Post on 18-May-2015
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Collection Care:Environmental Factors
Part I: Temperature and HumidityValinda S. Carroll
Agents of Deterioration
High Temperature
Humidity Light Pests and Fungi Handling Air Pollution
Environmental Monitoring
Determine current conditions for storage and exhibition, before investing in conservation treatment.
It would be a poor investment to conserve an artifact, only to return it to a damaging exhibit gallery or storage space. A monitoring program allows an institution to set preservation priorities according the specific needs of its collection.
Temperature and Humidity
Traditionally, museum climate control has been based on human comfort, the capacity of the heating and air conditioning system, and the limitations of the building.
Temperature and Humidity
Since 1994, published research has measured damage induced by climate fluctuations. In some instances, this research has resulted in wider “acceptable” climate ranges, adjusted for local conditions.
Collections housed in historic buildings may be required to tolerate slight seasonal climate fluctuations in order to preserve the buildings.
High Temperature
High temperatures may damage artifacts in several ways
Causes cross-linking (makes materials become brittle)
Causes chain scission (makes materials weaker)
Makes materials expand (causes splitting and delamination)
Lowers relative humidity (causes shrinkage and splitting)
Temperature and Humidity
Relative humidity is directly affected by temperature, so the two factors should be monitored together. An increase in temperature results in a decrease in relative humidity, because warm air has the capacity to hold more water. As the temperature is reduced, relative humidity increases until it reaches 100%, and water condenses on surfaces. This temperature is called the “dew point.” Dehumidification is important to avoid condensation. A relative humidity range of 35% to 55% is safe for most types of collections, and allows moderate temperature variations.
High Humidity
Excessive humidity is damaging to art and historic artifacts
Accelerates chemical damage
Promotes swelling, distortions, and delamination
Encourages mold growth Increases insect activity
(especially combined with high temperature)
Warped floor tiles
Measurement Tools
Handheld Thermohygrometer
Thermometer and Psychrometer
Recording Hygrothermograph
Electronic Data loggers Temperature and
Humidity Indicator Cards
Advantages
• Displays relative humidity• Displays temperature• Portable• Easy to use• Does not require
additional equipment or supplies
• Inexpensive
HandheldThermohygrometer
Disadvantages
• Does not record over time
• Does not store readings
• May be inaccurate, and difficult to calibrate
• User must purchase batteries
HandheldThermohygrometer
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 calibrating other instruments
More reliable and easier to use than sling psychrometer
Wet bulb
Aspirating Psychrometer
• Does not record over time
• Requires slide rule or mathematical calculations
• Does not generate documentation
Temperature and humidity guidelines from the psychrometer manual: The temperature range of 70-80F degrees for libraries and museums on this chart is aimed at human comfort rather than collection care.
Recording Hygrothermograph
Recording hygrothermographAdvantages• Shows temperature
Red temperature line
Recording hygrothermographAdvantages• Shows temperature• Shows humidity
Blue humidity line
Recording hygrothermographAdvantages
• Shows temperature• Shows humidity• Records over time: daily, monthly, or
weekly• User may calibrate to improve accuracy• Some types run without batteries or
electricity (wind-up clockwork mechanism)
• Easy to use• Paper chart provides long-term record
for comparison over seasons or years• Continues to work during staff vacations
or even during disasters
Toggle switch for 1 day, 7 days, or 32 days
Recording hygrothermographDisadvantages
• May require regular winding or battery replacement
• User must purchase paper charts and replacement pens
• Expensive• Paper charts require storage space• Red and blue ink lines may be faint and difficult
to see• Cannot shift between Fahrenheit and Celsius,
nor show dew point, etc.• Cannot be included in computer-generated
reports and presentations
Data logger
Advantages: 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 logger
Requires batteries Inexpensive models
cannot be read without computer
Inexpensive models must be downloaded individually, by hand
Disadvantages
Advantages
• 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 Card
Disadvantages
• Imprecise
• Not durable
• Does not record over time
• Cobalt blue salt is toxic (orange humidity cards are non-toxic)
Humidity Card
Advantages
• Inexpensive
• Easy to use
• Portable
• Small enough to fit into frame, exhibit case, or shipping crate
Temperature Card
Disadvantages
Temperature Card
• Imprecise
• Does not record over time
• Does not generate permanent documentation
• Not durable
End of Section
Please proceed to Part II: Light