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