Preservation

Comparison of Drying Techniques

Understanding Differences Between Vacuum Freeze Drying, Conventional Freezing, and Other Drying Methods

Vacuum/Freeze-Drying

Sometimes called "Freeze-Drying," though technically that is a different, less successful freezing process without use of a vacuum.

Don't confuse with simple freezing, a common first step in emergency response that allows time to arrange drying by any of the processes described here.

  • Efficient method dries large quantities of wet records by sublimation under vacuum.
  • Minimizes distortion, movement of soluble media, and blocking of coated papers.
  • Records in encapsulations or polyester sleeves need not be removed before drying.
  • Records can dry in their original containers reducing risk for disruption of original order.
  • Records come out of vacuum freeze-drying in the same physical configuration as they went into the freezer. If a book is warped going in, it will be warped coming out. Best results are achieved when materials are properly packed prior to freezing.
  • Vacuum drying chambers can be locked for security mandates. Security guards can be contracted to provide surveillance to the exterior of the chamber if needed.
  • Average drying time depends on wetness; 1-2 weeks.
  • No access to the records during drying.
  • Records can be vacuum freeze-dried at vendor's facility, or mobile vacuum freeze-drying chambers can be trucked to the wet records site. On site drying costs more than drying records at the vendor's facility.

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Dessicant Drying

  • This method results in the greatest distortion to the paper. It is labor intensive.
  • Records must be removed from their containers, spread on shelves to dry in warm dehumidified air, and periodically rotated to expose wet paper surfaces.
  • Items in encapsulations or within plastic sleeves must be removed to dry.
  • Records on coated paper must be separated and/or interleaved to dry without sticking or blocking.
  • Volumes must be fanned open to expose page surfaces to facilitate drying.
  • Records require more storage space after drying because of increase in volume from distortion.
  • Original order may be disrupted when records are removed from containers for drying.
  • Average drying time, 2-5 days depending on wetness.
  • Records are accessible during drying.
  • Records can be desiccant dried at vendor's facility, or desiccant drying equipment can be rented and installed on site.

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Air Drying

  • This method results in moderate distortion to the paper. It is labor intensive.
  • Records must be removed from containers and spread on surfaces to dry in air. The process requires vast surface areas covered with absorbent papers to lay records out. As records dry, they need to be turned and absorbent paper needs changing.
  • Items within plastic sleeves or encapsulations must be removed to dry.
  • Records on coated paper must be separated and/or interleaved to dry without sticking or blocking.
  • Volumes must be fanned open to expose page surfaces.
  • Original order of records may be disrupted.
  • Metal fasteners may rust or corrode.
  • Drying time depends on wetness of records, relative humidity, and amount of exposed paper surface.
  • Records are accessible during drying.
  • Records can be dried on site or at a location of the custodian's choosing.

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Cryogenic Drying

  • This method minimizes distortion to leather, vellum, and parchment by slowly drying materials with modified freeze-drying technology.
  • Drying time is longer and more expensive than vacuum freeze-drying.
  • No access to records during drying.
  • This process must be done at the vendor's facility.

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