Museum Environmental Conditions: A Sustainable Range?

Museum Environmental Conditions: A Sustainable Range? Stephen Hackney

Sustaining Collections Care The debate on the need for energy constraint by museums has been broadly welcomed by many conservation professionals from major UK and International Museums. This is to be expected since sustainability is central to any long-term conservation policy and current fossil fuel usage is recognised as unsustainable. A major energy cost for museums is to control humidity levels within galleries and stores. The need to protect museum and art objects from extremes and from rapid changes in relative humidity (RH) and occasionally temperature is well established and supported by research and experience, but not widely understood in detail. The boundaries of the safe range for each category of materials are broadly known. The current tightest standard for international loans of 50±5% RH and 20±2 C (or for some UK museums 55±5% RH) is based on the susceptibility of the most vulnerable types of objects in a collection, e.g. panel paintings, ivory, musical instruments and lacquerware.

Sustaining Collections Care In an age of relatively cheap energy, this target has been achieved largely by reliance on sophisticated air conditioning systems (HVACs), a practice that has been favoured for some time. However, experience has shown that such tight specifications are technically difficult to maintain on a continuous basis and have been challenging for most museums to meet at all times and in all spaces. The need to minimise risks of damage to borrowed objects whilst travelling and on loan and to preserve the museum’s reputation has meant that there has been a reluctance to challenge the use of the narrowest set of guidelines on an international level. As a consequence, large sums of money have been invested in HVAC systems, in part because they also efficiently provide comfortable temperatures for visitors (and staff). Discussion with engineers on the performance of air-conditioning systems is conducted through the medium of the relative humidity of the air, which is easily measured but is already one step removed from a conservator’s concern, which is for the moisture content of the materials in each object.

The museum environment The Gallery environment is controlled to preserve the moisture content of the collection, particularly objects made of wood, animal glue and bone, but also stretched canvas, paper and textiles In winter (to be precise, on cold days), buildings are heated for human comfort and this frequently generates dry conditions that can lead to materials cracking In summer, when the heating is off, the air humidity may be high, which causes hygroscopic materials to swell significantly and objects to distort. In extreme circumstances mould can also develop Extremes must obviously be avoided, but also, between the extremes, the conditions should be kept as stable as possible. Air-conditioning can provide this stability The National Trust has also demonstrated that historic buildings and their collections can be kept at relatively constant moisture content by allowing the temperature to fluctuate

The National Trust approach is potentially more energy efficient and involves less investment in equipment and interference with historic buildings, which of course is fundamental to their brief. It is fortunate that National Trust houses are closed in the winter, removing a major conflict between the requirements for visitors and for collections However, for museums that stay open all year round, and may well be housed in modern buildings, air conditioning has been seen as the best option For museums, localised humidification of specific exhibition galleries in winter is an alternative strategy. But for most museums their collections are spread across their entire site, including storage, which requires site-wide application of humidification. Humidity control has largely been seen as a solution to immediate physical factors and until recently insufficient concern has been given to long-term chemical degradation of objects. Another major benefit of centralised air-conditioning systems (HVACs) is that they can filter much dirt and gaseous pollution from galleries, in the past removing sulphur dioxide and more recently nitrogen oxides emitted from vehicle exhausts, particularly in cities.

Tension in a stretched canvas - rapid change in RHTheoretical evidence to support the susceptibility of materials to moisture change has been accumulated mainly over the last 30 years. Here, we see that a sudden drop in humidity (green line) causes a rise in tension (yellow line) in a stretched canvas at constant temperature (red line)

The conservation needs • The behavioural response of wood to humidity change has always been known by the craftsmen who work with it, but it became an issue in museums during the nineteenth century when expanding collections were displayed in large public buildings heated by coal-fired boilers with piped hot-water systems. • The National Gallery was able to prove that its collection was better preserved in the (unheated and dehumidified) quarries of North Wales than at its London site. • On the question of installing air-conditioning, Thomson noted that there was insufficient information to specify tolerances for RH and argued that the National Gallery should therefore build the best system possible at the time, which was plus or minus 5 percent. • He recommended the mid-point of 55% RH in part since that was the average humidity in London over the year and therefore most easily (and economically) maintained • Others have adopted these unintended ‘standards’ despite Thomson’s observation that “there is something inelegant in the mass of energy-consuming machinery needed at present to maintain constant RH ... Something inappropriate in an expense which is beyond most of the world’s museums”

