Organics
Organic materials are made up primarily of carbon-based compounds. These derive from plants or animals. On terrestrial archaeological sites it is rare to find organic artefacts in a good condition. This is due to the available oxygen in the burial environment which causes organic materials to biodegrade. However, because the remains of the Mary Rose were covered in silt, restricting the access of oxygen, the organic artefacts have survived.
Wood
The majority of the artefacts in the collection are made from wood. This includes the bowls used by the crew, the arrows fired during warfare and the nit combs. However, there are a multitude of different tree species represented in the collection. Oak, ash, elm, and box are most common. But we also have long bows made from yew, parrel balls from walnut and spoons from maple. The majority of native species plus some non-natives are found within the collection.
Due to the activity of anaerobic bacteria within the silt burial layer, parts of the cells of the wood have been eaten away leaving a weak microscopic structure. These voids within the cells fill with water which keeps the object’s shape and form. Removing this water would cause the wood to shrink, crack and warp. To be able to dry them for display we replace the water with a bulking agent called polyethylene glycol (PEG). This involves placing the objects into tanks and filling them with water. PEG is then added with the concentration increased slowly over time.
The process of drying for the wooden artefacts is called freeze drying. The PEG filled objects are placed within a large chamber and frozen. Moisture is then removed through a process called sublimation which turns the solid ice into a vapour, while under pressure. This vapour then condenses within the colder condenser chamber. Freeze drying is a more controlled method of drying than air drying because of this.
Leather
Leather is made of the tanned hides of different species of domesticated animals. There are many examples of leather objects in the collection: shoes, jerkins, mittens, pouches, and bags. The majority of the leather artefacts were also treated using Polyethylene glycol. Lower concentrations were used, and treatment times were considerably less. Freeze drying was the preferred method for drying these although careful monitoring was required to avoid over-drying. Once treated the conservators worked with objects like the shoes and boots to reconstruct them for display.
Textiles and hair
Some remarkable surviving objects are made of textiles and hair. We have socks, ribbons, bandages made of cotton, silk and wool and brushes made of horsehair. Textiles were carefully washed to remove sediment and other impurities. Ultrasound baths were often used for this. the items were then slowly air dried between cotton towelling.
Inorganics
Inorganic materials are those that do not come from plants or animals; metals, stone, glass, and objects made from clay like bricks and ceramic jars. Salt in the sea is the major problem for inorganic artefacts. This can cause cracking of ceramics through its crystallization in drier atmospheres and begin corrosion processes with metals. Desalination is a treatment method often used for inorganics to remove salts from within.
Metal
There were many different types of metal objects found on the Mary Rose, including: breech loading guns and shot made of iron, cast bronze cannons, brass aiglets (for the end of shoelaces), lead weights, pewter plates, and gold coins.
Gold is a very unreactive metal and so doesn’t require the in-depth conservation process that other metals do. Almost two-thirds of our metal artefacts are made of iron. Iron is most problematic after burial in the sea due to the chloride which can cause aggressive corrosion. In the 1980s a treatment method called ‘hydrogen reduction’ was used for some of the iron objects including guns and shot. This method of treatment was controversial though because it didn’t follow the conservation principle of minimal intervention. Instead, the high temperatures used, altered the metallurgy. More recently alkaline washing techniques and electrolysis have been the preferred treatment choice to remove the chloride. Alkaline chemicals are used to stabilise the iron while the treatment is ongoing. A pH lower than 10 (basic to acidic) will aid corrosion. These are long processes and full removal of the salt can’t be guaranteed.
Copper and its alloys (bronze and brass) were also washed to remove salts. However deionised water was used instead of alkaline chemicals. Electrolysis was chosen for the large cannons whereas cascade washes and hot water baths were used for the smaller artefacts. Many have been coated in conservation grade coatings to protect them further from the atmosphere.
Ceramics and glass
Ceramics (objects made from clay) and glass underwent desalination, to remove salts from inside the material. Salts left within the objects will dissolve in moisture from humid environments and then crystallise when drier. This continued process of dissolution and crystallisation can cause cracking internally and flaking of the surface.
Desalination treatments involve washing the objects in cascade baths of water to flush the salts from within. Measuring the salt level of the solution regularly helped identify the end point. Controlled air drying followed.
Concretions
Concretions are hard, concrete-like products that forms where iron is present. They are made up of sediment and silt from the seabed and are usually found around iron artefacts like guns and shot. As they formed around the iron objects, they produced a barrier layer protecting the iron surface. Lumps of concretions were also raised from the wreck site. These were x-rayed to determine the contents before being worked on by conservators to remove the concretion and reveal what was inside. For the smaller iron objects, like the blades and arrow tips, their resistance to corrosion was far lower and so by the time the concretions had fully formed the blade had completely corroded away. It is only from x-rays that we can see the outline of the shape of some of these mineralised remains.
Composites
Artefacts in the collection tend to be made of more than one material; these we call composites. Treating them depends on which materials are present. Some combinations are worse than others though. Wood and iron together are the most problematic. Polyethylene glycol accelerates the corrosion of iron while alkaline solutions degrade wood. Usually in situations like this we will try and separate the materials, if possible; if not, we’ll prioritise the treatment of one of the materials. This will either be the predominant material or more significant.
