Archival properties and graininess of photographic films

I have not (as usual) been keeping up very well with the writings on the
distlist.  I finally was skimming through the last (embarrassing number
of months of) DistLists and came across Andrew Lyne's questions about
the "archivalness" of films and the later response.  I guess I should
throw in my 2 cents.

I should first try to clarify Monica Hoskins' comments about Kodachrome
film.  She described it as a "dye transfer process on b&w emulsion."
This description does not quite describe the important difference about
Kodachrome film although the description is basically correct.  In
common color processes (C-41, E-6 and virtually all regular color prints
excluding Ilfochrome (nee Cibachrome)), the process is "chromogenic" --
literally "giving birth to color".  Kodachrome is also a chromogenic
process I might add.  All of these materials (excluding Kodachrome)
contain dye-precursors or dye couplers in their emulsions.  They are not
dyes.  There is at least one sensitive layer for each of the three
primary colors.  Upon exposure, only the layer (or layers) sensitive to
the required color of light is exposed.  For negative materials, a
special black-and-white developer is used.  This developing agent will,
like a regular developer, only develop silver that has been exposed.
Thus only certain areas of each of the (minimum three) emulsion layers
are developed to form silver.  In the process, the developer in these
areas also changes (it oxidizes) and it is the oxidized developer in
these areas that bond to or "couple" with the dye couplers that actually
form the dye.  Thus the dye is formed during the development process and
did not exist before.  In reversal processes like E-6, two developers
are used--one to develop the negative and the other to both develop
the positive and to form the color dyes.  Now the problem with this
process is that since the couplers are in the emulsion from the time of
manufacturing and don't wash out, unused couplers remain in the emulsion
after processing.  Anything that will chemically react with these
invisible couplers to form a visible material will cause staining.

Kodachrome is different because it does not contain the couplers when it
is manufactured.  These couplers are introduced during processing.
There are no residual couplers left in the film after processing.
Unfortunately, the phrase "dye transfer" was not such a good word
choice.  Technically, dye transfer only refers to Kodak Dye Transfer --
a dye imbibition process in which color dyes are transferred from a
matrix to a receiver.  In Kodachrome, there are no dyes that move
anywhere, just the dye precursors. The sharpness is achieved in
Kodachrome by the ultra thin layers.  It is said that Kodachrome
contains as many as 22 separate emulsion layers, although all are
micro-thin.

The longevity of all chromogenic materials is limited by the fact that
out of all the thousands of dyes available, only dyes that can be formed
during processing can be used.  This greatly limits the possible dyes
that can be used.  Compare this with Ilfochrome--a silver dye bleach
process in which all of the dyes are in the material.  During
processing, unwanted dyes are selectively bleached out.  Silver dye
bleach processes are much more stable in the dark and a little more
stable in the light than chromogenic materials.  Incidentally, there is
Ilfochrome transparency film available. It has been used in the
microfilming industry for a number of years. Although it has a greatly
stable image, it is unfortunately not practical to use for regular
filming.  Since light must pass through all of the dye to expose the
various layers, it has an ISO speed of about 8.

So where does that leave us?  I have never tested Kodachrome myself, but
Henry Wilhelm (who probably knows more about color stability than anyone
else in the world--manufacturers included) has presented some data.
In his latest article (1990) he estimates that to the same end point
(20% dye loss of least stable dye) that Kodachrome will last about 260
years to 20% yellow dye loss while Ektachromes will last between 75 and
80 years to a 20% cyan dye loss under "real-world conditions".  However
this is not all that should be considered.  Ektachromes (and all other
chromogenic slide films) slowly develop stain growth while Kodachrome
doesn't.  And just when you believe that Kodachrome is the end all film,
I will quote from Henry:

"In spite of Kodachrome's unequaled dark-storage dye stability and total
freedom from stain formation, it has the worst projector-fading
stability of any slide film on the market.  Kodachrome is a very good
illustration of how a dye, in this case magenta, can have very good
dark-fading stability but comparatively poor light-fading stability
(with some dyes, the opposite is true).   Kodachrome is a great film to
use if projection can be avoided; but if projection of originals
sometimes is a must, and time or money keeps you from routinely
duplicating originals, you will probably be better off with Fujichrome."

