If you are reading this section on humidity as a threat to collections, you must also read the section on temperature, because they are intimately linked. Warm air can hold more humidity than cold air, therefore changes in temperature cause changes in humidity. Often, it is the change in humidity that has the greater impact on collections preservation, but most Alaskan institutions do not have dedicated humidity control as part of their building equipment. Typically, Alaskan museums suffer high humidity in the summer and low humidity in the winter, as seen in the graph in figure 1.
For a long time, the climate ideal for museums was 50% relative humidity (RH) and 70°F with very little fluctuation. Those targets were trying to replicate conditions of a slate quarry in the UK where valuable collections of the British Museum were kept successfully during WWII, and the allowable fluctuations were based on the limitations of the HVAC equipment at the time. This has been problematic because it costs money and fossil fuels to achieve that target, and many buildings simply cannot achieve it. This is especially the case in our challenging Alaskan climate with facilities often very far away from HVAC repair specialists. In addition, there have not been enough scientific studies on real materials to prove this is the correct narrow target, in part due to the huge number of variables involved. The more recent thinking about museum climate involves an understanding that deterioration happens in different ways (biological, chemical, mechanical) and that you have to balance the risks and benefits for each one. Very low RH slows down biological and chemical risks, but increases risk of mechanical damage, for example. The beginnings of a shift in our conception of appropriate collections preservation climate began in the mid-1990s, but it has taken longer to reach the dawning of a new era. It is helpful to consider the three ways humidity causes damage:
Some people will say that RH is more important than temperature, and others will say just the opposite. It seems like the answer depends on what kind of damage you want to prevent. If it is biological (mold, pests, bacteria etc), you need to keep your RH low. There is an easy-to-remember rule of thumb: mold tends to flourish if you have stagnant air in a dark place over 70% RH and 70°F for at least two days. Insects and other pests need moisture to live, and will reproduce faster at higher humidity. Chemical usually refers to rate of deterioration, such as things becoming more acidic or corrosion happening. Many chemical reactions involve water that can be provided from the air, so many deterioration processes are made worse by high humidity. Mechanical damage is strongly connected to humidity from movement caused by shrinking and swelling as moisture content goes up and down. For this kind of damage, “proofed fluctuations” are part of the professional literature. Since the 1990s, it has been recognized that most things in museums have routinely experienced conditions between 30% and 60% RH before they came into our collection and have coped with that stress already. There are exceptions, such as repaired items and newly made artworks. Most museums that do not have equipment specifically designed to humidify and dehumidify spaces try to keep humidity within the 30-60% range. This is because below 30% risks damage from shrinkage, cracking, and embrittlement while above 60% risks mold. Let’s think about those extremes more specifically…
Many Alaskan museums experience very low humidity for limited periods in the middle of winter. This can cause damage from embrittlement, shrinkage, and cracking. Examples include cracking tusks and teeth (figure 2) and splitting skins on drums or kayaks (figure 3). In both these cases, there are different physical materials in the same object that react to humidity differently. Teeth and tusks, for example, contain both organic components that absorb and release moisture and inorganic minerals that do not. The stress from the organic material shrinking and swelling can cause cracks. Objects made of different materials that are fixed together, such as drums and kayaks, also suffer from these forces at extremes of humidity. Furniture joins and veneers are another example.
In the 1980s, it became popular to place a small cup of water inside a display enclosure to increase humidity. While this seems like a good idea, in practice it is risky because the act of frequently moving and refilling the cup may accidentally dump water and the fluctuations from the cup drying out could be worse than the small amount of good it may do. Without monitoring, it is doubtful the cup is elevating the enclosure to an appropriate degree. If it is dampening the interior too much, there is risk of mold. If used at all today, this practice is occasionally seen in gift shops, and unfortunately with a poor idea of the intent. Sometimes it is even placed with completely inappropriate artworks, such as small metal sculptures that could suffer corrosion. Using portable humidifiers in a collections space involves similar challenges. Without a device to monitor the humidity level, portable humidifiers can cause more harm than good. The old adage “you cannot manage what you cannot measure” applies here. See the section on temperature for descriptions of devices that monitor temperature and humidity.
