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Mechanical Damage

Understand how changes in temperature and humidity in your spaces affect the risk of mechanical damage to objects that absorb moisture.

Table of Contents

What the Metric Measures

The mechanical damage metric estimates how much your storage environment makes objects swell and shrink. Lower values mean lower risk.

Hygroscopic materials absorb moisture from the air and release it again. Wood, paper, parchment, textiles, ivory, and adhesives are all hygroscopic. These materials change size as their moisture content changes. Repeated swelling and shrinking can cause cracks, warping, splits, and flaking paint.

The metric is based on the mechanical damage metric developed by the Image Permanence Institute (IPI). It pairs equilibrium moisture content (EMC) with percent dimensional change (%DC).

Property Value
Applies to Hygroscopic materials, such as wood, paper, and textiles
Measures Effect of the environment on the risk of mechanical damage
Primary driver Change in relative humidity (RH) over time
Interpretation Lower is better
 

Metal corrosion and mold risk respond to how damp a space is. Mechanical damage responds to how much the dampness changes. A space that stays damp at a steady level can rate well for mechanical damage and poorly for metal corrosion.

How the Metric Is Calculated

The metric converts lagged temperature and RH readings into EMC, finds the lowest and the highest EMC in the selected period, and converts the difference between them into a percent dimensional change.

  1. Collect temperature and RH readings from the sensor.
  2. Calculate a 24 hour running average of temperature.
  3. Calculate a 30 day running average of RH.
  4. Convert each pair of averaged values to %EMC using the United States Forest Products Laboratory (USFPL) equations for an average species of wood.
  5. Select the lowest and the highest %EMC value in the period.
  6. Multiply the difference between those two values by the dimensional change coefficient for an average species of wood.

The result is the percent dimensional change (%DC) for the period.

The running averages model the time lag between a change in the room and the response of the objects, as the impact of environmental changes on an object's moisture content are not immediate.

Notes:

RH has a much larger effect on EMC than temperature. A space that holds a steady RH keeps the metric low, even when that RH is high.

Over a short period, the metric is usually low. The 30-day RH average moves slowly, so a few days cannot produce a wide %EMC range. Compare periods of the same length.

Interpreting the Values

Two conditions set the rating. The %EMC values for the period must stay between 5% and 12.5%, and the %DC must stay low.

%DC Rating Meaning
0.5% or less Good Low risk of mechanical damage
More than 0.5% and up to 1.5% OK Moderate risk of mechanical damage
More than 1.5% Risk High risk of mechanical damage
 

Bands from: Image Permanence Institute. IPI's Guide to Sustainable Preservation Practices for Managing Storage Environments, Ch. 6. Rochester; Image Permanence Institute.

If the lowest or the highest %EMC in the period falls outside the range of 5% to 12.5%, the rating is Risk, whatever the %DC is. Conditions outside that range carry their own risks, and the dimensional change calculation is less reliable there.

A %DC of 0.5% corresponds to an %EMC range of about 1.5 percentage points. A %DC of 1.5% corresponds to an %EMC range of about 4.5 percentage points.

On the Analytics page, the mechanical damage card shows the rating. Point to the rating to see the %EMC range and the %DC for the period.

Tip: If the metric is in the Risk range, review the RH history for the period. A large swing in RH or a repeated cycle between damp and dry is the most likely cause.

Limitations and Considerations

Important: The metric is an approximation of the risk of mechanical damage, not a direct measurement of damage.

  • The EMC and dimensional change equations describe an average piece of wood as selected by the USDA Forest Products Laboratory Wood Handbook. Other hygroscopic materials respond at different rates and by different amounts.
  • The metric uses only temperature and RH. It does not include the material, the age or condition of the object, or the way the object is constructed or restrained.
  • The metric reports the widest %EMC range in the period. One damp stretch and one dry stretch set the value, even if conditions were steady for the rest of the period.
  • A short period usually produces a low value, because the 30-day RH average changes slowly.
  • The metric applies to the environment the sensor measures. Conditions inside enclosures or display cases may differ.
  • A low value does not guarantee that mechanical damage will not occur.

Caution: Always interpret preservation metrics together with knowledge of your collection.

Related Articles

The mechanical damage metric is one of several preservation metrics in Conserv Cloud Analytics.

Additional Resources