Microchemical tests for indigo pigment

We are currently conducting pigment identification tests on 17th and
18th century Spanish Colonial objects that have been repainted
throughout 19th and 20th centuries.  In investigating ground indigo
pigment, (C16H10N2O2) we found that it decomposes nicely in nitric
acid, forming yellow (isatin) precipitate and is insoluble in
diluted HCl, as cited in the literature. Optical properties are the
typical coppery-luster blue in reflected in polarized light
microscopy  showed that the pigments have an IR slightly above 1.66
and are weakly pleochroic.   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.  The pigments are not attached to carriers but rather appear
to be the ground up dry dye material.  Why should the pigment not
beach out.  We have freed the pigments from their binders with both
DMF and 3M NaOH and still, no bleaching of the pigment particle
coloration takes place.  I feel like there is some piece of critical
information regarding the formation of the ground pigment particles
from the dry dye stuff that we are missing.  Or is there another
blue pigment that does not lose color or dissolve in HCl and turns
yellowish and dissolves in nitric?

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                  Conservation DistList Instance 12:73
                  Distributed: Friday, March 12, 1999
                       Message Id: cdl-12-73-006
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