble salts | Porous materials, archaeological metals, ceramics, stone, mixed objects | The risk is salts plus humidity swings; abrupt drying and the wrong rinse make the damage worse |
| Metal corrosion | Iron, copper alloys, lead, zinc, aluminium | Activity depends on moisture, salts, oxygen and composition; appearance is not a diagnosis |
| Leaching and dissolution of the glass network | Archaeological glass | Water alters the surface layers and weakens the glass; cleaning it until it is clear is not an option |
| Hydrolytic degradation of organics and polymers | Leather, textile, wood, paper, adhesives, coatings, some composites | The material may be weakened from within; the damage shows up on drying, bending or cleaning |
| Mechanical damage on drying | Wet organics, composite objects, layered items | This is not hydrolysis; the risk is irreversible shrinkage, cracks and delamination |
The full treatment is in water, salts and hydrolysis, which also explains why desalination is a controlled treatment rather than a soak.
When to stop
There are situations where the correct action is to do nothing:
- Rarities, hallmarks, enamel, organic remains on metal.
- Active corrosion that a soft protocol does not stabilise. Hiding it under wax achieves nothing — the cause needs treating.
- You aren't sure what metal you are holding. A photo card and a question to someone who knows is cheaper than a universal acid.
- The object falls into two cards at once and their “must not” lists contradict your plan.
In short
- Three questions first, then the card, and only then a method.
- One card is one group of materials; there is no general process here.
- Insets, enamel or organics present means no chemistry.
- Lead, tin, zinc and aluminium are not treated with the iron or copper protocol.
- A layer microns thick survives neither polishing nor electrolysis.
- A before photograph, a log entry and a date for the next check are part of the work, not paperwork: monitoring after conservation.
When to call a conservator
Organics and metal in one object, enamel and stones at risk of dissolving, a museum-grade or legally significant piece, active corrosion that grows despite a soft protocol.
Stopping at “still intact” beats finishing at “almost worked”. If the object looks archaeologically significant, read the find procedure first.
External orientation
Bi-metal, insets and stones are exactly where it pays to look at the museum approach: Canadian Conservation Institute — Metals. A doubtful piece is better handed to a conservator than finished off yourself.