The climate of the Vasa Museum - problems in coordinating the museum object and the museum climate.

Birgitta Hafors, Vasamuseet, Box 27131, S-102 52 Stockholm, Sweden


The planning of a permanent museum building for the Swedish 17th century warship, Vasa was on the agenda from the beginning of the project. For several reasons the new building was postponed until the early 1980-ies .

As the Vasa museum was predestinated to become a major monumental building the Government decided that the construction of a permanent Vasa museum was to be preceeded by an architects competition. Such a competition was advertised on the 7th of December, 1981.

The importance of the climate for the long term conservation of the Vasa hull and wooden artifacts was well acknowledged by the Swedish Maritime museum that was then in charge of the Vasa.

The technical equipment that was needed to keep the climate stable and on a level that was defined from the needs of the preserved Vasa wood was therefore a very important item in the museum and the architects competition included the design of the airconditioning system in the exhibition hall of the permanent Vasa museum.

The design of the airconditioning system started in 1985. The new museum opened in June 1990 and the airconditioning system was put to the test. The climate is monitored by automatic readings every ten minutes of RH and temperature values from twenty one stationary and ten mobile RH/temperature measuring units. The RH and temperature values are fed into a computer system.

The control parameters of the airconditioning system are set to see to the maintenance of both the Vasa and the museum building and to allow for the activities of the museum.


After the newly raised 17th century Swedish warship Vasa on the 24th of April 1961 had been towed into the Gustav V drydock in Sweden's naval shipyard in Stockholm she was put down on a specially built floating pontoon . On this pontoon a superstructure made of aluminum sheets was built as the first housing for the Vasa (figures 1 and 2). The pontoon superstructure was first and foremost intended as a space for the preservation treatment of the Vasa but since the museum director was convinced that publicity was desirable and important for the finance of the project the pontoon superstructure was also equipped with visitors galleries in two levels around the walls inside the housing. The total space volume of the building was 15 000 m3 and it was certified to take 400 visitors at a time. People came in guided groups but were also allowed to walk by themselves as long as they wished in the very special museum of the Vasa housing.

Figure 1 The Vasa hull on the pontoon during transport from the Gustav V dock to the construction site for the pontoon super structure

Figure 2 The pontoon super structure with the Vasa hull inside during transport to Wasavarvet

Along with the preservation treatment a cautious reconstruction work began. First there was some discussion about the feasability of demounting the whole of the hull and treat the separate timbers in a tank to get a better and quicker conservation process. This idea was soon abandoned and spray treatment was chosen. Some of the constructional timbers had however come loose of the hull and had been brought up by the divers along with the adornment pieces. For those it was decided that they were to be put back into the hull construction to be treated together with the rest of the hull. The main purpose of this was to diminish the risks of tension that might be the result when drying the hull if the treatment or the drying situation were not similar in the different parts.

This however, soon made the housing too small for the Vasa and during a period from the autumn of 1968 to the spring of 1969 the pontoon superstructure was lengthened and made higher. This added an extra 2 000 m3 to the volume of the building. Before the construction work was done two smaller pontoons were added to the original main pontoon to keep stability when the housing became larger. The added space allowed for the protruding cut water part of the ship to be reconstructed and a plastic replica of the figure head lion could be mounted. The added height of the building also made it possible to complete the stern. To give an idea of the stern adornment a plastic replica of the national coat of arms was mounted on the stern.

Planning for a permanent museum building

The temporary character of the pontoon superstructure was a reality from the beginning. Construction of a permanent museum building was on the agenda of the Board of the Vasa that was the organization responsible for the Vasa project during the years 1959 until the 30th of June 1964. The task was inherited by the Swedish National Maritime Museum that took over responsibility for the Vasa on the 1st of July 1964.

As long as the preservation treatment went on the temporary housing was the best way to keep the Vasa. Soon however, signals came about events that had an impact on the construction of a permanent Vasa museum. The first one was as early as in 1969 when it was decided that the Naval Shipyard was going to move from the situation in the middle of the Stockholm habour leaving a site that could be used for a permanent Vasa museum

Nothing came out of this and the temporary Vasa housing went on taking as many visitors as between 250 000 and 400 000 per year during the seventies and the number grew during the eighties to become 450 000 to 550 000. The quality of the housing as suitable for the Vasa came into dispute and there were suggestions that a new temporary museum should be built. Tests with tracing gas were made that showed a substantial leakage of the construction. Condensation of water on the walls and ceiling had been noticed that endangered the preservation work during the latter part of the treatment. This would consist of a slow drying with gradually diminishing treatment.

