From: Katharine Thayer [mailto:firstname.lastname@example.org]
Sent: Saturday, December 22, 2007 10:23 AM
Subject: Re: archivalness of gum
On Dec 21, 2007, at 9:13 PM, Dave S wrote
Same with indigo which originally was made from indigo
plant. It has a
beautiful purple tone, but it is also fugitive. Today's indigo is a
mixture of prussian blue and quinacridone red. Those who
are used to
true indigo sometimes complain that the synthetic indigo is too
colorful (the true indigo is more muted), but this can be
by adding just a touch of black; so today's indigo is also
a hue name.
Well, yes and no. :--)
The original indigo that was made from indigo plant and was
imported to Britain from India essentially disappeared
around the turn of the 20th century when it was replaced by a
synthetic pigment, PB 66, also called indigo, that was
supposed to be more permanent than the original indigo, but
proved not to be. But at least it was easier to
obtain. PB 66 was in use until fairly recently in artist's paints.
Winsor & Newton was the last to use PB 66 in watercolor
paints but discontinued it last year or the year before, so
now I no longer have to caution people to be extra cautious
about paints named "indigo" to make sure they don't contain
PB66, an essentially fugitive pigment.
Now all paints named "indigo" are convenience mixtures, as
you say, but it's not true that they all contain the pigments
you name above; in fact in a quick look through my sources I
can't find any made of
prussian blue and quinacridone red. Do you know of such a paint?
Most "indigoes" are lamp black or carbon black mixed with a blue;
the blues vary all over the place from pthalo to indanthrone (PB60)
to prussian. For a while a few brands were adding alizarin
but none of them do now because of its impermanent nature.
Winsor & Newton adds beta quinacridone to pthalo and lamp
black to comprise their "indigo" but beta quinacridone isn't
quinacridone red (PB 209), or even quinacridone rose (gamma
quinacridone); it's a deep bluish purple quinacridone.
This discussion just underscores how important it is to
identify pigments by their number; it's the only unambiguous
way of referring to pigments.