Vasa's New Climate-Control System

Maintaining a Stable Environment: Vasa’s New Climate-Control System

EMMA HOCKER

An extensive upgrade to the air- Introduction ship is not open to the general public, museum staff regularly go onboard for conditioning system of the Vasa The Vasa Museum in Stockholm, research or maintenance purposes. Museum in Stockholm is playing an Sweden, houses the seventeenth-century Although the largely anoxic (oxygen- warship Vasa, the largest and best pre- instrumental role in preserving the deficient) burial conditions in the Stock- served wooden ship ever salvaged from seventeenth-century Swedish holm harbor had generally favored the seabed and conserved. The warship, wood preservation, there was sufficient warship Vasa. adorned with hundreds of painted oxygen available in the murky waters of sculptures, was commissioned by King the harbor immediately after the sinking Gustav II Adolf, who had ambitions to to allow micro-organism degradation of dominate the Baltic region. It was thus the outer 3/4 in. (2 cm) of wood. In order a huge embarrassment when the ship to prevent shrinkage and collapse of sank unceremoniously in Stockholm these weakened wood cells once the ship harbor on its maiden voyage in 1628. was raised, a material that would diffuse Salvaged in 1961, the ship underwent a into the wood and take the place of the pioneering conservation program for 26 water in the cells was needed. The mate- years.1 In late 1988 the conserved ship rial chosen was a water-soluble wax, was floated on its pontoon into a dry polyethylene glycol (PEG), which was dock through the open wall of the pur- sprayed over the hull in increasing con- pose-built Vasa Museum, which has centrations over a 17-year period, fol- since become the most visited maritime lowed by a 9-year period of slow air museum in the world. Although the drying, during which the relative humidity (RH) around the ship was gradually reduced from about 90% to 60%.2 Built predominantly of oak, the ship is a monumental structure, the equiva- lent of a 7-story building; it weighs ap- proximately 900 tons. The hull is 226 ft. (69 m) long, including the bowsprit; 63 ft. 6 in. (19.4 m) high at the stern; and 105 ft. (32 m) to the top of the existing masts (Fig. 1). Unlike a wooden building, however, a ship is designed to sit in water, where its weight is evenly sup- ported. Exhibiting such a complicated, curved structure on dry land is problem- atic, and Vasa currently sits on a steel support cradle of 18 pairs of stanchions connected by large I-beams. Wooden wedges between the stanchions and the hull must be adjusted periodically in order to provide good contact and even support. The enormous weight is therefore concentrated at point loads, which has resulted in the sagging of the weak- Fig. 1. The hull of Vasa on display in the purpose-built Vasa Museum in Stockholm, as seen from the viewing galleries at level six. Photograph by Karolina Kristensen, all images courtesy of Swedish ened wood structure between the stan- National Maritime Museums. chions and the crushing of the keel.3

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Fig. 2. Plan of the Vasa Museum. Shaded regions signify areas outside the Fig. 3. North-south section through the museum. Illustration by Fred climatized spaces. Illustration by Fred Hocker. Hocker.

