Benzimidazolone yellow in watercolor paint

I use only # 1 ASTM lightfastness rated paints in my watercolor
illustrations.  A paint manufacturer rates their Azo
Yellow/Benzimidazolone Yellow (PY 151) as ASTM # 1.  My most recent
botanical illustration was primarily painted with this paint. I was
sent a copy of a page from the 2001-2002 Edition of the "Wilcox
Guide to the Best Watercolor Paints" stating that PY 151 is not
lightfast when used as a watercolor paint.

The paint manufacturer states that the ASTM has revised their
standards and does rate PY 151 as #1.  Do any of you have the most
recent ASTM guidelines on pigment lightfastness and if so, could you
let me know about the use of PY 151 as a watercolor paint?

Carol Wickenhiser-Schaudt
Botanical and Scientific Illustrator
Nautilus Fine Art Studio
21602 Park York Drive
Katy, Tx  77450


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                  Conservation DistList Instance 14:37
                 Distributed: Tuesday, January 9, 2001
                       Message Id: cdl-14-37-021
                                  ***