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 humid- ity (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 build- ing, 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 there- fore 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 3 4 APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 41:2-3, 2010 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.