The conservation needs • Inelegant it may be but it has worked very well in eliminating events that cause obvious damage • 55% RH was also adopted in BS5454 as the recommended mid-point for archive and libraries. A relatively high humidity helps maintain flexibility of documents, reducing handling damage, but high RH increases the rate of chemical degradation of cellulose by hydrolysis and has no benefits for the majority of unused stored objects • For this reason BS5454 is currently being revised to reduce the moisture content of stored archives. • This raises the first question, is 55% the best mid-point for our collections and what happens if we lend an object to a US museum that takes 50% RH as its mid-point? • More questions follow. How well can we measure RH? Is our RH meter properly calibrated? Is our air-conditioning system being operated well? What happens when it breaks down? • What about mixed collection? Metals may corrode at 55% but remain stable at low RH.

An acceptable range? • Air conditioning systems have been adopted in many countries; particularly those with extreme climates. To attract loans galleries have set high standards • As a result, we no longer see damage to a collection that can be attributed to humidity, except in the areas that are not conditioned or when systems have failed • Should we therefore widen tight specifications until we start seeing damage? How would we assess any damage? • What we will not easily see is the subtle long term deterioration, physical, chemical and biological, that accumulates over years • Does it matter? What are our ultimate aims? • Can extreme thresholds be better defined? I will attempt to answer this one.

Quantity of water in the air Understanding relative humidity At low temperatures there is very little moisture in the air, even at high relative humidity At high temperatures there is much more moisture for the same relative humidity

Moisture content of materials At any specific temperature there is a relation between equilibrium moisture content and relative humidity. At high RH wood can absorb much more moisture. At the low end and middle range of RH the response is close to linear.

The Mechanical Properties of Materials The mechanical properties of some museum materials have been measured. Stress strain graphs are used to find the elastic modulus of a material For comparison, steel showing typical elastic and plastic regions (left) and “Naples yellow” measured by Mecklenburg showing elastic and some plastic behaviour of pigment at 50% RH (right) These are externally applied forces but we can use the response to understand the behaviour of materials to changing moisture content.

Lower Humidity Threshold People are fairly insensitive to low humidity and buildings in the UK are not humidified for human comfort. It has long been established that brittleness can lead to disfiguring cracking in hygroscopic or mixed material objects and mechanical studies have quantified this to some extent. Materials contract at low humidity and have increased elastic modulus. In other words, they may become brittle and stressed, possibly beyond their breaking load. Wood is the obvious example but, paintings on canvas also contain a glue-size layer that becomes very brittle (and strong) at low humidity. Wooden panel paintings may also contain glue size. At what humidity does cracking occur: 40%RH, 30%RH, 20% RH, 10% RH? Does it require only one dry event for an object to be irrevocably cracked? For an object that is already cracked, will further cracking occur?

Research by Mecklenburg, Michalski, Hedley, Young and many others has improved our understanding of the mechanical behaviour of materials, particularly paintings. This work has all been carried out since Thomson’s recommendations Best information based on the behaviour of new wood suggests that cracking and warping of wood is not likely above 30% RH. However, failure is likely if the wood is constrained by inflexible joinery or rigid material that creates stress concentrations. Cradles are well known to cause painting panels to crack. Paint can also constrain wood. Strains induced by moisture change in the hygroscopic material (e.g. wood, glue and canvas) may lead to cracks in less responsive materials A dried oil paint film is hygroscopic, but much less responsive than wood. At low RH an old oil film is more brittle than it is at 50% RH. At very low temperatures most materials are more brittle too Materials become more brittle with age.

