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A-1671 December 2001 2
Introduction
Microscopic Blemishes
Some processed silver gelatin microfilms in
storage from two to twenty years have developed
microscopically small colored spots or blemishes.
The fogged leader at the outside of the roll is most
frequently affected by the blemishes, which are
generally red or yellow in color and are smaller in
size than the image characters (for example, a
typewritten letter reduced 20X) on the microfilm.
The spot size does not increase, but the spots
proliferate on the microfilm and can appear in
image areas. A more detailed description of the
blemishes and of the techniques used in inspecting
microfilm is given in National Bureau of Standards
Handbook 96.
1
The spots are caused by local oxidation of imaged
silver, resulting in the formation of minute deposits
of yellow- or red-colored colloidal silver.
2,3
Possible
oxidizing agents entering from outside the roll of
microfilm are aerial oxygen, whose action on the
film is strongly accelerated by moisture, and
atmospheric contaminants, such as hydrogen
peroxide, ozone, sulfur dioxide, hydrogen sulfide,
and nitrogen oxides, all occurring in industrial
atmospheres. Assuming your microfilm was
processed to established quality control standards,
microscopic blemishes (redox) may still occur due
to the oxidation of the metallic silver image,
depending on storage or use conditions.
It is the opinion of image stability experts both
within Eastman Kodak Company and outside of
Kodak that the LE-500 designation for properly
manufactured, processed, and stored silver gelatin
microfilms is conservative and may be expected to
outlive the designated Life Expectancy of 500
years. It is also, however, recognized that our
world is changing. The increasing number of
commercial solvents, auto exhausts, ammonia
fumes, paint fumes, solvents, heating gases, and
deteriorating cardboard and paper are only a few
of the many sources of oxidants and peroxides
affecting silver. These oxidants and peroxides are
believed to be the cause of redox blemishes.
Improper storage conditions also accelerate the
formation of redox blemishes.
Once the deterioration of the microfilm occurs,
it cannot be reversed. Silver film duplication,
molecular sieves, and Kodak Brown Toner can be
used to stabilize the microfilm and extend its useful
life under varying storage and use conditions.
Methodology
Molecular Sieves
Molecular sieves, or desiccants, are chemical
compounds that act as absorbers of water,
hydrogen peroxide, and other oxidants. These
sieves are also used to offset the vinegar syndrome
caused by the decomposition of acetate film
support
4
. The sieves are available in the following
packet formats from a company called Multisorb,
with the following part numbers:
Part # Film Format Content
41 ag 43 16 mm 3.5 grams 400 (1 gal)
41 ag 47 35 mm 7.0 grams 200 (1 gal)
41 ag 51 105 mm 12.5 grams 125 (1 gal)
Rule of thumb: The sieve size is 2% of the weight
of the roll of polyester-based film or 4% of acetate-
based film.
It should be noted that the sieves need to be
replaced at some interval, depending on storage
conditions. The use of molecular sieves is cited by
the American National Standards Institute (ANSI) as
another procedure to reduce redox blemishes
caused by high humidity conditions and oxidants in
the air. For more information or to obtain molecular
sieves, call Multisorb at 1-800-445-9890 and ask for
Customer Service.
Kodak Brown Toner Treatment
Brown toning is a chemical treatment that changes
the films processed silver to silver sulfide which is
much more resistant to humidity and oxidants.
Kodak Brown Toner will not degrade the quality of
the microfilmed image. After brown toning, areas of
redox blemishes may be clear or change to a more
neutral color. Although the name brown toner
suggests a browner image after toning, this is not
true for microfilm. In fact, after brown toning, areas
of density may appear blacker or more neutral in
color. Brown toning does not affect the D-min or
clear areas of the film when washed properly.
The first documented evidence of redox blemishing
occurred in the early 1960s. One recommendation
from the resulting investigation promoted the use of
low concentrations of potassium iodide in the fixing
bath (0.2 - 0.5 grams per liter).
3
This has been
shown to provide a good degree of protection
against redox blemishes. Kodak Microfilm and
Prostar Fix solutions contain this stability-enhancing
iodide. A second recommendation from this study
was gold or selenium toning of the microfilm.