Image change in photographic negative

Many thanks for the replies to my original inquiry, reproduced here
for those who did not see it.

>Can any of you photo types enlighten me?  The Louisiana State
>Archives has several large format (8x10) negatives taken on the
>scene of Huey Long's assassination.  On several of them there is a
>shifting of the negative image to a positive one.  The result is a
>negative in some areas and a positive transparency on other areas.
>On one the shift occupies roughly one half of the entire negative.
>Others are less affected by this phenomenon, and still others--from
>the same collection--are not affected at all.

Many of you have requested additional information.

The negatives were taken at the scene of the assassination of
Senator Huey P. Long in Sept. 1935.

The negatives are silver gelatin on cellulose nitrate.  They are 8
inches by 10 inches.

There is no silver mirroring or density discoloration on the
negatives.

No oxidized material is on the surface of the cellulose base which
can be removed by contact.

Although specific information about the previous storage conditions
is not known, some inferences can be made. Some negatives in the
collection, including both negatives demonstrating this
positive/negative "shift" and those not demonstrating it, indicate
storage in conditions of high heat. This is Louisiana, so if there
was high heat, it follows that there was high humidity.

Doug Harrison
Conservator
Louisiana State Archives

                                  ***
                  Conservation DistList Instance 12:94
                   Distributed: Friday, June 4, 1999
                       Message Id: cdl-12-94-010
                                  ***