Photographs (and paper documents) suffer from inherent chemical deterioration, generally exhibited by fading (called "chemical fade"). Fluctuations in temperature and humidity accelerate that deterioration process; that's why storage is such an important conversation to have.
Most "cultural collections" (business archives, household, social clubs and institutions such as nonprofit organizations) are stored in acidic cardboard boxes and acidic envelopes (from the photo labs where they were developed.) These acidic enclosures also accelerate the deterioration process.
Physical wear and tear is also a concern. Rifling through photos without using care and wearing protective cotton or Nitrile gloves (http://www.loc.gov/preservation/about/faqs/photographs.html#gloves ) can cause dents, dings, rips, and tears, especially with fragile photos from the late 1800s that were mounted on thick cardboard (called the substrate) which is now falling apart. Fingerprints leave oils that can etch into the emulsion over time.
Additionally, photographs stacked against each other can cause abrasions, scratches, emulsion separation and chemical transfer (http://www.alternativephotography.com/wp/essays/platinum-print-discussion
Paper documents contain resin, alkali, lignin and other ingredients (brighteners, etc.) added during the pulp-making process (for binding) that cause the substrate to deteriorate. If not protected (or buffered) these acids can migrate to other papers and photographs.
At Preserving The Past, we take the approach of re-housing each artifact using an acid-free support board (conservation board purchase through a museum supply vendor) and polyethylene chemically inert sleeve. Each element is given a catalogue number, or unique identifier. This is the same number as the digital file once it has been scanned, useful for cross-referencing. This is not expensive but it's not entirely inexpensive either. As a less expensive alternative to this process interleave each photograph with acid-free archival tissue (http://www.archivalmethods.com/product/archival-tissue); then place the photos and letters in an appropriately sized acid-free box. We work with Archival Methods, who make their own archival supplies here in Rochester (coincidentally) with quality control in place ensuring they are acid-free.
The suppliers I recommend:
Hollinger Metal Edge
Carr McLean (in Toronto)
Pick up an Abbey pH Pen at an art supply store (or from these suppliers) to test products purchased at consumer-level stores. The term "acid-free" is NOT legislated so even if the label says "acid-free" it may not be; use this pen to test before you purchase or return if you've already purchased.
Rather than consumer "photo boxes" we suggest using the metal edge archival box from a reputable museum supply company, which is acid-free and has more support. (Bankers boxes purchased at most office supply stores are not acid-free.)
President and Founder
Preserving The Past, LLC
2290 East Avenue
Rochester, NY 14610
(585) 271-4774 NY
(818) 317-7147 LA
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Preserving The Past, LLC offers best practice services in organizing, archiving, digitizing and preserving historical artifacts safeguarding them for future generations. Instead of returning photographs, film, videos and other documents back to your clients to inevitably go into acidic boxes, hot closets and garages—all of which accelerate inherent deterioration—consider partnering with us to offer additional services. Visit www.preservethepast.com and feel free to call me directly at: firstname.lastname@example.org or (585) 271-4774.
See also Preserving Your Family and National Treasures: Archiving, Conservation, and Preservation (Pat McNees, Telling Your Story)