Re: archivalness of gum
On Dec 21, 2007, at 9:13 PM, Dave S wrote
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.