Preservation

Efficacy of Various Drying Methods

Hilary A. Kaplan and Kathleen A. Ludwig
Document Conservation Laboratory
National Archives and Records Administration

Records were placed upright on plastic shelving, supported by vertical polyvinyl chloride (PVC) pipes inserted at regular intervals. Bound materials could be opened and supported upright by the poles of the drying rack. Corrugated boards and blotter paper were inserted to support the drying materials, but proved inadequate. As the records dried, distortion occurred. Though holdings originally filled two boxes, four boxes were required to accommodate the size of cockled dried materials following dehumidification-drying.

Dehumidification-drying has been successfully employed in situ for slightly damp materials where items are dried in place, without removing them from their shelves or from their box. Though our experimental records were wet, not damp, we were anxious to see how well the dehumidification system already set up on-site could successfully dry wet items in situ. Would they eventually dry? How long would it take? Would mold grow under such dry ambient conditions?

An archivist at the NARA facility took daily measurements of the materials being dried in situ and recorded the data using a Delmhorst P-2000 moisture meter. This instrument works optimally when the papers to be measured are between 21°- 32° C (70°-90° F.) It measures the relationship between moisture content and electrical resistance using a scale of 4.3-18% moisture content. Five to seven percent (5-7%) moisture content is considered dry. The Rotronic™ data logger was placed next to the boxes being dried in situ to record environmental conditions. Moisture content percentage was recorded daily to track the drying progression.

The in situ dehumidification drying trial ended on the 13th day of the project when the rented drying equipment was returned to the vendor. Wet boxes did not dry during this period. A core area of the box measured initially exceeded the upper range of the moisture meter's 18.2 % moisture content and remained at that level for the duration of the project's test period. Only the sides and outer ends of the boxes were beginning to dry by day 13. The least wet box diminished from 16.0 to 15.3% moisture content.

The temperature in the drying chamber fluctuated from 21 -37°C (70-90°F) and the humidity was within 5-20% throughout the thirteen-day period. The data-logger corroborated our impression that working conditions for staff within the chamber were oppressive. Staff found the hot, dry conditions too uncomfortable for productivity and often turned off the equipment and raised the loading-dock doors when actively loading and unloading the chamber. The system was, however consistently turned back on at night when staff presence was not required.

Ideally, this system is designed to be loaded, sealed, turned on, and completed before personnel would need to spend time within the chamber. But the circumstances of this situation dictated the process be completed as quickly as possible due to administrative concerns that materials be readily available and financial constraints--such as fuel to run the freezer trucks and personnel. As a result, the cycles were modified from the original plan design recommended by the equipment vendor.

Our attempt at dehumidification in situ was not successful and confirmed recommendations found in the literature; dehumidification in situ is not a suitable approach for drying wet materials. Mold grew on a variety of substrates. These wet samples were eliminated from further comparison, bagged and discarded.

Commercial Off-site Drying: Issues of Intellectual Control

Drying records on-site repeatedly demonstrates a level of intellectual control that does not always appear attainable when items leave the repository. The use of labeled plastic "milk crates" is one means to safely transport many small volumes to an area prepared for drying. Though space-consuming to store, these crates are sturdy, can be used again and again, and are not terribly susceptible to damage from water or weight. If necessary crates with damaged materials can be placed directly into a freezer for drying.

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