Volume 20, Number 8
Dec 1996

Patents for Low-Cost Deacidification, Rapid Retrieval in Remote Storage Facilities

Dr. Donald K. Sebera (recently retired from the Library of Congress) has been awarded two patents:

5,393,562. Method of Preserving and Storing Books and Other Papers

5,537,760. System of Low-Temperature, Low-Humidity Preservation Storage and Accelerated Retrieval of Books and Other Papers.

Both are assigned to LC.

Number 5,393,562 utilizes very low concentrations of gaseous ammonia, continuously maintained in the storage area of library or archive, to establish and maintain near neutral values of paper pH. An older gaseous ammonia technique (the Kathpalia process) treats individual batches of acidic paper with high (ca. 10,000-100,000 ppm) concentrations of gaseous ammonia that takes the paper to alkaline pHs of 9-11 but which must be repeated at intervals because the paper gradually loses ammonia to the storage atmosphere and returns to its initial acidic condition. The storage area, especially in the early period following treatment, smells strongly of ammonia.

As shown long ago by W.J. Barrow, any pH greater than 7.0 does little, if anything, to reduce the rate of acid hydrolysis of paper cellulose; keeping paper pH between 6.0 and 7.0 provides 95% of the protection afforded by pHs of 7.0 or greater. The Sebera process is applicable to storage areas (especially closed or limited access) in which the storage atmosphere is continuously maintained at 1-10 ppm ammonia. Under these conditions even the most acidic papers will have their pHs raised to the 6.0-7.0 level (non-acidic papers can go to a pH of 8.5 or so) and so 95% of the benefit of deacidification to alkaline pH values is attained. At these 1-10 ppm concentrations the ammonia is below or just at the level detectable by odor and is completely safe within OSHA and other guidelines. If the storage volume is reasonably tight, only small amounts of ammonia need to be continuously introduced to maintain the 1-10 ppm levels so the process is economic. Books removed from the storage environment will, as in the Kathpalia process, lose ammonia but when returned to storage will regain their near-neutral pH.

Information about current plans to implement this process should be addressed to Dr. Chandru Shahani at the Preservation Directorate of the Library of Congress.

The second patent addresses one major practical problem associated with employing low-temperature, low-relative humidity storage conditions to reduce the rate of paper deterioration-the relatively long time required for the paper temperature and moisture content to increase to levels which eliminate the hazards of condensation and inflexibility/stress concentration. The reason long periods of time are required for rehydration (especially for books, but significant as well for manuscripts in boxes) is that the molecules of water in the reconditioning atmosphere are impeded in their passage between the sheets of paper to the dry celluose sites by repeated collisions with the molecules of air already occupying that confined space.

In the Sebera process, the cold dry book or manuscript box is removed from storage and placed in a sealed enclosure, the chamber evacuated to a low pressure of 10-25 torr to remove most of the air, pure water vapor is introduced for an equilibration time of 5-15 minutes, air is reintroduced to return the chamber to atmospheric pressure, and the contents removed for immediate use. The paper will have been hydrated to the point of moisture equilibrium with reading room environment (typically 40-60% RH) and near room temperature. The principle by which this rehydration equilibration occurs in minutes rather than days is that the pressure reduction step removes most of the air molecules blocking the movement (which is extremely rapid) of the water molecules to the hydration sites of the paper cellulose. The temperature is also raised because of the release of energy when cellulose is hydrated (typically in the range of 15-20°F for each 1% increase in moisture content of the paper).

The process is applicable not only to long-term low-temperature, low-relative humidity storage but to rapid rehydration of water-soaked books treated by freeze-drying techniques. Freeze-drying chambers can readily be modified to provide for water introduction at the end of the drying process.