Microchemical tests for indigo pigment

Dale Paul Kronkright <dkronkright [at] mnm__state__nm__us> writes

>We are currently conducting pigment identification tests on 17th and
>18th century Spanish Colonial objects that have been repainted
>throughout 19th and 20th centuries....

There are a couple of questions here.  Artificial indigo kicks in
about 1870. You have not stated which literature sources you have
depended upon.

I would suggest Allen's Commercial Organic Analysis, both the 1906
and 1912 editions.  There are some differences.

>Here's the catch: In indigo dyed paper
>and cotton fibers, the blue coloration is quickly lost when treated
>with 5% NaClO bleach.  The pigment particles, both in our
>contemporary controls and in the samples, do not lose coloration in
>bleach.

Smalt would not react much.

See:  The materials & Techniques of Painting, by Kurt Wehlte, (1975,
trans. Ursis Dix), pp. 149-50.

One of the questions is how certain you are about the contemporary
controls.

Jack C. Thompson
Thompson Conservation Laboratory
7549 N. Fenwick
Portland, OR  97217
503-735-3942  (voice/fax)

                                  ***
                  Conservation DistList Instance 12:74
                 Distributed: Thursday, March 18, 1999
                       Message Id: cdl-12-74-005
                                  ***