High relative humidity can cause a number of problems, including mold, corrosion, swelling, distortion, and sticky surfaces. Mold is perhaps the most common concern with high humidity. Mold spores are everywhere at all times as a normal part of indoor and outdoor environment, but they are usually dormant and harmless. Alaska has hundreds of different kinds of molds, but the vast majority are not active below 70% RH at room temperature. A good rule of thumb that humidity is above 70% and the temperature above 70°F more than two days is very likely to cause mold. Our “proofed fluctuation” metric of 30-60% takes into account the idea of microclimates throughout a space. If the datalogger in the middle of a room reads 60%, there are likely pockets of dead air and higher humidity in localized areas like corners and low shelves along exterior walls (figure 4). Mold often has a white fluffy look (figure 5) but there are several other things that might also look like mold, such as salts, fatty spew, insect debris, and pesticides. To help understand the differences between the various white substances that form on artifacts, see the website “What’s That White Stuff? Caring for Alaskan Artifacts.”
High humidity can be addressed with the temporary use of a portable dehumidifier. These devices collect water from the air and store it in a reservoir that must be emptied regularly or connected via hose to a drain. It is not uncommon for a high humidity situation to be made worse by an overflowing dehumidifier. Vigilant emptying of the accumulating water is crucial. Likewise, the effort is questionable without a way to monitor the environment and record the actual relative humidity to identify when the danger has passed or if the dehumidifier is helping at all. Another option used to control high humidity is silica gel. These crystals of water-absorbing material are sometimes found in little packets shipped with medications or electronics. Unfortunately, silica gel is often used incorrectly. The proper amount to use is based on volume of space to be dehumidified and how damp the environment is to begin with. Once the crystals have absorbed a specific amount of moisture, they no longer work. They must be in an enclosed space with a limited volume of air, or else they will become saturated very quickly. Silica gel is typically colorless and has no visual cue that it is no longer working unless some (more-expensive) color indicator silica gel is mixed in (figure 6). This kind of silica gel is blue when ready and pink when it is no longer working. An enclosure like an exhibit case can be conditioned to a safe humidity with silica gel and the use of some indicator crystals and a humidity card can identify the point at which the silica gel needs to be reconditioned. Baking in an oven can drive off the moisture and make it ready to use again. However, in practice this is very labor intensive and requires diligent monitoring that many museums cannot provide.
One example of a material that can have trouble at both low and high RH is glass. Sometimes early recipes for glass manufacture were incorrect, and the resulting glass had problems from the beginning. If a material has deterioration problems as part of its very nature, the term “inherent vice” is used. In Alaskan collections, we most frequently see this kind of problematic glass as beads. At high humidity, moisture causes alkaline components to leech out to the surface and make the glass slick and moist-looking. This is called “weeping” glass. At low humidity, the glass is dehydrated and can crack, causing “crizzling.” The beads we most often see with this problem tend to be larger beads, around the size of a cherry pit, and in our collection the cracked and sugary-looking surfaces of crizzling are more common than weeping (figures 7-9).
Many architects and engineers have little experience with museum environments. Controlling indoor humidity is challenging in the Alaskan environment. Two resources that help museums and building contractors talk about humidity are the Image Permanence Institute and the ASHRAE manual. The Image Permanence Institute (a non-profit research lab at the Rochester Institute of technology) has done amazing work (much of it grant funded by the NEH, the Mellon Foundation and IMLS) to quantify the risk to various kinds of collections from different temperature and RH conditions. They developed the widely-used PEM2 datalogger, and its online software called eclimtenotebook. If you are an Alaskan institution, the Alaska State Museum has a few PEM2 dataloggers we loan out to help get a grip on your preservation environment. Data is uploaded to the eclimatenotebook website, which has both free and subscription-level resources for making graphs and understanding the data. ASHRAE stands for the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. They have been a national standard for building systems for 125 years. Chapter 24 of the 2019 edition is for Museums, Galleries, Archives, and Libraries. Skimming this chapter can help museums understand what engineers are talking about, and coming to the conversation with quantitative data from a preservation source like IPI can help engineers understand what museums need.
Questions? Contact Us! The Alaska State Museum has an outreach mandate to help provide advice and expertise to museum professionals and other caretakers of Alaskan material culture.
Email Ellen at ellen.carrlee@alaska.gov or fill out our online contact form.
