Green streaks on oil pastels

An artist client has brought me something I have not seen before,
does someone have an insight to the problem.

On two of 5 oil pastels the artist still retains, thin green streaks
(1/4" X 3 "; 3/16" X 2"; 1/8" X 1") have appeared over predominately
heavily applied areas.  The streaks do not appear to have a natural
source because they follow the media-application strokes and are not
round or blotchy.  The magnified surface of the streak is lumpy and
has an oily sheen.  The brand of oil pastel's is Caran d' Ache,
Neopastels.

The source may be an exudate of the oil media, but this is not
clear.  A likely source for the color, a copper compound, would
appear (checked with Customer Service at a big Arts Materials
Supplier) to be inconsistent with oil media intended not to harden
on the shelf.  When removed from the frame, the paintings smelled
heavily of linseed oil. This does not fit with the concept of a
non-drying oil, but the smell resembled its characteristic odor.

The streaks are not confined to one color and have been observed in
white, purple, yellow, brown and blue.  A green streak did not
change color when thinned or partially removed from the surface of
the painting.  A swab used to remove a portion of a streak, dry,
appeared not to pick up the green color, but did turn brown. This
could be due, however, to the mixture of the purple pigment and
green streak.  Two conditions seem consistent: 1) the streaks appear
to be over the thicker portions of the media and are less common in
hand-rubbed (thinner) areas and 2) most of the streaks rested on a
direct crayon-applied surface, showing the groves of direct
application.

Thorough examination revealed that in some of the broad white areas,
a very light green haze "seems" to be developing below the surface,
within the media layer.  This occurrence is faint and difficult to
confirm, but two viewers seemed to observe its presence.

The oil pastels are part of a series of 15 that were painted in a
variety of locations, 1-2 years ago.  The artist has five of the
images and only two show the green streaks.   The two effected
paintings were done in dry hot climates using the same materials and
techniques.  All the paintings were executed using the same brand of
oil pastels, on the same medium weight Arches paper and over a one
year period using the same techniques by the artist.  They are in
glazed frames on an outside masonry (red brick) wall that gets sun
(on the image) in the morning. The artist lives about one mile from
the ocean.  This suggests a possible heat or moisture based cause.
The artist does not know if works held by the owners are showing the
same condition.

Has anyone seen a similar problem?  Has anyone seen problems with
discolorations of the surface of oil pastels of a different type
(excluding dust and dirt accumulation)?  Has anyone ever seen a
green discoloration on similar artwork (excluding copper-based
materials)? Did the green discolorations have a defined source?
Thanks!

Timothy Vitale
Conservator and Consultant
408-684-2731 office
408-335-2541 studio
Fax: 408-684-1847

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                  Conservation DistList Instance 11:28
                Distributed: Tuesday, September 23, 1997
                       Message Id: cdl-11-28-006
                                  ***