There was no new temporary building but repair and enlargement of the ventilation installation were made and the walls inside the pontoon superstructure were painted with a special paint that adsorbed the water condensing in the building (figure 3 and figure 4).

This was the situation until the permanent Vasa museum was constructed.

Necessary qualities of a museum buildning for the Vasa ship

The design of the new museum building made it necessary to state the conservation parameters for the Vasa wood. This was discussed by the board of specialists and their decision was that moisture ratio is the relevant parameter for the long term conservation. The moisture ratio of the wood had to be kept stable over time. Because of the height of the museum building the technical difficulties of keeping the same values of the climatic parameters on all levels were taken into account and two lowest levels of moisture ratio were established. A 10% moisture ratio lowest level was allowed in the upper parts of the hull and in the lower parts the lowest moisture ratio level was set to 11%.

It also was stressed by the specalists' board that no condensation was allowed in the building. This was stated because of the situation in the pontoon superstructure and was meant as a protection for the Vasa. As it came out condensation parameters also put some restrictions upon the climatizing of the building during the sometimes rather cold Swedish winter period.

The architects' competition

On the 23rd of July 1981 the government's decision came that a permanent Vasa museum was to be constructed. The Swedish government gave the civil service department of public buildings the dual commission to carry on planning and preliminary design of a permanent Vasa museum and also to organize an architects' competition for the design of the museum building.

The architects' competition addressed itself to architects in all the Nordic countries. A rather substantial number of 384 architects sent in models of their ideas of the most suitable building for the Vasa. A number of the contributions were shown to the public in a special exhibition. There was quite a number of different approaches although the area for the building site and the function of the building were given parameters. The classification of the very different contributions became a hard match for the jury but eventually two architects became winners of the first prize. One was a Danish architect who had called his contribution "In a hundred years the prize will be forgotten about" the other was a Swedish team with a museum model rather efficiently called "Ask" which means "box".

These two first prize winners both got the opportunity to work for a period of time on their projects to bring them into a state which could be judged also from technical and economical standpoints. After this second round the jury scrutinized the two projects again with this new information. They were also judged from the viewpoint of the impact of the buildings on the city silhuett which was observed from several outlooks by the jury.

The final result was that the commission to design the Vasa museum went to the Swedish architect team G ran M nsson and Marianne Dahlb ck. Their contribution with the rather prosaic name "Ask" was explaind by the jury as follows "With its colours, its pennants and stays, eaves, bridges, canopy and lantern the building has a strong and happy quality. The great merit of the project is its lively expression which is altogether devoid of sacrality. Instead of this it has a touch of the baroque period spectacle. With the indicated qualities it has in a subtle way captured the nature of the area (Djurg rden)"

Construction of the permanent museum building

In October 1986 preliminary works started to prepare for the permanent museum building. The dry dock that was going to be the berth of the Vasa ship was widened and made deeper. The foundations of the rest of the irregularly shaped building were constructed. Those works took a litttle more than a year.

The foundations lain the more spectacular construction work began with an inauguration ceremony that was held on the 2nd of November 1987. The ceremony was performed by His Royal Highness Prince Bertil and the highlight of it was when the foreman of the concrete works placed some chosen objects, together with a plaquette with a text stating the date and the origin of the objects, in a niche and completed the concrete layer on top of it. This leaves a message for those who one day will tear down the present museum to construct a new and even better one for the Vasa.

The walls of the museum but the one facing the water were completed during 1988 and in November that year water was filled into the drydock to prepare for the reception of the Vasa.

During the construction period the Vasa was not available for visitors during a period of eight months from September 1988 when a special encasement was constructed around the hull. The encasement was very narrow but it contained all the necessary equipment to keep the climate the same as it was in the pontoon super structure.