These are the factors that lead to museum objects cracking and we can immediately perceive that there is a need for detailed information on individual objects. In addition, shear forces generated between layers of different materials such as between an oil priming and a glue/canvas layer may cause cupping and flaking of paint. Although the principles are well understood, direct correlation between flaking paint and low humidity is not good. This is because there is a delayed response to air humidity change between the different components of the object and the actual moisture content of the materials is not known. Cracking may occur some time after exposure to an extreme environment and it will not be immediately visible, until it widens or absorbs dirt. Even with an object in a brittle condition, cracking may be initiated by an impact it. In normal examination practice we cannot discriminate between the various factors An oil painting on stretched canvas tends to crack after 100 years (depending on its history).

This is partly due to embrittlement from cross-linking and oxidation of the paint but also the effect of relaxation of the canvas. Low humidity may be the final straw that leads to cracking. Consolidating furniture with water based glue may reform a crazed original glue introducing new tensions Conversely it has been argued that objects that have been kept in varying conditions may be damaged but when consolidated with adhesive may be proofed against further damage Conclusion: A lower limit of RH of 30% is safe for simple new structures in very good condition. However, museum objects may already have incipient cracking and weak points that make them more sensitive than new wood. Chemical changes in wood can also make it more brittle. These are the objects that will be at risk. Real works of art may fail because of inherent constraints, flaws and poor adhesion. Keeping the RH above 40% is advisable.

High humidity threshold The presence of liquid water from a leaking roof or rising damp in a wall is an obvious danger. Condensation on cold, poorly insulated, walls is a frequent problem in cold climates. If the temperature of a surface falls, the air may reach saturation. At very high humidity there may be mould damage. To develop, mould requires free moisture. When there is no free water, at high but constant RH, mould will survive but will not spread. An object at, say, 16.5oC brought into contact with air at 20oC and 80% will experience condensation at its surface. In the tropics, although the RH is often very high, the walls of buildings are frequently warmer than the air inside. Condensation therefore does not occur so readily. Many paintings survive in non-conditioned buildings in the tropics without serious mould damage. For enclosed objects, high humidity events may be triggered by high temperatures. A large wooden object enclosed and equilibrated with air at, say, 15oC and 80% if heated by sunlight may temporarily saturate the air around it. Mould requires time to grow and should not therefore be a problem in a well maintained museum.

There are examples of paintings that have been kept in castles and churches at a constant 70% RH and constant low temperatures for years, remaining in relatively good condition. At high RH, materials swell much more through moisture absorption, leading to significant changes in shape and to local internal stress when constrained. Materials also become more flexible and therefore more capable of absorbing strains. Cracking is less likely if all the materials are in equilibrium. Blanching in paint films occurs when the medium has swelled so much with water vapour or free moisture that the bond between the pigment and medium breaks, allowing light to be scattered at the interface. This is likely to occur only at very high RH, for instance, in the conditions described for mould growth. Research has shown that stretched linen canvas loses tension as the RH increases until it reaches 75-80% RH. Beyond this point the woven canvas becomes tighter again. This is an inevitable feature of its woven nature. At 80% RH animal glue becomes very swollen and loses its cohesion altogether. Objects containing animal glue should therefore definitely not be taken above 80% RH.

Chemical degradation from hydrolysis (including much light fading) is increased by the presence of water. It causes the deterioration by depolymerisation of cellulose in canvas, paper and wood as well as the polymer in some adhesives. It is also implicated in the degradation of paint films to cause metal soaps, protrusions, blanching and embrittled paint films. There is no RH threshold for chemical damage and it does not usually lead to sudden failure, and for these reasons it is not given enough attention Hydrolysis rates are increased dramatically by acidity. Metals are present in many objects and sensitive to corrosion at high RH Conclusions: Paintings should be kept below 70% RH to avoid physical and biological risks. This can only be reliably achieved at constant temperature. Chemical degradation from hydrolysis and corrosion will be significantly worse at this RH (compared with lower RH). A collection containing mixed objects should have a general level less than 60% RH perhaps with objects that are sensitive to corrosion kept in conditioned enclosures

