Digital BW, The Print

>Surface area? With the particles spread that much thinner over the
> same area, there would be more particles exposed to the light/air.

Surface area of a pigment is a critical factor.  However, most of the
surface areas of the pigments used appear to be about the same within any
class of pigments.  Limits are set by the specific gravity of the particle,
the viscosity of the carrier and other factors.  Carbon is actually one of
the heaviest used, and thus it must also be one of the smallest to stay in
suspension.  However, it's so tough, that is OK.  But within carbon, the
smaller particles are, I believe, the more neutral ones. 

The reputation of the lighter inks as faster-fading seems to have applied to
dyes more than to pigments.  With pigments, the surface area of the particle
is the same no matter what the distribution of the particles.  That is, when
I dilute LK to make a lighter ink, the particle size does not change.  With
dissolved dyes, when the more dilute ones crystallized on the paper fibers,
the lighter inks ended up with thinner coatings.   

However, the dilution of the lighter inks still has some application that is
analogous to the thinner coatings of dyes.  There seems to be quite a bit of
evidence that pigments in the middle of a pile are protected from the
"breeze" and oxidize more slowly, even though the pile of pigments is such
that oxygen can easily reach the surface of the particles.

It seems odd at first, but there appears to be quite a lot of evidence that
the air speed at the particle surface affects the oxidation rate.  On the
other hand, how many of us have blown on a bed of coals to get a fire going
again?  That may even be a significant part of what glass over the image
does.

With the UT sepia toners, it's obvious why they are the weaklings of the
inkset -- they have yellow and R800 red in them.  The red is presumably
better than the old magenta, but it's still not in the carbon or cyan class.
(The R800 blue used in the neutral ink may be a phthalocyanine like the
cyan.)  I never use sepia for serious printing.  Carbon is the warm end of
what I use for work I want to last.  But, IJM's "sepia" was supposed to be a
pure carbon.  So, I have no idea why it didn't perform better.

Paul
www.PaulRoark.com 









> -----Original Message-----
> From: DigitalBlackandWhiteThePrint@yahoogroups.com
> [mailto:DigitalBlackandWhiteThePrint@yahoogroups.com] On Behalf Of
> dfaprinting
> Sent: Sunday, July 24, 2005 5:22 PM
> To: DigitalBlackandWhiteThePrint@yahoogroups.com
> Subject: [Digital BW] Re: Removing color content(was:Wilhelm 2400 data )
> 
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Paul Roark"
> <paul.roark@v...> wrote:
> >
> >
> > > ... Much to Bill Bergh's dismay, the IJM's Carbon
> > > Sepia inkset, which he designed as supposedly pure carbon, fell
> > > slightly short of the other sets that do have color pigs mixed
> into
> > > them for hue, in their RIT tests. There seemed to be some positive
> > > symbiotic effect on longevity.
> > >...
> >
> > I never understood that either.  With the UT inksets the pure
> carbon is the
> > strongest in my fade testing.  Wilhelm also seems to be endorsing
> the
> > concept that carbon is the strongest of the pigments (relative to
> those in
> > the UC inkset).  That said, the cyan is extremely strong also.
> >
> > Paul
> > www.PaulRoark.com
> 
> Surface area? With the particles spread that much thinner over the
> same area, there would be more particles exposed to the light/air.
> Well that's my guess anyway. Doesn't really explain why a mix works
> better.
> 
> Also brings up the question of BO prints. Do the lighter patches fade
> more quickly than the darker patches? That would be one way to check
> the surface area theory.
> 
> 
> 
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