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Subject: Interleaving Copper green pigments

Interleaving Copper green pigments

From: Tom Chase <chaset>
Date: Saturday, January 27, 1996
This is in reply to that from Cor Knops about discoloration of paper
due to copper green pigments.  This phenomenon is well known in Near
Eastern "Persian" Manuscripts and also in Far Eastern (Japanese)
paintings, where it is called "rokkusho burn" (rokkusho being the
Japanese for malachite). I believe that it arises because copper is
a transition metal and can have two different valence forms; the
change from Cu+2 to Cu+1 (reduction) requires little energy and can
promote the oxidative breaking of the bonds in the cellulose
structure.  (This is coming out of deep memory from someone who is
not an organic chemist, and who has done no bibliographic research
on this topic recently.)

We have seen the effect on *lots* of objects, especially on Asian
Paintings (where it is generally limited to browning of the support
paper) and on "Persian" miniatures, where the paper can be gone or
(particularly nasty) green borders eaten away around text on a page,
making it come out.

We have also discussed the possibility of research on the
phenomenon; a possibility that came to mind was immobilizing the
copper with benzotriazole so that it could not continue to further
damage the paper. The questions of acidic solutions or by-products
came up, and we decided that it wasn't worth going further with this
idea.  The damage seemed to have already been done.  If, however, it
is a continuing problem in printed books, perhaps it's worth taking
up again.

Most of these processes require water, so the suggestion of keeping
things dry seemed to have some merit; however, many of the
miniatures with borders eaten away don't show any evidence of
excessive moisture.

It's a fertile field for research; I seem to remember seeing a
recent paper in German that seemed to have a lot of the answers.
Perhaps someone out there has *all* the answers to this problem?  If
not, maybe we could do a little more research.

Tom Chase
Department of Conservation and Scientific Research
Freer Gallery of Art/Arthur M. Sackler Gallery
Smithsonian Institution, Washington, D.C.

                                  ***
                  Conservation DistList Instance 9:57
                Distributed: Wednesday, January 31, 1996
                        Message Id: cdl-9-57-002
                                  ***
Received on Saturday, 27 January, 1996

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