Digital BW, The Print

Last night I took a look at the paper Harvey (SKID Photography) has noted in
a few postings.  The paper is called, "Stability Issues and Test Methods for
Ink Jet Materials," and is by Barbara Vogt, Department of Image Engineering,
University of Applied Science, Cologne.  The url is
http://www.geocities.com/mortenryhl/index.html

I found several things to be interesting and new to me.

First, dye (and pigment) "aggregation" has significant fade-reducing
effects.  What this refers to is the dye crystals clumping together, so as
to present less surface area per volume.  The less surface area there is per
volume of colorant, the less area the oxidizing agents have to attack.  That
is really the primary advantage pigments have over dyes.  The pigment
particles are huge compared to the small dye crystals -- at least when the
dye particles are not "aggregated."  However, with aggregation, the dyes
might be able to approach pigment stability.

Dye aggregation can be induced by several methods.  One technique is to
reduce the dye's solubility via a co-solvent.  (I've been told -- but I
don't know if it is authoritative -- that Piezo ink uses a co-solvent.  I
wonder if there is such a thing as too much aggregation?)

Another way to induce aggregation is by having a less basic pH and add
salts.  (I'm told Lyson is acidic.)

But, the most important way to increase aggregation is to increase dye
concentration in the drying dot.  This is why the light/"photo" inks in the
6-ink printers fade so much faster.

Optical brighteners can affect stability.  They absorb UV and re-emit some
of the energy as visible light.  The dyes can absorb energy from the
brighteners as well as from the light.

Anti-oxidants can be added to paper to help, but they may be very ink
specific.  (I'll feed my paper some vitamin e.)

Humidity can de-aggregate dyes.  Accelerated testing can dry the paper
artificially, thus giving an impression of better longevity than would be
the case in normal display.

Air flow (like A/C) is a very significant cause of accelerated fading.
Ozone may be a cause, but it is less certain than the mere air flow.

The different dyes and pigments are affected by the various fading factors
in different amount.  For example, cyan is relatively stable under glass,
but very unstable with unfiltered light, airflow, ozone and NO2.

Very little catalytic fading was observed.  The exceptions seemed to be when
the dyes were on glossy paper and there was significant air flow.

Overall, and not surprisingly, the pigments did very well, and the dyes did
poorly in the testing.

I think most of what is in this paper has been disseminated in general
terms.  However, the two things that really stuck in my mind in terms of
their relative significance: (1)  the extent of aggregation is a very major
factor in whether an ink fades; and (2) airflow is a big cause of fading.

Paul
http://www.PaulRoark.com