Most old photographs from the last century (80%) are albumen prints, made with thin paper coated with egg white. (The photographer had to coat it himself with silver nitrate just before use, and mount the print on board afterwards to keep the thin paper from curling or being damaged in handling.) Despite the fact that they were all gold-toned and have survived in great numbers, they are more unstable than most silver photographic materials. They are particularly prone to fading and yellowing.
Albumen prints, when new, are a rich purple-brown with white highlights and full detail. After deterioration--and this describes at least 9 out of ID surviving prints--they are yellow-brown with yellow highlights, and faded (lightened) shadows; detail in highlights is lost.
A three-year research project under James Reilly, author of The Albumen and Salted Paper Book (Light Impressions, 1980), was carried out at BIT to investigate the effect of all the factors usually implicated in such deterioration. The investigators found to their surprise that the culprit was not poor processing, but high relative humidity. Fading, they found, increases dramatically when the RH is over 60%. For storage, 30-40% is recommended.
Another big surprise was that contact with alkaline storage materials accelerated the yellowing. Even contact with the mild alkalinity of buffered paper produced yellowing. But acidic storage materials were no better, because they produced fading. In dealing with albumen prints, therefore, one has to use neutral pH storage materials.
In the two years since these research results were announced, several suppliers have made such materials available. Since the materials cannot use calcium carbonate to achieve chemical stability, they have to be pure--no alum, lignin, hemicelluloses or other reactive substances. The customer has to rely on the reputation and statements of the supplier for assurance that these elements really are absent, because there is no easy way to test for presence of lignin or hemicelluloses. The Barrow test kit has an indicator for lignin, but even when it is fresh and accurate, it only indicates the presence of large amounts of lignin (over 20% or so).
Alkaline storage materials are harmless to albumen prints as long as they are not placed in direct contact, since buffering materials are not volatile and do not migrate even when they are in direct contact except in high humidity. They are also not known to affect other kinds of photographs. The main cause of trouble seems to be the egg white. Other sorts of emulsions, such as gelatin and collodion, are not affected.
That leaves relative humidity as the biggest problem in keeping albumen prints. Whatever control system is used, even the most primitive and improvised, it is essential to have a way of monitoring temperature and relative humidity (temperature has a powerful effect on RH in addition to its own effect on deterioration, so it has to be watched too).
There are two reasons for monitoring. One is short- term and the other is long-term. The long-term reason is that the records, accumulated over a meaningful period of time, serve as convincing (reportedly, almost overwhelming) arguments for capital expenditures for improved air conditioning systems. The short-term reason is that they enable the curator to take measures that will at least prevent the worst damage during humid periods, by renting dehumidifiers, moving valuable parts of the collection to drier rooms, and so on.
Monitoring can be done with a variety of measuring instruments, the best (for convenience and completeness of the record) being the hygrothermograph, which runs for a week without attention. For detecting dangerously high levels of RH, however, a relatively simple device has been recently found to be reliable and long lasting: paper impregnated with cobalt chloride. The salt turns from blue to pink as it gains moisture, and can be used to make indicator cards for the full range of RH. This was reported by Vincent D. Daniels and S. E. Wilthew last year ("An Investigation into the Use of Cobalt Salt Impregnated Papers for the Measurement of Relative Humidity," Studies in Conservation 28 (2), 80-84, May 1983). An American supplier's name is listed: Humidial Corporation, 465 Mt. Vernon Avenue, P0 Box 464, Colton, CA 92324.
James N. Reilly, "Albumen Prints: A Summary of New Research About Their Preservation." Picturescope 30(1): 54-56.
Debbie Hess Norris, "The Proper Storage and Display of a Photographic Collection." Picturescope 31(1): 4-8, 1983.
Robert A. Weinstein and Larry Booth. Collection, Use, and Care of Historical Photographs. Nashville, AASLH, 1977.