Blocked negatives

Here is my response to Ann Dodge's request about blocked negatives. This
treatment should really be done by a conservator with experience with
photographs; feel free to edit this according to what you know about the
distlist membership.

The negatives may be adhered to their paper enclosures for two reasons.

First, the negatives may be degraded cellulose nitrate. Cummings et al.
describe five stages of nitrate deterioration. In the first stage, the
negative has an amber color and the image fades. In the second, the
gelatin emulsion adheres to enclosures or neighboring films--probably
your problem. In the the third stage the film contains gas bubbles and
gives off a noxious odor of nitrogen oxides. In the fourth stage, the
film becomes an oozing mass, and in the fifth the film becomes powdery.
The film may be in the second stage.

The second possibility is that the film has become wet in a flood and
has blocked together. The film could be on any kind of base, cellulose
nitrate or safety (ie. cellulose acetates or polyester).

The film may also have both problems--I have examined negatives that
were obviously water damaged, but the film had also degraded where it
had become wet. This isn't surprising, as nitrates deteriorate by
hydrolysis.

I have treated negatives with all of these problems. The negatives could
be carefully separated with water/alcohol solutions. The idea is to have
enough of a water concentration to swell the gelatin, but not enough so
the gelatin dissolves away. This procedure works best for negatives that
have blocked due to flooding. However, I have not had a good success
rate with the degraded stage 2 negatives, as the gelatin becomes
increasingly soluble in water. Cummings notes that at stage 3, only
portions of the negative can be recovered. For the latter negatives, I
have worked under the microscope to mechanically lift out individual
paper fibers that were stuck to the photograph--but I had no success
finding an image underneath.

I had the best results when removing glassine enclosures from stage 1
nitrate negatives, c. 1918, that had been flood damaged. The technique
was worked out by Debbie Hess Norris when I attended "Preventive care of
Historic Photographic Prints and Negatives, Part 1".

Separation of negatives is a risky procedure for treasured original
negatives. My work on the degraded nitrates was a last ditch effort to
retrieve information from low priority material. I would suggest that
only conservators familiar with and trained to work with photographic
materials should try this treatment. In addition, there are health risks
to be aware of. Cummings' article mentions separating films in carbon
tetrachloride, a solvent that has joined the carcinogen list (and the
pantheon of conservation no-nos) since the article first came out. The
present water/alcohol solutions require the use of a fume hood, solvent
proof gloves, and protective eye-wear.

So for diagnosis and treatment, I would suggest consulting with your
local photographic conservator.

References:  There are many good, recent articles on nitrate negative
deterioration.  The work cited is:

    James W. Cummings, Alvin C. Hutton, and Howard Silfin "Spontaneous
        Ignition of Decomposing Cellulose Nitrate Film" Journal of the
        Society of Motion Picture and Television Engineers 54 (March
        1950): 268-274.

One of the most useful recent references is the following, ghostwritten
by Debbie Hess Norris:

    United States. National Park Service. 1990.  Curatorial care of
        cellulose nitrate negatives.  Appendix M in Part 1 of Museum
        Handbook.  Washington, D.C.: National Park Service.

Betty Walsh
Conservator
B.C. Archives and Records Service
655 Belleville Street
Victoria, B.C. V8V 1X4
Canada
604-387-3686

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                  Conservation DistList Instance 8:16
                 Distributed: Thursday, August 25, 1994
                        Message Id: cdl-8-16-001
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