Work is underway to document the enormous public interest in the ship. absorb and desorb moisture from the structure as it sits today and to examine During the 1990s visitor numbers ex- atmosphere. Humidity fluctuations can the ship’s movement in relation to sea- ceeded the original predictions of therefore lead to moisture transport sonal changes in order to design a new 600,000 per year by 33%, and since within the wood, which causes both support system. Another future task is 2004 visitor numbers have steadily physical and chemical problems. Too to replace the mild-steel bolts that were increased, mostly due to the museum’s high a relative humidity (over 70%) can inserted in the 1960s to hold the ship’s deliberate marketing strategy combined promote the growth of mold and other timbers together, a measure that will with more cruise-ship arrivals in Stock- micro-organisms on the wood, as well improve the hull’s structural integrity. holm, which culminated in almost 1.2 as make the surface of the PEG sticky. million visitors by 2009. Fire-safety These changes in turn increase the ship’s Entering the Display Case regulations restricted the number of weight, thus increasing the stress on the visitors inside the museum at any one support structure. Too low an RH will The museum was designed with an time to 1,440, but this number was only cause shrinkage and cracking of the internal airspace of about 3,708,000 ft.3 recently enforced; as a result of the extra wood, which is compounded in a large, (105,000 m3) and with a maximum visitors, the climate system was often three-dimensional structure, where ceiling height of 119 ft. 10 in. (36.5 m). overloaded during the summer. The complex wooden joints are subject to The hull is placed towards the east wall, museum is understandably more popu- extra strain. Regular monitoring of the with the bowsprit pointing towards the lar on rainy days, and wet clothing and ship between 2001 and 2003 using a main entrance to the north. Exhibit large numbers of exhaling visitors there- total station revealed seasonal move- areas are concentrated on the port side fore create very humid conditions, ment and twisting of the hull, mainly to the west (Figs. 2 and 3). The museum which the air-conditioning plant must due to humidity fluctuations.4 is entered via double air locks from out- deal with. Visitors to the Vasa Museum One of the effects of moisture trans- side or single air locks from the offices essentially enter a huge display case, and port caused by fluctuating RH has only and restaurant. This setup presents an maintaining the environment in this air been recognized in the past decade, pri- unusual challenge in climate manage- volume is no easy task. While light levels marily through visual evidence found on ment, as it results in a very large air can be minimized and dust removed, the the ship’s timbers. Sulfur compounds in volume that must be maintained at very control of temperature, and especially the polluted waters of Stockholm harbor close tolerances of temperature and RH, in such a large building volume remained in the wood, and upon expo- humidity but subjected to high traffic. requires special measures. sure to air they reacted with iron from This situation is further complicated by corroded bolts and moisture in the the enormous heights necessary to Consequences of an Unstable Climate museum environment to form a range of accommodate the masts and tops (the acids and other harmful compounds, round platforms at the mast heads). A stable RH is an essential part of the which can potentially deteriorate the Naturally, temperature gradients occur, long-term preservation of archaeologi- wood and reduce its mechanical especially in the warmer summer cal wood treated with polyethylene strength. A secondary problem is that months, which in turn produce RH glycol (PEG). Not only is organic mate- cycles of wetting and drying in the gradients, with more humid air concen- rial particularly sensitive to fluctuations museum environment have caused these trated at lower levels. and extremes of humidity and tempera- compounds to be drawn to the surface One aspect not fully appreciated ture, but the conservation agent, PEG, of the wood, where in drier conditions when the museum was built was the is hygroscopic, meaning it will readily they precipitate as a range of acidic iron MAINTAINING A STABLE ENVIRONMENT 5

