U of S | Mailing List Archive | alt-photo-process-l | Re: archivalness of gum

Re: archivalness of gum

On Dec 22, 2007, at 8:53 AM, Dave S wrote:

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.

But surely it has to be accurate in at least one case, in order not to be completely nonsensical. Your original statement was "Today's indigo is a mixture of prussian blue and quinacridone red." My question wasn't whether you know it to be 100% accurate in all cases (although to make logical sense the statement would have to be assumed to be generally true); my question was whether you know of ANY such paint. If there's no paint called "indigo" that consists of prussian blue and quinacridone red, then the statement not only isn't "100% accurate in all cases" but is categorically false.

Hue names are useless for gum printing, because they tell you nothing about the pigments involved; if you don't know and understand the pigments you're using, you're losing out on one of the crucial control variables of gum printing in my experience. So discussing colors in terms of hue names just confuses and misleads people, and I can't see any point in it.
Katharine






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.


Dave


-----Original Message-----
From: Katharine Thayer [mailto:kthayer@pacifier.com]
Sent: Saturday, December 22, 2007 10:23 AM
To: alt-photo-process-l@usask.ca
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
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