The ship was raised and placed on a reinforced concrete pontoon, and supported by temporary shoring struts. Excavation of the interior took place from May to September 1961, raising over 40,000 separate objects of different materials. All the while, the hull was sprayed with harbour water to prevent the wood from drying out. In November the same year, the ship was moved to a temporary building for conservation at the Wasa shipyard, where conservation was to take place.
When waterlogged wood is allowed to dry out without treatment, severe cracking and shrinkage can occur. The wood may look sound, but the wood cells are weakened by bacterial decay. Some substance is therefore required to replace the water in the cells and provide internal support. After examining a number of possible materials, the conservators chose the synthetic polymer polyethylene glycol (PEG) to treat Vasa's wood. PEG is a synthetic wax that is soluble in water; it is a common component in cosmetics, such as lipsticks and face creams, and is widely used in the pharmacy and food industries. Experiments showed that a PEG solution could diffuse into the wood structure to replace the water, and prevent cracking and shrinking.
While PEG impregnation works well for objects in tanks, treating a ship the size of Vasa presented a huge challenge and the only option was to spray the PEG over the ship. PEG spraying began in April 1962. Initially the work was done by hand which was time-consuming and not very efficient. It took five men five hours to spray the entire surface of the ship. More efficient spraying was achieved in 1965, when an automatic spray system was installed, with 500 spray nozzles directed over the inside and outside of the ship. The PEG concentration was gradually increased from a low concentration of 10% and ending with a ca 45% solution. Boron salts were added initially to prevent micro-organism growth but later also to neutralize acids.
The conservators continued testing various types of PEG over the years, and PEG 4000, 1500 and 600 have all been used on the ship. The PEG ran over the hull's surface, was collected in tanks and re-used. The spray treatment lasted 17 years, from April 1962 to January 1979, followed by another 9 years of slow air-drying. A final surface layer of PEG 4000 was applied as physical protection to the ship and metled into the surface with hot-air blowers.
Vasa's conservation began as a huge experiment but the pioneering research by Vasa's conservators has paved the way for numerous other shipwreck projects around the world.
Apart from the ship hull, more than 40,000 objects were found with Vasa. These were stored in water tanks until conserved at the purpose-built conservation laboratory at Beckholmen.
The wooden objects were placed in stainless steel tanks filled with PEG 4000 solution with a 2% solution of boric acid and borax to prevent the growth of micro-organisms. The solution was heated to 60° C and the concentration of PEG was gradually increased daily, until a final concentration of 40-45% was reached. Generally the same types of wood and similar sizes of objects were treated together, as these factors affect the length of treatment. Impregnation was considered complete after about 18-24 months, once the weight of the objects stabilized.
The objects were then gradually air-dried for 6-12 months, in the hold of a large ship, Menja, moored next to the conservation facility, where the relative humidity was gradually reduced from 90% to about 60%. Finally a protective surface treatment of PEG 4000 was applied to the objects and the excess removed using hot-air blowers.
Some fine-grained wood species, such as beech and alder, did not respond well to this treatment. Instead, objects made of these wood spices were left in water for another decade, until freeze-drying techniques were developed.
Drying is a critical stage in the conservation of organic materials, as the surface tension of water as it evaporates can be strong enough to collapse cells already weakened by bacterial decay. Developed in the 1970s, freeze drying is a method to remove water by changing it from its solid form (ice) directly to its gas form, thus avoiding the damaging liquid phase. Freeze-drying is often combined with PEG pre-treatment.
About 500 smaller wooden objects and most of the 450 leather pieces that were found with Vasa were freeze-dried. After washing in distilled water, the objects were placed in a 5-10% solution of PEG 400. Once their weight stabilized, they were frozen to –20° C in a commercial freezer, and then transferred to the freeze drier. Under a slight vacuum, heat was slowly applied, causing the ice to vaporize. Treatment was considered complete once the objects' weight stabilized. Objects treated with early freeze-drying methods, including wooden spoons, bowls and jars, and even a game of backgammon tend to be very light in weight and quite fragile, due to the low molecular weight PEG, but their size and shape have been preserved, and their colour and surfaces are closer to those of natural wood.
Since the 1970s, there has been a lot of research into freeze drying techniques combined with PEG impregnation, and it is today one of the most successful and popular methods available for conserving organic materials, the only limitation being the size of the freeze-drying chamber.
Conservation of metal
Seventeenth-century iron technology produced three types of iron – cast iron, wrought iron and steel. The cast iron on board Vasa, about 850 cannon balls and cooking pots, survived in reasonable condition. Cast iron becomes saturated with carbon from the furnaces during manufacture, and contains about 6-7 times as much carbon as wrought iron. The carbon forms a crystalline skeleton inside the object, which retains the size and shape of the object, even though all of the iron can corrode away.
In comparison, wrought iron has a carbon content of less than 1%. Without the carbon content, Vasa's wrought iron did not survive as well under water. Only the lowest part of the gudgeon strap holding the rudder to the stern, the anchors, and a few fragments of the wrought iron bolts holding the ship together survived.
Vasa's iron objects were conserved by heating to 600–800° C under hydrogen gas, in a process called hydrogen reduction. The hydrogen reacts with the corrosion products (rust) forming metallic iron, while water is given off as steam. The reduced iron objects were then sealed in paraffin wax to prevent moisture and oxygen causing new rust. One drawback of the method is that the temperatures used are high enough to change the internal metal structure of the object, destroying any evidence of the production technology.
Copper, bronze, lead and pewter objects were also found on board Vasa. Three bronze cannon, which were missed from the 1660s salvage, were found on the gun decks towards the stern. Four thousand copper coins were also found, including a handful of silver coins, as well as lead and tin objects such as pump parts, fishing weights, ca 9000 musket shot and pewter plates. These metals do not corrode as readily as iron under water, and were in good condition when found. These objects were mechanically cleaned and air-dried.
Other finds, such as bone, ceramic wares and rope were found with Vasa. Although new conservation treatments were tested, the majority of these materials were rinsed in changes of fresh water and allowed to air-dry without further treatment.
Remarkably, six of Vasa's original sails and two smaller boat sails were found folded on the orlop deck. Made of hemp and flax, vegetable fibers that would not normally survive, the sails were extremely fragile, the fibers disintegrating when touched. To remove the sails from the storage locker on the ship, the archaeologists were forced to cut the pile in half and slide iron sheets under each pile to allow them out to be lifted of the hatches.
Conservation began in October 1962. As the sails were so fragile, they could not support their own weight and had to be cleaned and straightened under water, in large, shallow trays. Gradually the silt and dirt were removed and the various layers were separated.
Like wood, the fibers of waterlogged textiles can also collapse through the surface tension of water, causing uneven drying and shrinkage. Vasa's sails were dried through changes of alcohol and then xylene, a solvent with very low surface tension, but they were still extremely fragile and required strengthening.
A new method was devised by Vasa's conservators, whereby a close-weave fiberglass fabric was painted with a number of layers of an acrylic solution, with the same refractive index as the glass fiber, which made the backing fabric almost invisible. The fragile sails were then attached to this backing support using a lower concentration of the same acrylic solution. Conservation of Vasa's sails took over a decade to complete. Today at the museum, we have the remains of 650 square meters of sails, most kept in drawers in our store rooms and one sail on display in the ship hall.