Collection Care:Environmental Factors Part I: Temperature and Humidity Valinda S. Carroll
Agents of Deterioration High Temperature Humidity Light Pests and Fungi Handling Air Pollution
Environmental MonitoringDetermine current conditions for storageand exhibition, before investing inconservation treatment.It would be a poor investment to conservean artifact, only to return it to a damagingexhibit gallery or storage space. Amonitoring program allows an institution toset preservation priorities according thespecific needs of its collection.
Temperature and HumidityTraditionally, museum climate controlhas been based on human comfort, thecapacity of the heating and airconditioning system, and the limitationsof the building.
Temperature and HumiditySince 1994, published research has measureddamage induced by climate fluctuations. Insome instances, this research has resulted inwider “acceptable” climate ranges, adjustedfor local conditions.Collections housed in historic buildings maybe required to tolerate slight seasonal climatefluctuations in order to preserve the buildings.
High TemperatureHigh temperatures may Causes cross-linkingdamage artifacts in (makes materials becomeseveral ways brittle) Causes chain scission (makes materials weaker) Makes materials expand (causes splitting and delamination) Lowers relative humidity (causes shrinkage and splitting)
Temperature and HumidityRelative humidity is directly affected by temperature,so the two factors should be monitored together. Anincrease in temperature results in a decrease inrelative humidity, because warm air has the capacityto hold more water. As the temperature is reduced,relative humidity increases until it reaches 100%,and water condenses on surfaces. This temperatureis called the “dew point.” Dehumidification isimportant to avoid condensation. A relative humidityrange of 35% to 55% is safe for most types ofcollections, and allows moderate temperaturevariations.
High HumidityExcessive humidity is Accelerates chemicaldamaging to art and damagehistoric artifacts Promotes swelling, distortions, and delamination Encourages mold growth Increases insect activity (especially combined with high temperature) Warped floor tiles
Measurement Tools Handheld Recording Thermohygrometer Hygrothermograph Thermometer and Electronic Data loggers Psychrometer Temperature and Humidity Indicator Cards
Handheld ThermohygrometerAdvantages•Displays relative humidity•Displays temperature•Portable•Easy to use•Does not require additionalequipment or supplies•Inexpensive
Handheld ThermohygrometerDisadvantages•Does not record overtime•Does not store readings•May be inaccurate, anddifficult to calibrate•User must purchasebatteries
Aspirating Psychrometer •Compares wet bulb and dry bulb temperature to find dew point and relative humidity •Contains light to make thermometers easier to read
Aspirating Psychrometer Uses a fan to evaporate moisture from cloth on wet bulb Can assist in calibratingWet bulb other instruments More reliable and easier to use than sling psychrometer
Aspirating Psychrometer • Does not record over time • Requires slide rule or mathematical calculations • Does not generate documentation
Temperature and humidity guidelinesfrom the psychrometer manual: The temperature range of 70-80F degrees forlibraries and museums on this chart is aimed athuman comfort rather than collection care.
Recording hygrothermograph Advantages •Shows temperature •Shows humidityBlue humidityline
Recording hygrothermograph Advantages•Shows temperature•Shows humidity•Records over time: daily, monthly, orweekly•User may calibrate to improve accuracy•Some types run without batteries or Toggle switch for 1 day,electricity (wind-up clockwork 7 days, or 32 daysmechanism)•Easy to use•Paper chart provides long-term recordfor comparison over seasons or years•Continues to work during staff vacationsor even during disasters
Recording hygrothermographDisadvantages•May require regular winding or batteryreplacement•User must purchase paper charts andreplacement pens•Expensive•Paper charts require storage space•Red and blue ink lines may be faint and difficultto see•Cannot shift between Fahrenheit and Celsius,nor show dew point, etc.•Cannot be included in computer-generatedreports and presentations
Data loggerAdvantages: Records over time Small enough to fit into exhibit case or packing crate Data can be loaded into standard spreadsheet software to generate reports May be less expensive than hygrothermograph
Data loggerDisadvantages Requires batteries Inexpensive models cannot be read without computer Inexpensive models must be downloaded individually, by hand
Humidity CardAdvantages • Small enough to fit into frame, exhibit case, or shipping crate •Inexpensive unit cost •Does not require batteries •Does not require computer or slide rule
Humidity CardDisadvantages • Imprecise •Not durable •Does not record over time •Cobalt blue salt is toxic (orange humidity cards are non-toxic)
Temperature CardAdvantages•Inexpensive•Easy to use•Portable•Small enough to fit into frame,exhibit case, or shipping crate
Temperature CardDisadvantages•Imprecise•Does not record over time•Does not generate permanentdocumentation•Not durable
End of SectionPlease proceed to Part II: Light