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Ancient shipwrecks are preserved by controlling change, not by simply drying them out. Newly recovered wood is kept saturated and dimensionally stable; soluble salts are washed from metal and ceramics; corrosion and structural collapse are monitored; and each object is dried, impregnated or stored according to its material, decay state and future environment. The most important rule is immediate: keep waterlogged wood wet until a treatment plan is ready, because uncontrolled evaporation can cause severe shrinkage, cracking and warping.
Why a shipwreck can deteriorate after it leaves the seabed
Burial underwater does not leave a wreck unchanged. Bacteria and other organisms can consume parts of the wood’s cell walls, leaving voids and weakened structure. Water fills those spaces and can temporarily support the remaining framework. While the timber is submerged, it may retain its archaeological shape. Once exposed to air, evaporation removes that support. The weakened cell walls can collapse, causing shrinkage, distortion, splits and surface loss.
Marine water also loads objects with soluble salts. Chlorides trapped in iron can continue driving corrosion when the object encounters oxygen and moisture after excavation. Porous ceramics and glass can hold salts in their pores; as water evaporates, salts crystallize and exert stress that can produce cracking or flaking. The conservation problem therefore changes at recovery: a stable-looking object may become chemically active or physically unstable within hours or days.
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Conservators stabilize the wreck in stages. Recovery teams keep materials wet, record condition and separate materials that need incompatible treatments. Laboratories then remove salts, replace or support water in degraded wood, dry objects under controlled conditions and design storage or display climates that limit future reactions.
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Waterlogged wood: the standard conservation workflow
1. Keep recovered timbers saturated
Immediately after raising, timbers should remain saturated with water and protected from uncontrolled evaporation. Wet storage is not a final treatment; it is a holding measure that prevents the rapid collapse that can occur while the conservators assess the wood.
The Mary Rose project illustrates the scale of this logistics problem. More than 3,000 loose timbers were removed into wet storage, and approximately 700–800 were considered suitable for reuniting before conservation treatment. The hull was covered with water-saturated foam and sprayed with seawater. Later, the museum changed to recycled, filtered fresh water. Its warning is explicit: “Without spraying, the wood could have shrunk by anything up to 50%, warped and cracked.”
Wet storage must be managed as carefully as the later chemical treatment. Staff need to prevent exposed surfaces from drying, keep spray or immersion systems functioning, and inspect for local drying, biological growth and physical movement. Large structural members may need support while saturated so that their own weight does not introduce new distortion.
2. Impregnate weakened wood with PEG
Polyethylene glycol (PEG) is a water-soluble synthetic polymer used as a bulking agent. In degraded wood, PEG moves into voids and cell lumens, replacing water and supporting the remaining cellular structure. The aim is not to restore the original wood chemically; it is to prevent the shrunken, weakened framework from collapsing as water is removed.
Application depends mainly on scale. A complete hull may be sprayed because a tank large enough for immersion is impractical. Smaller timbers and objects can be placed in tanks, allowing the concentration to be increased gradually as PEG diffuses into the wood. Concentration, temperature, circulation and exposure time must be controlled so that the polymer reaches the interior rather than forming a treated shell around an untreated core.
The Mary Rose selected PEG for its hull and used spraying for this reason. The method allowed treatment of a structure that could not realistically be submerged in a single tank, while maintaining controlled delivery over a large surface.
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3. Dry slowly, or use freeze-drying when evaporation is unsafe
After impregnation, drying is still a conservation operation. Air-drying removes water by evaporation and must be controlled to limit movement and internal stress. Freeze-drying is used when evaporation would collapse especially fragile cell structures. The PEG-filled object is frozen, then water is removed by sublimation: under reduced pressure, ice changes directly into vapor instead of passing through a liquid phase that could pull delicate structures together.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallFreeze-drying is not automatically superior. It requires specialized equipment, substantial energy and careful control of freezing, pressure and heat transfer. It is selected when the object’s structure and treatment history justify that intervention. At the Vasa project, fine-grained beech and alder objects did not respond well to the earlier treatment and were later freeze-dried.
4. Allow for very long treatment schedules
Large archaeological structures can require years or decades rather than a short laboratory cycle. The Vasa Museum reports that its spray treatment lasted 17 years, from April 1962 to January 1979, followed by another nine years of slow air-drying. Wooden objects were treated in tanks with PEG 4000; the concentration was gradually increased to 40–45%, and the solution was heated to 60 °C.
Those figures describe the Vasa project, not a universal recipe. Species, density, dimensions, decay profile, PEG formulation, circulation and the intended storage climate all affect a treatment schedule. A large ship’s hull and a small cup-sized object should not be assigned the same concentration curve or drying rate merely because both are made of wood.