Temperature The temperature within a museum or historic house is related to human comfort rather than conservation concerns. Temperature ranges are frequently specified, for instance in loan documents. This is mainly to prevent overheating of materials, but low temperature constraints should be expressed more carefully to reflect the needs of the object rather than assumptions about building comfort levels. At 17C the chemical reactions that degrade works of art operate approximately half as fast as at 22C. At 27C they are twice as fast again, doubling approximately for every 5 degrees Celsius, depending on the material. Low temperatures are therefore more desirable from a chemical point of view. 10C is a safe lower limit in most cases, provided the moisture content remains stable Museum directors should not be afraid of lowering temperatures for conservation reasons Below 10C some materials may become brittle or glassy, for instance, surprisingly an acrylic paint might crack around zero C. Except in winter transit this is not likely to be a problem

Galleries can be zoned by performanceTate Britain is the most problematic of the Tate sites with a wide range of conditions: 3 major air-conditioning systems and many uncontrolled areas.Only the centenary development (dark blue areas) provides reliable international standardsThe collection is displayed across the sites apparently seamlessly but in fact there are constraints on where works can be displayed. Major loans exhibitions are displayed in the Linbury galleries By contrast, the Duveen galleries are only used for the most RH/T insensitive works

Microenvironments • Because of Tate Britain’s complexity and Tate’s frequent display changes and loans most individual works in the collection will be exposed to a range of conditions some outside tight parameters. • To counteract this, for many years we have had a policy of protecting works of art by establishing a microclimate around each one. • At its simplest this involves putting paintings behind glass (with backboards) and sculpture or other objects in display cases or vitrines. • This has been shown to be extremely successful, even in unconditioned areas • Controlling the moisture content of objects in a microenvironment is clearly very much more efficient than controlling the air humidity in a large and sometimes leaky gallery • This approach has allowed us to hang glazed panel paintings in the unconditioned original galleries. Of course, not all objects can be enclosed.

Blue line, RH inside (behind) a glazed framed painting on display in gallery 9 over 6 months compared green line, with gallery RH, showing stability provided by frame, backboard and glazing. Also reverse exponential adjustments to equilibrium with two ambient levels of gallery. Frame must have been dry when fitted and gallery RH altered in November

Broader parameters have been put into practice for air-conditioning • Relative Humidity (RH) mid-points have been broadened to the range 45 to 60% allowing a 2.5 percentage point change per month. • Adding +/- 5% variation, the RH level maintains the object between 40% and 65% throughout the year. • Similarly, the midpoint temperature (T) has been allowed to vary annually between 20C and 23C, changing 2/3C every month, with +/- 2C variation maintaining the object between 18C and 25C. • By changing the mid-points of the air-conditioning systems to be closer to ambient conditions throughout the year the heating and moisture control costs will be reduced. • The new parameters have been applied to all collection conditioned spaces except loan-in exhibitions areas. • It has been assumed that materials will not be affected by long term (annual) cycling of RH

Benefits and Cost savings The Psychrometric chart helps us to see how energy costs can be reduced. Here we see the humidification cycle of external air at 60% and 3C brought into a museum and raised to 50% and 20C The wider annual variation should bring the midpoint closer to exterior conditions and therefore reduce air treatment costs. The wider specifications are being tested in practice to discover how much savings can be achieved. Day-to-day operation of the system remains the same and staff remain committed to gaining the most from their Building Management System (BMS) controls. The fabric of the buildings is less exposed to condensation in walls.

Some further ways of achieving savings unconnected with revision of existing conditions Leakage of air from buildings or from a conditioned area into an unconditioned one is wasteful and could be improved at Tate Britain and Tate Store, although with some interference with circulation Reducing the fresh air component in under-occupied conditioned galleries or stores by the use of carbon dioxide sensors might also bring savings Slowing air flow also brings benefits to the collection by reducing air exchange with the objects There is the possibility of further lowering of winter temperatures at Tate Store below 16C A new underground external store has been occupied. Stable humidity and very stable low temperatures (14C) can be achieved throughout the year.

