Conservators who research preservation environments urge those of us caring for cultural heritage collections to develop a more sophisticated set of target ranges for both temperature and humidity. The simple guideline of 70 degrees Fahrenheit and 50% relative humidity does not reflect optimal preservation conditions. The ideal depends upon the collections materials; for instance, a lower RH of 35-45% is best for metals storage. Where human comfort is less of a factor and when RH can be controlled, colder is usually better. As a further complication, experts are now thinking in terms of acceptable ranges of RH, with the understanding that allowances for seasonal fluctuations are best for a more holistic, institutional and environmental approach.
Even with these more complex and contingent ideas about collections storage environments, RH levels can readily creep into the danger zone during a North Carolina summer. Once the RH level tops 65%, mold will grow. The cobalt humidity indicator card that sits on my office bookshelf serves as a reminder of this threat. This is the type of inexpensive humidity indicator that we give out at our collections care workshops, and although there are many other higher tech environmental monitors on the market, these strips are great for both micro-environments and low-budget institutions. Today, as the outdoor temperature tops 100 degrees, the lavender circle is at the 60% mark.
If your institution’s climate control system is operating at maximum capacity and still cannot maintain an RH at around 50%, then you need to take extra precautions during this season of high humidity levels.
1. Inspect exhibit and storage areas: of course, you’re doing this already, but in summer you may need to walk through twice a week to watch for the tell-tale fuzzy white growths on wood and brown splotches on paper.
2. Install fans: Air circulation is the best thing you can do to prevent mold spores from settling on artifacts.
3. Light storage areas strategically: Natural light is a good deterrent for mold growth but has damaging effects, especially for paper and textiles. For some types of materials, light can be part of the solution; even periodic use of artificial light can help.
3. Remove dust covers from furniture: Although collections care guidelines recommend these, they will inhibit your inspections and give mold dark, still arenas to colonize. If you must use covers, tyvek is preferable in that it will not absorb moisture from the air.
4. Consider adding silica gel to archival storage boxes and exhibit cases. Because boxes are porous, you will need quite a bit of silica gel to dessicate.
5. Look into equipping your space with a dehumidifier to use seasonally: Make sure you have the facility and/or the staff to drain the collected water as needed.
6. When possible, isolate moldy objects from the rest of the collection. Freezing is an option to kill the mold. Vacuuming in the open air, preferably with a hepa-filter vacuum is an effective treatment, but unless RH levels can be controlled, mold will re-grow.
Keep cool and fungus free!