Efficacy of Various Drying Methods
Hilary A. Kaplan and Kathleen A. Ludwig
Document Conservation Laboratory
National Archives and Records Administration
Selecting an Appropriate Drying Method: Conclusions
Preparations for all drying activities involve staff time, whether in direct recovery, or in administering activities associated with an off-site commercial process. For security purposes, records may need to remain on-site or be handled only by staff before being sent for vendor services. If housings no longer maintain adequate structural support, water saturated records may need to be handled for re-boxing and re-labeling before freezing and shipping.
Air-drying is useful for small numbers of damp materials. It is attractive because records do not need to leave the repository, and it allows for item by item retrieval or inspection for special handling needs--optimal control during manipulation if necessary, provided that expertise is available. Air-drying is best for plastic coated materials.
Air-drying also has drawbacks. It is very labor-intensive. Time is needed to layout and re-assemble records and remove damp or wet papers from plastic enclosures. Air-drying requires large areas of surface space. While potentially less expensive than commercial drying services, costs for air-drying include supplies such as absorbent and interleaving materials, security charges, and energy expenses for de-humidifiers and fans. Costs associated with staff time carrying out recovery actions in place of regular work cannot be underestimated. Even if routine tasks are not interrupted, staff may be displaced as workspaces are occupied by drying activities.
Though air-dried items are treated in-house, there is still a chance for disruption of original order. The possibility for mold is ever-present, and cockling without restraint during drying results in an increase in volume of dried materials, and potential need for re-boxing or rebinding. Coated papers will irreversibly block without interleaving and extensive handling may exacerbate damage.
Dehumidification-drying is suitable for damp, not wet materials. Like air-drying, on-site dehumidification-drying is advantageous from an access and security standpoint, but results in a distorted product with greater corrosion from metal fasteners. Dehumidification-drying may be more suitable than air-drying for large quantities of damp materials. This process also holds promise for drying slightly damp items in situ, i.e., without removing them from their housings or locations.
Vacuum freeze-drying is the drying method of choice for large quantities of wet materials. It results in the least amount of distortion, precluding the need to re-house or re-label most materials. Freezing immobilizes water to minimize corrosion from metal fasteners. Soluble media will not move once the document is frozen or during subsequent drying, and coated papers do not stick. Even items enclosed in plastic need not be removed from their original housings or enclosures, minimizing disruption of original order or potential loss. Existing labels are preserved.
While advantages generally outweigh disadvantages, vacuum freeze-drying also has downsides. No access to materials during the frozen state or drying process is available. Your materials may be included with other records in the drying chamber. While the cost for freeze-drying appears comparable to other processes, materials must be kept frozen, and the logistics of keeping materials frozen during transport is an additional cost factor. This may make freeze-drying small quantities of materials expensive. Increase delamination in cellulose nitrate coated architectural drawings was noted.
Being an informed consumer about the recovery services purchased will help make the best value of limited resources and ultimately yield the most satisfying results from a water-related incident. As a result of our experiences, we have learned how important it is to be as precise as possible at the outset of a project about expectations for a final product. Proprietary processes may offer outstanding final products, but caution is best exercised to ensure that your needs and specifications for what are acceptable and unacceptable methods and materials are assiduously followed. Regardless of technique used, many of the issues raised will be useful in the decision-making process so critical in determining the most appropriate action for a particular group of materials. While the information we accumulated in our review of drying processes has repeatedly proved fruitful, we wish to emphasize that there is no substitute for a building-wide risk assessment and preparedness plan.
The commercial drying industry continues to be responsive to the preservation community, tailoring, improving, and expanding their products and services. We are grateful for their steadfast cooperation in working to minimize loss to cultural property from water-related disasters.