Some consequences We need to move the collection from site to site and within each site. The collection is therefore already exposed to broader conditions than the ideal There is uncertainty in predicting the effect on the collection of a broader range of gallery conditions and new research will be necessary in key areas, in particular, into the rate of response of objects and the relaxation of stress The current policy of preventing rapid changes of conditions by, wherever possible, enclosing the object in a frame or vitrine should be continued. Objects protected in this way will not respond to most short-term changes Where protection is impossible or undesirable for display (or interpretation) and the object is in a category known to be particularly susceptible to changes in RH or T, some compromise on display or special facilities will be required With broader parameters it will be more critical to reassure lenders that we remain within spec. This will require more monitoring and better presentation

Loans agreements These energy savings have not been applied to Tate loan exhibition spaces. To extend it to exhibitions would require the agreement of a wider group of museums and owners who lend to and borrow on a regular basis Air conditioning has led to the development of international RH/T expectations. In many countries, air conditioning is essential: e.g. USA, Canada, Russia, Eastern Europe, Japan and tropical countries These informal ‘standards’ are a point of contact, creating consensus and trust, and supporting international exhibitions and loans As a condition for loan, some lenders demand very tight RH/T conditions and galleries seek to satisfy lenders. In part this is because sometimes fragility may be given as the only reason to refuse a loan of a valuable object, whereas the truth may be more complicated

Strategy for agreement on a loans policy • Energy saving proposals will be received positively if they satisfy the concerns of each stakeholder: exhibition curator, conservator, architect, engineer, facilities operator. • This involves establishing a debate between the parties and the sharing of responsibility for all aspects of the problem. • An interim agreement has been made between many UK national museums. • The agreement needs to have an international consensus • The Museum Directors’ meeting and Bizot group have promoted the agenda • Various conservation meetings including the London IIC meeting 2008, ICOM Delhi 2008, AIC Milwaukee 2010, Boston 2010 have discussed these issues • The outcome is likely to involve compromises for all parties: a small increased risk to the collection, some display restrictions, architectural constraints and the reviewing some gallery activities

The next challenges Collections care is now expressed in a form that does not assume air-conditioning: 40-60% RH and 18-25oC Temperature The interim guidelines are compatible with National Trust conservation heating controls which can be applied to most traditional buildings. There is now leeway for imaginative solutions for future new buildings and operations Improved thermal insulation and more thermal mass tends to stabilise the climate within a building and should be incorporated into all new building Reducing solar gain and skylight heat loss in a building can also bring savings We are fortunate that some existing museum buildings are the right kind of construction

Gaps in knowledge and research needs Research is required on the rate of moisture exchange between the air and materials in collections The rates of stress relaxation for each material need to be investigated A better definition of a higher temperature threshold is needed Better modelling of the complex interplay between layers and components of museum objects would be useful If vitrines and enclosures are to be used more in future, better materials that do not off-gas will need to be used in their construction. Architects and engineers need to develop imaginative new buildings that deliver over the long term and that are not energy hungry

Summary The UK is fortunate that it does not have an extreme climate, although one model suggests that global warming will lead to warmer summers and cooler winters. The greatest gains can be made by reducing temperatures in winter. This reduces heating load and humidification requirements for the collection It may also put constraints on exhibition scheduling, visitor comfort, exhibition interpretation or architectural design We must remember that countries with extreme weather have less to gain, since they will need to continue to use central air conditioning systems e.g. Canada, Russia, continental climates, tropical countries, monsoon or desert regions. Those countries that continue to need air conditioning will require us to prove that our energy saving proposals are acceptable before they lend us works of art.

The use of microenvironments, wherever possible, can achieve results sometimes better than air-conditioning. It is important to think in terms of moisture content and not relative humidity. Conservators accept that their concerns need to take into account sustainability and that gallery conditions should not be so tight as to restrict imaginative solutions In the last 30 years enough experience has been gained of museum environmental control to enable conservators to be confident with adequate conditions rather than the very best for all occasions If an international loans agreement following the interim guidelines can be achieved confidence can be preserved removing the concern that exhibitions may not receive important loans When it comes to compromise between the needs of visitors, interpreters, architects and conservators, value judgments are required. The outcome should therefore be based on a wider debate that includes the public and all stakeholders. THE END

Museum Environmental Conditions: A Sustainable Range?