Re: archivalness of gum

[Katharine Thayer <kthayer@pacifier.com>]

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 fixed easily 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 crimson,  
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.
Katharine