Corrosion of brass mats in cased photographs

In Conservation DistList Instance 10:33
Gary W. Ewer <garyewer [at] poweramp__net\|[gt    ]\|, writing on behalf of Nicola
Longford <njmlt [at] aol__com\|[gt    ]\| wrote:

>What is the general interest about brass mats and their
>corrosion? Do you know of anyone who cleans and "touches" brass
>mats up for presentation? I am especially interested in any
>information about their manufacture which would assist in the
>development of conservation treatment recommendations for brass
>mats in the near future.

In the case of daguerreotypes the brass mat nearly always cause
corrosion on the silvered surface of the photograph when both
materials are in contact. (Probably it is an electrochemical
reaction). So, to stop it a layer between is useful from any
non-dangerous material. (I.e. photo/silversafe paper.)

The decomposition of the cover glass of daguerreotype preserver is
resulting corrosive materials (visible: yellow/greenish disgusting
drops on the inner surface of the glass plate). This material is
dangerous for the daguerreotype plate and for the brass mat also.

Usually these copper parts of the dag. preservers have a protection
layer against corrosion (and probably to get a more decorative
color). In a simple case it is a (shel)lack layer but often a result
of some chemical treatment. The hurts of these layers would be a
starting point of the corrosion process. Restoration/ethical
problem: after cleaning the recoloring of the surface to the
original looking is very difficult. (You need to know a crowd of old
receipts, but don't enough to get the original antique surface
sometimes...)

Balint Flesch
Budapest/Hungary. -

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                  Conservation DistList Instance 10:35
                 Distributed: Saturday, October 5, 1996
                       Message Id: cdl-10-35-009
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