Ferrotyping

Kurt Kuss and Mark Clarke recently posted queries regarding the
suitability of storing gelatin photographs in Mylar sleeves.

Mark Clarke <markey [at] gn__apc__org> writes

>Is it safe to place photographs in direct contact with their
>mylar/melinex enclosures? I have heard of an effect called
>"ferrotyping" which can happen to gelatine-coated
>photographs--apparently some sort of change in the appearance of the
>surface. Can anyone fill me in - what is "ferrotyping", and is it
>only a problem for gelatine coated material (which should then
>presumably have a barrier of a suitable safe paper.)

Ferrotyping (sometimes known as "glazing") is a technique used to
substantially increase the gloss on gelatin photographs.  It is
usually carried out by the photographer as a post-processing step.
The technique involves washing photographs in water to which a
wetting agent, like Photo-Flo, has been added.  Photographs are
removed from this bath and squeegeed, face down, onto a very smooth
surface (often a chromium-coated iron plate). The prints are allowed
to dry in contact with the plate.  Once dry, the prints should lift
themselves away from the plate.

Certainly if photographs were wet then dried while stored in the
Mylar envelopes there would be problems--possibly the least of which
would be changes in surface gloss.  If a collection might undergo
substantial temperature and humidity changes (like coming in and out
of cold storage), I would not recommend the Mylar.  Also, if the
relative humidity regularly got above 70% RH or so, Mylar might not
be the best choice (photographs should not be stored in conditions
which are regularly above 50% RH).

To my knowledge, Mylar sleeves do not present a ferrotyping risk for
gelatin prints under normal, controlled, storage conditions.  I too
have seen precautions on the use of Mylar for gelatin photographs,
but I have never seen evidence of a real threat--I would be very
interested to hear of previous bad experiences.

Paul Messier
Conservator of Photographs
Boston Art Conservation
617-738-7072

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                  Conservation DistList Instance 9:45
                 Distributed: Friday, December 1, 1995
                        Message Id: cdl-9-45-006
                                  ***