Fig. 4. An example of the fluctuating temperature and RH taken from three Fig. 5. Weight fluctuation in kg of an oak plank impregnated with polyethy- sensors on the ship in July 2003. lene glycol that has been stored on board the ship since 1996. sulfate salts, predominantly yellow in east wall near the ceiling (Fig. 3) and sponsible for the ship, it does not own color.5 The volume increase of the salts returned to the plant, where it was the building or air-conditioning system. as they crystallize has caused the surface filtered, dehumidified, and cooled to Persuading the landlord, the Swedish of the wood to spall off in places, result- about 45˚F (9˚C). For visitor comfort, National Property Board, that an up- ing in the loss of surface detail on carved about 30% outdoor air was included in grade was necessary was not an easy elements. Although it is likely that these the conditioned air, which was then task, especially as their sensors, which processes had begun after the ship was redirected into the museum. Onboard were located in the ducts providing con- raised, the results became particularly the ship an internal air-distribution ditioned air to the exhibit space, showed apparent after the wet summer of 2000, system, with air exchange of 247,200 the target values. Sensors placed by the which, combined with large numbers of ft.3/h (7,000 m3/h), was adapted from preservation staff directly on the ship, visitors, saw dramatic fluctuations in the the ducting used during the post-conser- however, deviated from these readings RH. An example of the museum climate vation drying. When the museum substantially. Although the landlord had from July 2003 taken from the climate opened in 1990, specifications of 60% already decided to replace the cooling sensors onboard the ship shows the RH RH and 68˚F (20˚C) were set for sum- plants with more efficient units, which at some points lower down on the ship mer months, and 57% RH and 62˚F would also improve airflow and air hull rise above 70% and then drop by (17˚C) in winter to reduce the risk of quality in the museum, it was the evi- 10% overnight (Fig. 4). condensation in the walls of the build- dence of the salt outbreaks that finally Once moisture was identified as a ing.6 These values corresponded to a persuaded the authorities in December problem, it was decided that the average target moisture content in the wood of 2002 that a major and expensive over- RH in the exhibit hall should be reduced 10 to 12%. Temperature and RH were haul of the system was needed. from 60% to 55%. The ship continues recorded every 10 minutes through 21 To assist with the planning and de- to dry out and is only now approaching permanently mounted sensors (17 on sign process, the museum hired a heat- a stable condition. Much of this climate the exterior and 4 on the interior of the ing-and-ventilation company as consul- history can be seen from the weight ship) and 10 portable sensors. The basic tants, and with their assistance a com- fluctuation of a PEG-treated oak timber principle of this system was to distribute prehensive evaluation of the existing from Vasa, which has been stored on conditioned air by convection and thus air-conditioning system was conducted, one of the lower decks since 1996 (Fig. avoid causing uneven drying of the which continued over the next two 5). wood through fan-assisted circulation. years. A gradient of around 10% RH The temperature and corresponding RH was found between lower decks and The Need for a New Climate-control gradients caused by the building height underneath the ship and high up in the System were accepted as inevitable. In practice, stern and masts. Large fluctuations were however, the system was under-dimen- also recorded at the forward starboard The museum’s original climate-control sioned and too often ran at full capacity. hull area near the entrance to the mu- system from the late 1980s was designed The RH targets were frequently ex- seum shop and the rear starboard area to handle 3,178,000 ft.3/h (90,000 m3/h) ceeded, and data typical of those seen in near the restaurant, areas where doors of air, distributed through 20 cylindrical Figure 4 were regularly recorded. were often left open to improve air drums situated around the hull on level Clearly one of the major measures in circulation and where unconditioned air two. The conditioned air was directed ensuring long-term preservation of the could enter freely. Public congregation upwards, creating a protective air cur- ship, both chemically and structurally, points were also examined, which tain around the ship rather than condi- was establishing a stable climate. Com- showed areas of strongly fluctuating RH tioning the entire exhibit space, and then plicating matters, however, was the fact at the viewing galleries on level six at evacuated through a large duct on the that although the Vasa Museum is re- the stern, areas where stronger lighting 6 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 41:2-3, 2010

