RE: archivalness of gum
It went through different generations. Some of them has Prussian Blue only.
Some added red. Later some uses thalo blue instead of prussian blue. The
fact is that real indigo (from indigo plant) has different hues too. Indigo
is the substance also used in lithmus paper. The hue changes from purplish
red to purplish blue depending on acidity/alkalinity. That is why in
gardening, when you deal with certain purplish-color flowers, you can change
the hue by altering the acidity/alkalinity of the soil.
That is why when indigo was substituted by synthetics, some people felt so
cool because the synthetics looked exactly like indigo (to them), but others
disliked it because they thought they were so different. The fact is
depending on where they bought (or made as some Chinese artists still made
their own indigo color in the beginning or last century), their indigo could
have different hues.
When I said indigo was a mixture of prussian blue and quinacridone red, I
didn't mean it to be that technical and 100% accurate in all cases. The
point is simply I was concurring with Judy that indigo is nowadays a
hue/tone name. As a mix, each manufacturer is free to use different pigments
in different proportions. One needs to check the content.
As to listing pigment by pigment numbers, that is always true in another
context. But here we are not talking about USING INDIGO PIGMENT. In fact,
the context is the opposite. We are talking about indigo not being a pigment
in manufactured watercolors but a hue/tone name.
> -----Original Message-----
> From: Katharine Thayer [mailto:email@example.com]
> Sent: Saturday, December 22, 2007 10:23 AM
> To: firstname.lastname@example.org
> 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
> 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
> 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.