Metal: desalination first, then corrosion control
Why iron needs special attention
Iron recovered from seawater can retain chloride in corrosion layers and pores. When oxygen and moisture become available after excavation, chloride can trigger aggressive corrosion. The object may continue to deteriorate even after visible seawater has drained away, which is why surface cleaning alone is not enough.
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Cascade baths and controlled drying
Mary Rose conservators use cascade baths for iron and other salt-laden objects. Objects are moved through water baths while salt levels are measured. As soluble salts leave the corrosion products, the readings help conservators judge whether desalination is progressing and when the treatment is complete enough for the next stage. After desalination, the object is dried in a controlled manner rather than left to dry rapidly in ambient air.
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The same principle applies to many marine metals: identify the salts and corrosion products, wash out soluble contaminants where appropriate, document the response and then dry under conditions that do not restart corrosion. Treatment remains object-specific because alloy, manufacturing history, corrosion morphology and residual salts vary widely.
Historical hydrogen reduction: effective stabilization with an evidential cost
The Vasa project documents a historical hydrogen-reduction method in which iron corrosion products were reduced under hydrogen at 600–800 °C. Treated objects were then sealed in paraffin wax. The heat can stabilize some corrosion products, but it can also change the internal metal structure and destroy evidence of production technology. For archaeological iron, that loss of metallurgical information can be as serious as the corrosion being treated.
High-temperature reduction should therefore be understood as a historical intervention with trade-offs, not a default prescription for every wreck. Conservators must weigh physical stabilization against the need to preserve manufacturing evidence, microstructure and residues that researchers may later study.
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At Vasa, copper, bronze, lead and pewter generally survived in better condition than wrought iron. The museum reports mechanical cleaning and air-drying for these metals. That experience does not eliminate the need for examination: copper-alloy corrosion can be chemically active, and lead or pewter can respond differently to salts and handling. The treatment decision should follow analysis of the object and its corrosion products rather than the metal name alone.
Ceramics, glass and other inorganic objects
Porous ceramics can absorb marine salts even when their surfaces look intact. During drying, dissolved salts migrate toward evaporation fronts and crystallize in pores. Crystal growth creates pressure that can cause cracking, spalling, powdering or loss of glaze and decoration.
Desalination is therefore a standard step for ceramics, glass and other inorganic finds. Rinsing or cascade washing in fresh water removes soluble salts; repeated bath changes and salt measurements help determine whether extraction is continuing. Once the salt load has been reduced, the object is dried gradually and monitored for movement, cracking and newly visible deposits.
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Washing is not a license to immerse every object indefinitely. Porosity, friable surfaces, soluble decoration, attached organic material and existing cracks can make prolonged immersion risky. Conservators select the water treatment, duration and handling method after examining the object’s condition and material associations.
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Shipwreck finds often combine wood, iron, copper alloys, leather, rope, ceramics and mineral deposits. A treatment that benefits one component can damage another. The Mary Rose identifies a critical conflict: PEG can accelerate iron corrosion, while alkaline solutions used for some metal treatments can degrade wood.
If separation is feasible and archaeologically acceptable, conservators may dismantle the object and treat each component with its own method. Separation is not always possible; corrosion, swelling, fasteners or historical assembly can make it unsafe. When components must remain together, the team may prioritize the predominant or most significant material and accept a compromise for the other. The decision should be documented because it affects future interpretation, display and maintenance.
How conservators choose among methods
There is no single “shipwreck treatment.” ICCROM frames conservation choices around the properties of the wood, the conserving agent and the future climatic conditions. Preventive conservation adds the wider collection context: risks in storage and display must be managed after the laboratory treatment ends.
| Method or stage | Best suited to | Main control problem | Equipment or energy implication |
|---|---|---|---|
| Wet storage and spraying | Freshly recovered waterlogged wood, including large hull sections | Preventing local evaporation, shrinkage and distortion | Reliable pumps, spray systems, wet-storage space and continuous monitoring |
| PEG tank impregnation | Small or medium wooden objects that can be immersed | Achieving penetration without a treated outer shell | Tanks, heated PEG solution, circulation and long treatment periods |
| PEG spraying | Large structures for which a full immersion tank is impractical | Maintaining even coverage over complex geometry | Spray infrastructure and long-term solution management |
| Controlled air-drying | Objects whose treated structure can tolerate gradual evaporation | Limiting shrinkage, cracking and warping during moisture loss | Enclosed environmental control and monitoring |
| Freeze-drying | Fragile, fine-grained or collapse-prone wood after suitable impregnation | Freezing and sublimating water without structural damage | Freezer, vacuum system, controlled heat input and significant energy use |
| Desalination baths | Salt-bearing iron, ceramics, glass and other inorganic objects | Removing soluble salts before drying | Bath tanks, fresh water, replacement cycles and salt testing |
| High-temperature hydrogen reduction | Some historically treated iron objects | Balancing corrosion reduction against metallurgical evidence loss | Hydrogen-atmosphere furnace operating at 600–800 °C |
The practical decision matrix includes material and decay state, object size, salt and corrosion load, treatment duration, equipment and energy, intervention level, compatibility with mixed materials, and the relative humidity planned for storage or display. A method that works in a laboratory can fail in a gallery if the future climate is unstable or incompatible with the treated materials.