on the fire regulations of the time, which for reasons of cost, but with the latest allowed a maximum of 1,440 visitors in upgrade, six columns 11 ft. 6 in. (3.5m) the museum, it was agreed that the sys- high connected to CA01 have now been tem should be slightly over-dimensioned installed in this area; they are guided by to accommodate 1,500 persons. More- sensors on the port side of the ship. over, based on the recommendation of These columns are capable of distribut- the consultant, who had particular ing conditioned air to a height of 52 ft. experience in designing medical facilities 6 in. (16 m) to create a “climate um- and hospitals, the museum required that brella” around the hull (Fig. 8). a reserve power supply should be installed in case of power failure. Sensors Construction of the new climate- control system took place between 2003 Temperature and RH monitoring is and 2004, during which temporary air- done by 42 sensors mounted directly on conditioning units were in operation at the ship hull, inside and out. One of the museum entrance. The principle of these sensors has no permanent place- the new system is similar to that of the ment but has proved useful when other old but with improvements; the major sensor readings need to be checked, as it plant, CA01, circulates conditioned air can be moved around the ship as neces- through the drum-shaped outlets under- sary. A further 15 temperature and RH neath the ship and through ducts inside sensors are used in other components of the hull, providing a protective curtain the system, one of which is placed on Fig. 6. Specially designed duct heads used to of conditioned air around the ship. The the exterior of the building, allowing provide efficient distribution of climatized air air is then removed via the large duct in comparison with external environmental inside the ship. Photograph by the author. the east wall and returned to the plant in conditions. The sensors, Vaisala HMP the basement. After advice from the 230, have tolerances of ±0.04˚F was also concentrated. Smoke visualiza- museum consultants to improve airflow (±0.15˚C) and ±1% RH. In order to tion tests showed that the airflow was inside the ship but also protect the wood minimize the number of cables on the uneven in the hull interior, especially on from drying too rapidly, the ducts on- ship, cordless sensors were investigated. the lower decks, where pockets of higher board the ship were shortened in some However, since they can easily be af- RH were created, and that the ducting cases and repositioned to allow at least fected by radio traffic, are slower to 7 was in poor condition and leaking. 3 ft. 3 in. (1 m) of free space around transfer information, and require more Much of this information had been rein- them. The specially designed duct open- work to replace batteries, hardwired forced by the evidence of the salt out- ings were retained, since they provide sensors were eventually chosen, each breaks, which were more numerous and efficient multi-directional distribution of connected to individual display boxes exhibited lower pH levels in areas ex- the conditioned air (Fig. 6). With leaks sitting on the ship, which show real-time posed to higher or fluctuating RH removed, the average airflow is now temperature and RH (Fig. 9). Certain 8 levels. The design and capacity of the 409.7 ft.3/h (11.6 m3/h), compared to sensors may be chosen to guide the sys- air-conditioning system were also exam- 190.7 ft.3/h (5.4 m3/h) previously, and tem, and should the average RH read- ined, as were the effectiveness of the air exchange is now doubled to about 8 ings from these sensors exceed the speci- supply ducts and the location of sensors volume changes per hour. fied values for more than 10 minutes, for controlling and monitoring the To combat problem areas in the automatic alarms sound in the land- climate. Computer simulations using museum, auxiliary systems have also lord’s and security-personnel offices. computational fluid dynamics analyzed been installed. As visitors tend to con- the effects produced by evening events gregate to look at the sculptures adorn- Monitoring the Climate or banquets and times of maximum load ing the stern, a second plant, CA02, A shortcoming of the old system was to the system, for example in July and delivers conditioned air through small that only a week’s worth of data could August, when visitor numbers are high- slits located in the walls of the visitor be stored on the landlord’s hard drive. est and maximum dehumidification is galleries on levels four and six, and Preservation staff received print-outs of required. ducts high on the south wall of the the sensor readings a week after they museum above the stern are connected were recorded, by which time it was too The New Climate-control System to plant CA03 (Fig. 7). Controlled by late to make adjustments. The advan- nearby sensors on the ship, these plants For the new climate-control system, the tage of the new system is that data may are designed to remove extra moisture. preservation staff stipulated a year- be accessed by logging onto the land- Since the ship does not sit at the center round temperature of 18.5 ± 1.5˚C (i.e., lord’s server, which shows climate data of the museum, there is consequently a 62-68˚F, 17-20˚C) and RH of 55 ± 4% in real time. Also, comparisons can be huge volume of air on the port side. (i.e., 51-59%), a figure comparable to made over longer periods, up to the Plans to include air-conditioning in this 10% moisture content in wood. Based previous two years. Diagrams can be area were struck from the 1980s designs MAINTAINING A STABLE ENVIRONMENT 7