Moving into the permanent museum building

Because the new Vasa museum was constructed with the main part of an old drydock inside the building it was possible to move the whole floating pontoon with the Vasa hull inside the temporary protection encasement into the museum (figure 5). This was a risky business although the distance between the position of the old museum and the building site was as short as some 500 metres. The encasement made a huge surface exposed to the wind and the day of transport had to be chosen carefully with the help of meteorologists to have as low velocity of wind as to provide a safe journey. The chosen day was the perfect for the task and on the 6th of December 1988 an assembly of invited people witnessed two tow-boats skillfully manouvering the precious parcel of the Vasa hull on the pontoon into the new museum. The freeboard on each side of the pontoon to the dock sides was 40 centimetres.

Figure 5 The temporary protection encasement containing the Vasa ship on its way into the Vasa museum

Finishing the construction of the permanent museum building

After the pontoon had been brought into the museum the drydock was emptied and sealed off from the surrounding water. The pontoon was carefully placed on plinths that were distributed on the bottom of the dock in order to give adequate support. The pontoon then became a permanent part of the new museum building.

The wall through which the Vasa made her entry into the museum was completed during the winter that followed . During the period of time from the 1st of May 1989 to the last of August that year visitors were allowed into the museum building while under construction to get a glimpse of the port side of the Vasa. This was made possible by ripping away the upper part of the lengthwise wall of the temporary protection encasement. The half finished museum building gave enough protection for the Vasa hull because the climatizing equipment was still working inside the encasement .

There were 228 000 people visiting the half finished museum building during the four months visitors were allowed on to the construction site.

It took another winter to finish the whole museum that was officially inaugurated on the 15th of June 1990.

Functioning of the permanent museum building

The purpose of the museum building is dual. The one very important assignment is to create and keep suitable levels of temperature and relative humidity for maintaining the object. The other is to give possibility to preferably as many people as would wish so to see the Vasa. It is a question of the right functioning of the right technical equipment but also of managing the reception of visitors and other activities of the museum.

Creating a suitable climate for the Vasa

To ensure a suitable climate for the Vasa ship a circulation system that handles 90 000 m3/h provides a fully conditioned air to the space around the Vasa ship. This system only works with recirculated air that is cooled or heated, dried or humidified if the temperature or the relative humidity are not in accordance with the preset control values. The air is distributed through twenty displacing outlets situated in an oval around the Vasa on the keel level. There is also a distribution system inside the Vasa.

The height to the upper deck of the Vasa is some ten metres, the stern rises to a height of about twenty metres and there is another ten metres before the ceiling is reached. Knowing that the source of air distributed around the hull is situated on the keel level it is quite understandable that there will be a temperature gradient that in its turn will create a gradient of relative humidity. This is the reason why different lowest levels of moisture ratio of the wood have been established at different heights. Stability of the once established equilibrium is however important and this calls for stability of the two gradients during the different phases of the day and night period, the different phases of weekdays versus weekends and stability during summer versus winter periods which will be the most difficult stability to be established.

The summer/winter climatic situation in the Vasa museum

The "normal" room temperature of 20 degrees C was chosen as museum temperature for the summer period. For the sake of keeping the building safe of getting damaged by low outdoor temperature it was desirable to lower the moisture content of the air during the winter. This could be done by lowering the temperature while keeping the relative humidity at the same level. However, to keep the equilibrium moisture ratio of wood at a chosen constant level at a lower temperature the relative humidity has to be lowered as well.Thus there was some extra benefit for the building. As a lowest temperature that might be acceptable to the museum visitors and of course the museum personel on duty in the museum the figure 17 degrees C was set.

There was not enough documentation about the equilibrium of moisture ratio of the polyethylene glycol treated Vasa wood to the climatic parameters so the graph for fresh spruce wood was chosen in order to establish the preliminary values of the control parameters. The higher of the lowest moisture ratio levels, 11%, chosen for the equilibrium situation in the Vasa wood was taken as guideline. Considering the chosen temperature levels the value of 60% relative humidity for the summer situation and 57% relative humidity for the winter situation were read from the equilibrium moisture ratio graph for fresh spruce. Thus the pair of temperature/relative humidity values of 20 degrees C/60% RH and 17 degrees C/57% RH became the first summer/winter pairs of control values of the museum climate.