As for the black-and-white question.  Our feeling is that generally
black-and-white films are fairly comparable in stability.  The theory
says that fine grained images should be less stable than coarse grained
images just because of the relative surface to volume ratios.  However,
I have never seen any evidence of this.  Processing, however will make
quite a difference.  At this point I will apologize for being long
winded and especially to "old timers" on the distlist who have heard me
talking about this before.

Follow the manufacturers instructions when processing and post-treat in
gold, platinum, sulfur or (lots of) selenium.  Probably the biggest
shock that ever came out recent information is the fact that excessive
washing of photographs will make images **LESS** stable and that a small
amount of retained hypo is beneficial.  This myth about hypo came
originally from the Fading Committee of 1855.  In their final report,
they did find that the photographs examined were poorly washed and that
was the cause of fading. However, with the thin papers used, it took
very little washing to remove the hypo.  Only intentional poor washing
produced the results seen.  A small amount of sulfur attack on a printed
out image produced a very nice tone and also a gain in density richness.
Unfortunately, more than a small amount of sulfur rapidly fades the
photograph to nothing.  Photographers were intentionally using bad hypo
or not washing just produce the nice effect but not realizing the long
term effect.  The committee also stated:

"The Committee find that there is no known method of producing pictures
which will remain unaltered under the continued action of moisture and
the atmosphere in London."

Further they state, "They find that pictures may be exposed to dry
sulphuretted hydrogen gas for some time with comparatively little
alteration...."

Authors writing on the subject of permanence copied only the information
about hypo retention from the report and completely ignored the part
about air pollution.  Subsequent generations of authors continued to
write about hypo retention until today, all deterioration problems are
blamed on bad processing.  In fact today, we find that of the thousands
of photographs examined here at IPI, we rarely find deterioration from
hypo retention. Virtually all of the fading seen in photographs has been
caused by air and moisture.

Furthermore it has been proven at Kodak, Fuji and here at IPI that a
small amount of hypo will make photographs MORE stable against air
pollutants and that overwashing decreases the stability.  This is a
"Goldilocks" effect though in that too much and too little hypo can be
bad, but there is an amount that is "just right".  Since we can't
recommend that hypo be put back into well washing images we recommend
post-treatment of the image to produce a more stable material than
straight silver.  During the course of research that led to the
development of IPI SilverLock (tm), a number of chemical treatments were
studied.  Ultimately, both gold and platinum work well (silver is
exchanged for more noble metals) in improving the resistance of silver
images to pollutants.  Sulfur works extremely well and selenium works
well if enough is used.  Selenium has the property of split toning.
High density areas are converted in silver selenide extremely well,
while the lower density areas are hardly converted at all. Therefore,
unless a heavy shot of selenium is given to an image, middle and low
density areas will still be vulnerable to attack.  Film images have the
nice property that their silver morphology is such that it is difficult
to produce a significant tone change when treating with any of these
materials.  Photographic prints tend to change in tone much more easily,
but it has been shown that the right toner/paper combination can produce
very stable images with minimal tone change.

In a nutshell:

1) Use Kodachrome for images that won't be projected and will be kept in
the dark.

2)  No other practical transparency will match Kodachrome in dark
stability.  (Ilfochrome is too slow to be practical.)

3)  Use something like Ektachrome or Fujichrome if things are being
projected.

4)  Ultimately I would recommend cold storage for slide masters.

5)  All black and white films are generally of approximately equal
stability.

6)  Processing has a major effect on stability.

7)  Follow the manufacturers processing instructions.  Don't waste the
effort and water to overwash since it will decrease resistance to
atmospheric attack.

8)  Post-treat all silver images in platinum, gold, sulfur or selenium
for maximum permanence.

Doug Nishimura
Image Permanence Institute
dwnpph [at] ritvax__bitnet
dwnpph [at] ritvax__isc__rit__edu

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                  Conservation DistList Instance 7:12
                   Distributed: Monday, July 19, 1993
                        Message Id: cdl-7-12-001
                                  ***