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Storage and display after treatment
Conservation does not end when an object leaves a bath or drying chamber. The planned storage or display environment must be chosen during treatment design. Relative humidity, temperature changes, light, handling and air pollutants can all influence corrosion, dimensional movement and salt activity.
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For a mixed collection, preventive conservation means managing risks across the storage system rather than relying on one object’s treatment history. Staff should record the material, treatment chemicals, salt-removal endpoint, drying method, observed changes and environmental requirements. Display cases and stores then need monitoring that can reveal drift before damage becomes visible.
There is no universal relative-humidity number in the documented project examples. The appropriate set point and allowable fluctuation depend on the object’s materials, treatment and condition, and should be established by the responsible conservation team.
A practical recovery-to-display sequence
- Stabilize at recovery. Keep wood saturated, support fragile structures and prevent wind, sun or heated air from drying surfaces.
- Document before intervention. Photograph, measure and map joins, corrosion, salts, coatings, biological deposits and associated materials.
- Sort by material and compatibility. Separate components only when it is safe and does not destroy archaeological information.
- Assess decay and salt load. Examine wood density and collapse risk; identify corrosion products; measure soluble salts in bath water; inspect ceramic porosity and cracking.
- Choose the least damaging effective treatment. Select spraying or tank PEG for wood, freeze-drying for structures that cannot tolerate evaporation, and desalination before drying for salt-bearing metals and ceramics.
- Control the transition to dryness. Use measured drying rates, environmental monitoring and physical supports. Do not substitute an uncontrolled room for a drying chamber.
- Record compromises. For composites, note which material was prioritized, why separation was or was not possible, and what risks remain.
- Set the future environment. Define storage and display conditions, install monitoring and schedule inspections for corrosion, salt efflorescence, cracking and movement.
Common mistakes and how to avoid them
Letting wood “dry temporarily”
Even a short unplanned drying episode can create irreversible movement in highly degraded wood. Keep spray or immersion continuous and provide backup water and power arrangements.
Treating the surface instead of the interior
A clean-looking timber or iron object may still contain water, chloride or untreated decay beneath the surface. Penetration, bath readings and condition examination matter more than appearance.
Using one recipe for every object
Species, density, dimensions, alloys, corrosion products and composite attachments differ. Vasa’s PEG concentration and temperature, for example, document one major project rather than a universal specification.
Ignoring archaeological evidence
High heat can alter metal structure, while aggressive cleaning can remove manufacturing traces. Stabilization must be balanced against the information an object carries.
Stopping environmental control after exhibition
A treated object remains vulnerable to moisture fluctuations and renewed corrosion. Storage and display monitoring are part of preservation, not an optional final step.
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| Project detail | Documented figure or practice | Why it matters |
|---|---|---|
| Mary Rose loose timbers | More than 3,000 moved into wet storage; approximately 700–800 suitable for reuniting before treatment | Recovery logistics and reassembly decisions begin before chemical conservation |
| Mary Rose hull | Water-saturated foam and seawater spraying, later recycled filtered fresh water | Continuous wetting prevents uncontrolled evaporation in a structure too large for a tank |
| Mary Rose drying risk | Uncontrolled drying could cause shrinkage of up to 50%, with warping and cracking | Wet storage is a structural safeguard, not merely a cleaning step |
| Vasa hull treatment | PEG spray treatment from April 1962 to January 1979, followed by nine years of slow air-drying | Large wrecks may require multi-decade conservation schedules |
| Vasa wooden objects | PEG 4000 concentration gradually increased to 40–45% in tanks heated to 60 °C | Tank impregnation uses controlled concentration and temperature rather than a single bath |
| Vasa iron treatment | Hydrogen reduction at 600–800 °C, followed by paraffin-wax sealing | Historical stabilization can carry a risk of changing metallurgical evidence |
The essential principle
Ancient shipwreck preservation is a managed transition from a water-supported, salt-loaded environment to a stable museum environment. Keep wood wet until its treatment is ready; use PEG or freeze-drying only when the object’s condition and scale justify them; desalinate metals and ceramics before controlled drying; treat iron with special attention to chloride; and handle composites as compromises rather than as single-material objects. The best method is the one that preserves both the object’s physical integrity and the archaeological information it contains, then keeps those gains safe through appropriate storage and display conditions.
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