Efficacy of Various Drying Methods
Hilary A. Kaplan and Kathleen A. Ludwig
Document Conservation Laboratory
National Archives and Records Administration
In order to better understand how various drying methods would affect a sampling of archives materials, the effects of four drying approaches were observed and compared on sets of similar materials. Our study was performed using expendable items donated by a number of the Archives' custodial units. These disposable paper-based records were diverse, though they did not represent the entire universe of media and supports found within the Archives. Sufficient examples were available to create seven sets of paper records, filling two records storage boxes, measuring .06796 cubic meters. The four processes compared were air-drying, dehumidification drying, vacuum freeze-drying, and a vacuum thermal freeze-drying proprietary process. The following paper conveys how we carried out this study; observations made; and what we have learned along the way.
Air-Drying involves drying records at room temperature, in-house. Typically, materials are dried in a prepared workspace and spread out on, or interleaved with, absorbent papers. In some instances materials may be dried under restraint in a stack of weighted blotters. In dehumidification-drying, also referred to as desiccant-drying, materials are dried by introducing dried (dehumidified) air at very low relative humidities, often below 15%, and circulating that dried air with fans in and around the drying chamber. Air temperatures vary throughout the drying process, but usually are in the range of 26-37° C (79-99° F).
Vacuum freeze-drying is generally recommended for large quantities of wet or damp materials. Materials must be in a frozen state when entering the vacuum freeze-drying chamber and remain frozen throughout the drying process. Items are placed within a high vacuum at temperatures below freezing while cycles of controlled heat are applied. This process causes frozen water to sublimate to a vapor without passing through a liquid stage-advantageous in minimizing feathering and bleeding of soluble media. It allows coated materials to dry without blocking, and results in minimal distortion. The process can be performed on-site by vendors equipped with mobile vacuum chambers, or items may be sent to a drying facility. Drying time depends on the wetness of the materials but can usually be accomplished in less than two weeks. ).
The fourth method we set out to investigate was vacuum thermal drying. Vacuum thermal-drying has been recommend for wet or damp materials. Materials can be placed into the chamber either frozen or thawed and are dried above 0° C. Materials are dried in a chamber under a vacuum with heated air. Generally recommended for un-coated papers, vacuum thermal drying is often cited as a cost-effective option for materials of low intrinsic value. This procedure is known to distort paper, causes coated records to block, and exacerbates the feathering and bleeding of soluble inks. Drying time is usually shorter than vacuum freeze-drying, but this will depend on initial wetness.).
We did not, in fact, investigate this process as described, though this was indeed the service we intended to purchase. An absence of clearly understood terminology resulted in obtaining thermal vacuum freeze-drying, not thermal vacuum-drying. This discrepancy only became obvious once materials were returned and did not display the characteristic effects anticipated--blocking of coated paper, feathering and bleeding of soluble inks. ).
The results from thermal vacuum freeze-drying, however, provided a useful sampling for comparison with other methods evaluated. Thermal vacuum freeze-drying is similar to vacuum freeze-drying in using reduced pressure and controlled heat to vaporize water, but it also employs a patented procedure to compress subjected materials into shape. Such proprietary processes may not be universally available, and like other technologies, may be specific to particular locales. ).