Fig. 7. Ducting at the stern of the ship, part of auxiliary plant CA03. Photo- Fig. 8. Auxiliary vertical ducting on the port side of the hull. Photograph by graph by the author. the author. generated from any combination of ciably better than in previous summers. revealed to be reading 3% too low sensors and time periods, from hours to Indeed, the system has functioned so about 6 months after installation. The years. Past data can be saved as dia- well that the average RH for the rest of manufacturer’s theory was that due to grams or generated from the back-up the year is now 53 ±2% at all parts of the unique chemical composition of files, which the landlord forwards the ship. Daily RH fluctuations at any wood in the Vasa, “unknown sub- weekly to museum staff as text files. sensor are reduced to around ±2%, stances” were saturating the sensor helped in 2008 by the introduction of membranes and distorting the readings. Initial Results and Lessons Learned more energy-efficient lighting in the Although air samples and samples of the museum. membrane were sent for analysis, the The new system came on line in May While preservation staff can view results were inconclusive, as the manu- 2004 and was trimmed in July 2004 data, any technical adjustments to the facturer was unwilling to reveal the full with visibly effective results (Fig. 10). system must be done by the landlord, chemical composition of the membrane. The almost 10% RH gradient over the and so it has been necessary — and This predicament has been solved by height of the ship was halved immedi- productive — to develop a smooth investment in a portable sensor with a ately, and by the end of summer 2004 working relationship with the techni- chemical purge function; that is, the the data curves appeared to be converg- cians. Regular meetings are held to sensor may be heated to 320˚F (160˚C) ing even more, averaging about 52 to discuss pertinent issues and to fine-tune for 2 minutes to burn off any built-up 55% RH. Since 2004, efforts have been the system to everyone’s benefit. A long- deposits on the membrane that might concentrated on fine-tuning the guid- term goal has been to be more proactive reduce its ability to absorb water ance parameters, both to improve en- than reactive in moderating the climate. molecules and reduce its sensitivity. This ergy efficiency and to achieve the best A sensor that measures carbon dioxide portable sensor is sent annually for balance of factors for the ship; for levels from visitor respiration has always calibration. Each spring before the peak example, adjusting the balance between been part of the climate-control system, tourist season, all sensors on the ship are the humidifiers, de-humidifiers, cooling but since September 2009 the visitor individually adjusted to match the newly units, and distribution fans. At first, counter at the museum entrance has calibrated portable sensor, which is sensors near the bottom of the ship, been incorporated into the control chemically purged each morning. The where freshly conditioned air emerges, parameters. It is now possible to get landlord then uses the same sensor to were chosen to guide the main plant; advance warning of potential large loads calibrate those in the plant. This solu- however, a better overall climate has and in theory prepare high-traffic areas, tion avoids the impractical and expen- now been achieved by changing two of such as the stern galleries, which may sive alternative of sending all sensors to these primary sensors for two halfway need to be dehumidified in advance. the manufacturer for annual calibration. up the hull, which are more sensitive to For all the sophistication and success ambient changes in museum climate. of the new system, however, traditional Costs and Environmental There is still a tendency in summer techniques of measuring RH are still Considerations months for the RH and temperature employed. Thanks to regular measure- bands to spread in reaction to higher ments by preservation staff using an The upgrade of this sophisticated sys- external temperatures and greater num- Assmann psychrometer (a type of tem has not been cheap. Installation bers of visitors, but these data still fall wet/dry bulb psychrometer), the newly costs are estimated to be about 50 within acceptable levels and are appre- installed RH sensors inside the ship were million SEK, with another 30 million 8 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 41:2-3, 2010

tourism to Stockholm helps to explain system, but since 2007 this effect ap- why this enormous financial outlay was pears to be stabilizing. approved by the state, which oversees Each year the landlord stages an the National Property Board. Costs overnight test of the back-up services by aside, the resulting stable climate has switching off the power supply to the exceeded the expectations of preserva- building. The reserves have so far re- tion staff, the technicians, and even the sponded automatically, with no de- consultants. That the height gradients tectable changes in the museum climate, and daily fluctuations in such a huge but this test is done when the museum is volume of air have been minimized is empty, with no respiring visitors to af- nothing short of remarkable. fect the climate. Two unplanned power One of the clear consequences of a cuts have also taken place in the last few more stable climate has been seen in the years during museum operating hours. salt-outbreak problem on the ship: since The reserves started up as expected, Fig. 9. On-board sensor and display box. Photo- 2004 no major new outbreaks have with little effect on the museum climate, graph by the author. been identified, and the pH of the exist- since these outages were not during peak ing precipitations has stabilized, strongly season. What is a concern, however, is SEK in consultant fees, etc. (a total of indicating that major moisture move- that should a power cut or system fail- US$8.8 million or 7 million euros). ment has been abated. This is clearly a ure happen in peak summer months Financial responsibility has been di- positive effect for Vasa’s wood chemistry when the museum is full of people, a vided; although the landlord paid for and, due to the size of the ship, likely stable climate cannot be guaranteed, the bulk of the installation costs, the the only practical measure that can be since the reserve systems can provide museum purchased the sensors and the taken in the short term to treat the only 40% of the main plant’s capacity. back-up generator and hired the consul- outbreaks. From a structural viewpoint, This is why, after five years of use, the tant, but it is now paying indirectly for the semi-annual geodetic measurement first major overhaul of the main plant, some of the landlord’s costs through of the hull form has not shown the large CA01, took place overnight in Novem- increased rent. seasonal movements exhibited with the ber 2009, historically the month with However, significant savings have older climate system, and less work is fewest visitors and when the load on the been made. The operating costs have required to adjust the wedges between system is at its lowest. actually been reduced, helped in large the hull and the support cradle. If struc- The popularity of the museum is a part by the landlord’s environmental- tural movement can be minimized, it mixed blessing. Although valuable policy goals to reduce greenhouse gases will assist greatly in the planning of a income is generated by more visitors, the and move to cleaner fuels. The oil- better support system. The ship contin- long queues outside the museum in peak burning furnaces have been replaced by ues to dry out, as can be seen from the tourist season have put pressure on the cheaper and cleaner electrical-powered weight change in the PEG-treated plank museum authorities to increase the heat pumps, and the three cooling units in Figure 5, probably due to the in- number of visitors allowed inside the are now cooled by water from the creased airflow established with the new museum at one time from 1,440 to nearby Stockholm harbor, thus eliminat- ing the need for the ozone-degrading substances in the earlier refrigeration units. According to the landlord, annual energy savings of 10-15%, or 400,000 kilowatt-hours, have been achieved.9 The system still draws in 30% fresh air, but only when it is needed for visitor comfort in the daytime, and so savings can be made after opening hours.

