Preservation Metrics

In the mid-1990’s IPI began work on computer modeling and the development of algorithms to measure the effects of the storage environment on collections. This research led to the introduction of the general quantitative model of chemical decay in organic materials called the time-weighted preservation index (TWPI), published in New Tools for Preservation: Assessing Long-Term Environmental Effects on Library and Archives Collections. Based on this concept, IPI developed the Preservation Environment Monitor® datalogger and Climate Notebook® software. During field trials of these tools, IPI developed additional metrics for environmentally induced mechanical damage, biological decay, and metal corrosion. These metrics were incorporated into the analysis performed by Climate Notebook and the eClimateNotebook.com website.

IPI’s Preservation Metrics® are described briefly below. Environments are rated as Good, OK, or Risk based on evaluation of temperature and relative humidity using the metrics. For a four-page overview on the metrics, download our handoout (PDF). For a more in-depth explanation, download Understanding Preservation Metrics (PDF).
 

Type of Decay: Chemical Change

Metrics Used:
Time Weighted Preservation Index (TWPI)

Interpretation:
Higher the TWPI, the better

TWPI Value (years)
Interpretation
≥75
Good
45-75
OK
≤45
Risk

Measures:
The rate of chemical decay as determined by the rate of spontaneous chemical change in organic materials.

  • TWPI integrates the T and RH values as they change over time into a single estimate of the cumulative effects of the environment on the rate of chemical decay.
  • TWPI is helpful as a quantitative comparison of the preservation quality of different storage locations or environments.

     

Applies to:
All Organic Materials (paper, textiles, plastics, dyes, leather, fur, etc)

 

Type of Decay: Mechanical Damage

Metrics Used:
Mininmum % Equilibrium Moisture Content
  (min % EMC)
Maximum % Equilibrium Moisture Content
  (max % EMC)
% Dimensional Change (% DC)
% Min EMC, % Max EMC and % DC
Interpretation
Min EMC > 5.0 AND Max EMC < 12.5 AND DC < 0.5
Good
Min EMC > 5.0 AND Max EMC < 12.5 AND DC < 1.5
OK
Min EMC < 5.0 OR Max EMC > 12.5 OR DC > 1.5
Risk

Interpretation:
Moderate % EMC is best ==> % EMC between 5% and 12 % maintains appropriate levels of moisture
The lower % DC, the better ==> fewer, smaller fluctuations between maximum EMC and minimum EMC

Measures:
Three aspects of moisture content that promote mechanical or physical damage:

  • Max % EMC:
    Is it too damp? Will paper curl? Will emulsions soften? Will wood warp?
  • Min % EMC:
    Is it too dry? Will paper become brittle? Will emulsions crack? Will wood chip?
  • % DC:
    How great are the fluctuations between the most damp and the most dry? Has expansion and contraction - from absorption/desorption of water - put physical stress on the collection materials?

Applies to:
All organic or hygroscopic materials (paper, textiles, plastics, dyes, leather, fur)

Most Important for:
Rare books, paintings, furniture, textiles; any bound or composite object

 

Type of Decay: Mold Risk

Metrics Used:
Mold Risk Factor (MRF)

Measures:
The risk for growth of the xerophilic mold species on collection objects or in collection areas.

 
Mold Risk Factor
Interpretation
≤ 0.5
Good
> 0.5
Risk

Interpretation:
Lower the MRF, the better

  • MRF greater than 0.5 indicates that mold spores are half way to germination, meaning half way to a vegetative mold state.
  • MRF greater than 1.0 indicates that mold spores have germinated, entering a vegetative mold state and visible mold could be actively growing.

 

*Note: There is no "OK" rating for mold growth. At a MRF of 0.5, conditions are appropriate for germination of spores. By alerting RISK of mold growth at germination, the user is aware of the potential of mold growth before any visible or vegetative mold will appear. This allows for time to react and prevent formation of vegetative mold.
Applies to:
All organic materials (paper, textiles, plastics, dyes, leather, fur) or inorganic materials with organic films

 

Type of Decay: Metal Corrosion

Metrics Used:
Maximum Equilibrium Moisture Content (% EMC max)

Interpretation:
Lower % EMC is better

% EMC (max)
Interpretation
≤ 7.0
Good
≥ 7.1 and ≤ 10.5
OK
≥ 10.6
Risk

Measures:
The effect of the environment on metal corrosion. The % EMC max represents the maximum amount of moisture that was present in hygroscopic collection materials. Because metallic corrosion is dependent on available moisture, the Max % EMC gives us an idea whether or not metallic objects (mainly ferrous metals) would corrode in such an environment.

Applies to:
Metals or materials with metal components