In 1970 the foundation of The Archaeological Research Centre (ARC) was formed at the National Maritime Museum to help cope with waterlogged remains of a wooden boat, a trading vessel dated to the 9th/10th century AD from the Graveney Marches in Kent. At that time there were no facilities in the UK,for the conservation of such a volume of waterlogged wood.
The ARC has continued research into all aspects of maritime archaeology,including the archaeology,history and ethnography of boats and has the largest facility in the UK for the conservation of waterlogged wood by the hot Polyethylene Glycol(PEG) method
Wood and other organic materials can be preserved in constantly wet terrestrial sites,or under the sea,especially when isolated by silts and sands. The preservation of organic materials is a complex process but with the development of anaerobic( i.e. without oxygen) conditions the conservation process brings wood and other wet materials to a dry and stable condition with the minimum of distortion by means of treatments that do not alter the internal structure or that may impede any future examination or re-conservation.
CONSERVATION OF WATERLOGGED WOOD.
The actual method chosen for a wooden object depends on the species of wood involved,its size,state of preservation,and any operational hazards. A technique extensively used in the ARC.and other conservation laboratories. is impregnation with a water soluble wax which can be obtained in various grades.In the ARC large timbers are treated by immersion,the so called”tank method”
The largest of these purpose-built tanks in the ARC is 8m in length so substantial timbers can be dealt with. After the timber is loaded the tank is filled with water and heated to the operational temperature of 60 deg C which is maintained throughout the treatment. PEG is added in equal amounts daily, until the concentration is considered high enough to ensure support of the timbers when they are cooled.
Alternatively, PEG can be used at room temperatures for the conservation of smaller objects made from more porous species of wood. I remember when working with the Worcester Archaeology at the Droitwich excavation many pieces from a Roman barrel with staves of silver fir and split oak hoops being treated in this way.
SMALL WOODEN OBJECTS.
When dealing with small wooden objects the range of treatment available broadens, for any operational hazards involved are more easily contained. Such objects can be dehydrated by exchanging the contained water for alchohols or other organic solvents which have low surface tension and higher volatility than water, can be allowed to evaporate with minimal disruption to the wood structure. Freeze drying achieves dehydration but by different means. Wood treated in this way is more normal in appearance.
Wood and Leather Buckets from The Mary Rose.
CONSERVATION OF OTHER MATERIALS FROM WET SITES.
Concretions may enclose an iron object, or the cavity resulting from its dissolution ,or an assembly of objects fused together .Maybe only the surface of an object may be obscured or disfigured by concretions or marine organisms. These are removed preferably by mechanical means using scalpels, vibrating needles or other hand held tools. However the chemical cleaning of organic materials may well be necessary.
This is not Iron but a pot showing concretions which need to be removed.
The concretions being removed using a vibrating needle.
Its possible that wood,leather and textiles may be contaminated by iron or some other corrosion products filling the pores or internal structure. After the cleaning there follows often and extended period of washing to remove and residues of chemical treatment and soluble salts that may be present. Iron is very sensitive to the presence of such salts and requires more than simple washing and there are a number of chemical and electrlytic treatmentd which may be required before the washing process.
Most materials are dried through dehydration with an organic solvent. Broken objects can now be reassembled and joined with a suitable adhesive,preferably one that is reversible. Fragile objects may require consolidation to ensure their durability and this may occur before or after any necessary reassembly. Objects which are substantial but sensitive to atmospheric moisture also require protection. Iron is commonly impregnated with a micro-crystalline wax, whereas other metals such as copper alloys are coated with laquers which contain inhibitors to prevent further corrosion.
When all conservation work has been achieved,depending on the material involved, then has to be stored in a controlled temperature as the object concerned will react to changes in temperature,humidity,light or contaminations in the atmosphere. Iron and other metals may corrode in constantly high humidities which may also support mould growth,textiles and painted surfaces can fade when exposed to high light levels. We can see why Museums have to ensure that an object is located in a controlled and appropriate environment whether in storage or on display is one of the most important factors in its long term survival.
Items from the Vasa Shipwreck Sweden.(Courtesy of the Vasa Museum)
The Upwich Brine Pit Droitwich.
Excavation of the Great Upwich Brine Pit Droitwich.
Remnents of the Mary Rose before Conservation.
The Vasa Vessel as it appears today after conservation treatment.