Conclusions The new climate-control system at the Vasa Museum has proven that a stable climate can be achieved in a huge volume of air — probably one of the largest internal public spaces in the world maintained at this close tolerance — but at a cost that most museums are unlikely to be able to fund. In this regard, Fig. 10. Temperature and RH readings during summer 2004, when the new climate-control system the role of the Vasa in attracting became operational. MAINTAINING A STABLE ENVIRONMENT 9

1,600. Since the air-conditioning system Acknowledgements 5. Yvonne Fors, “Sulphur-Related Conservation is dimensioned for 1,500 visitors, this Concerns for Marine Archaeological Wood” The author would like to thank Jacob Jacobson (PhD diss., Stockholm University, 2008), decision will require that the system be and Fred Hocker from the Vasa Museum’s 29–32. operated at maximum capacity for Preservation Unit for useful discussions and longer periods, with the risk that if there assistance with illustrations; project consultant 6. Birgitta Håfors, “The Climate of the Vasa Conny Lindqvist, Energo AB; and Ulf Bjaelrud Museum – Problems in Coordinating the is failure of any component of the main and Thomas Ericsson from the Swedish National Museum Object and the Museum Climate,” in plant, the carefully maintained stable Property Board for their insights and assistance Proceedings of the Third International Confer- climate cannot be guaranteed. The with this article. ence on the Technical Aspects of the Preserva- tion of Historic Vessels (San Francisco: Mar- difﬁcult decision of whether to close the itime Park Association, 1997), available online museum in order to preserve the climate Notes at http://www.maritime.org/conf/conf- hafors.htm. is unlikely to be taken, but failure to 1. Carl Olof Cederlund and Fred Hocker, Vasa take this decision might jeopardize the 1: The Archaeology of a Swedish Warship of 7. Conny Lindqvist, Vasamuseet Loggbok: careful and expensive attempts to pre- 1628 (Stockholm: National Maritime Museums Klimatprojektet 2002-2006, internal report to serve the ship. of Sweden, 2006). the Vasa Museum, Stockholm, 2006. 2. A full account of the conservation process 8. E. Hocker, L. Dal, and F. Hocker, “Under- EMMA HOCKER, conservator at the Vasa may be found in Birgitta Håfors, Conservation standing Vasa’s Salt Problem: Documenting the Museum in Stockholm since 2003, has a BSc of the Swedish Warship Vasa from 1628 Distribution of Salt Precipitations on the from the University of London in archaeologi- (Stockholm: The Vasa Museum, 2001). Swedish Warship Vasa,” in Proceedings of the cal conservation and an MS from Texas A&M 10th ICOM Group on Wet Organic Archaeo- 3. Jonas Ljungdahl, “Structure and Properties University in historic-building preservation. She logical Materials Conference (Amersfoort: of Vasa Oak” (licentiate thesis, Royal Institute has worked on conservation projects in the ICOM/RACM, 2009), 46–480. U.S., Bermuda, Denmark, and Turkey. She can of Technology, 2006), 2–3. 9. Personal communication with technicians be reached at [email protected]. 4. Ibid, 4–5. from the National Property Board, Aug. 2009.