It soon was noticed that although the temperature readings in the chamber of processed air followed the control value rather accurately the RH-values were a little below that value during the winter period. To compensate for this the control was set at the slightly higher RH value of 57.5 % first as a test from September 1993 to April 1994 and then on a permanent basis from the 15th of March 1995.

The quality of the conditioned air when leaving the air conditioning system is one thing and what happens at the surface of the wood is another. To know what is going on it is necessary to get information about the resulting temperature and RH values at the surface of the object and also a reaction from the wooden object itself. Information about the climate is registered at twenty-one permanently mounted and ten mobile RH and temperature sensors. Each of these sensors reads a value into the computer system every ten minutes. The actual climatic values can be read at any time but as a routine the values are processed on a weekly and a yearly basis.

To evaluate the summer versus winter situation at a particular spot of the Vasa wood the mean summer and winter RH and temperature values are read from the year diagram. The equilibrium graphs for different moisture ratio equilibria are consulted to establish the corresponding moisture ratio equilibria for the summer and winter periods at the chosen spot of the Vasa wood. If the two of them differ something may have to be done to equalize the summer and winter climates if the difference has made itself known to the wood. In this case it has had a registered impact on the movement of the wood that shows a yearly shrinkage/swelling behaviour indicating changing moisture ratio equilibrium (figure 6). In order to ameliorate the situation a somewhat higher RH value is desirable during the winter period. Therefore investigations as to how this would affect the building have been started.

The day and night and weekdays versus weekend stability of the climate

To get climatic information as to the day and night periods of the week and from different parts of the week four mean values of temperature and relative humidity are calculated for each weekly period. The day periods lasts from six o'clock in the mornings until eight o'clock in the evenings. The five weekdays comprizes one period and saturday and sunday the other.

This makes four periods of the week which have some differences of climatic values. These are however seldom of a magnitude that would mean different moisture ratio equilibria during the period of one week even in the summer when the Vasa museum recieves the main part of its yearly about 800 000 visitors.

The Vasa museum also functions as a space for special events. With some restrictions as to smoking and candlelight it houses rather substantial dinner parties mostly during the winter evenings. In the immediate neighbourhood of a large dinner party the temperature and RH readings can get rather high during the three to four hours it lasts but that will not affect the wood because of its retarded reaction.

Monitoring the reaction of the Vasa wood

The reaction of both wood and polyethylene glycol to varying humidity of the air will be that of absorption - desorption. This would affect the weight of the wood. It might cause shrinkage and swelling of the wood and also variations of electrical resistance. The hygroscopic character of polyethylene glycol could give the wood a more or less sticky or moist surface to be judged by the touch even before the added or lost moisture can be noticed by any other means.

The monitoring of the Vasa uses all those reactions. Since the beginning of the Vasa project the shrinkage that has taken place during the drying process has been monitored at about fifty measuring points each of them consisting of two stainless steel nails at a distance of between ten and fifteen centimetres. The pairs of nails are also used for measuring electrical resistance with an instrument specially constructed for this situation. A pattern has been established over the wooden surfaces both inside and outside the Vasa hull for measuring with a conventional electrical moisture ratio meter which is performed at regular intervals as well as the previously mentioned measurements.

Weighing is used as an indicator of stability of the climate inside the Vasa hull. Four small pieces of preserved material which are kept on the orlop deck are weighed regularly. Those are the quickest informators of the earlier mentioned summer/winter differences in moisture ratio equilibria.

Even the feel to the touch of the wooden surfaces is registered at regular intervals in terms of different degrees of wetness and stickiness. This is an extremely operator dependent procedure and the results from two operators should not be put together in the same diagram. The information from this investigation give the same result as the other ways of monitoring. Its added quality is that it might give a quicker answer to changes of the climate.

Managing the future maintenance of the Vasa ship

For the future manitenance of the Vasa ship (figure 7) control of the climate, control of the object and proper management of the use of the museum are of vital importance. The monitoring of the Vasa will hopefully be able to benefit from the technical development to reach a higher degree of automatization. Even so there are some functions that have to be handled of conservators and last but not least - it takes a skilled conservator to evaluate the information recieved from the measurements and to convert these into useful management instructions.

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