Digital BW, The Print

I just read a paper entitled, "Preservation of Ink Jet Hardcopies," (M.
Jurgens, 1999 http://www.knaw.nl/ecpa/PUBL/InkJ-300.pdf ) that has a number
of interesting facts that I had not seen elsewhere.  While the article is
aimed at conservators, it has good explanations of many facets of the
technologies we are using.  I've put some of my notes below.  My comments
are in brackets; page numbers are in parentheses.

I Ink Jet Printing [a general overview]

The brighteners used in many coatings are titanium dioxide pigments. (20)

II. Materials Analysis (29)

Pigments and Dyes (31)

The majority of pigments used in ink jet ink today are organic.  Organic
pigments are produced by chemically "stacking" organic dyes to form larger
particles.  Formerly water soluble dyes are stacked like wet glass plates on
top of each other, forming a crystal-like structure that precipitates out of
the aqueous solution. (31)

[This could explain the seeming confusion about whether the pigments we use
contain dyes.  They may be dyes that are simply "stacked."  Nowhere did I
see a description of a coated carbon particle.  I'm begining to wonder if
the color pigments have any carbon in them at all.]

Smaller pigment particles have better color, gloss and transparency but
worse lightfastness. (32)  [UC inks come to mind.]

The harder a pigment particle is, the more abrasive it will be in the print
head and the more it might endanger the integrity of the fragile nozzle
opening.  Organic pigments are generally softer than inorganic ones. (32)
[Metal added to black to make it darker might not be the best solution.]

Organic Colorant Classes (35)

Water soluble azo dyes will have a sulfonic acid group (-S03H).  Azo dyes
are the most common dye molecule types.  [Imagine what that acid does when
it hits an alkaline buffer.]

Copper phtalocyanine cyan dyes are counted among the most brilliant and
stable of dyes.  [The cyan pigment might be a "stack" of these.  It appears
to be the most stable in my fade testing.  The key to a good midtone, in my
view, is a pure carbon particle toned with cyan pigment.]

Chemically inert pigments employed in ink jet printing include carbon black,
diarylide yellows, metal salt reds, pthalocyamine blues, and mono azos.
[I'm not sure "chemically inert" implies resistance to being broken by UV
and visible light.]

Ingredients of Liquid Inks

Colorant

Pigment particles are large enough to scatter white light on their surface,
whereas dye molecules are much smaller than the wavelengths of visible light
and thus do not scatter any.

Substrates

Paper Media (38)

Bond Paper

The most common internal sizing is that of the tree resin called rosin,
which is mostly an organic acid. (39)

Archival Paper (40)

While the most stable paper is made from cotton, chemically purified wood
pulp, such as that used in fiber-based B&W photo paper, is also suitable for
long term archivability.

The use of rosin sizing would be counterproductive.  These papers tend to be
internally sized with synthetic alkaline substances ... buffers ... most
common calcium carbonate.

Coated Paper

These have better image quality, but ageing characteristics have not been
explored thoroughly. (41)

Paper-Plastic Combinations

RC based papers are likely to develop problems similar to those of
photographic RC papers.

Receptor Coatings (43)

[Typical single layer coatings include a number of ingredients.  These can
be complex mixes, which, of course, means that the interactions with inks
and other chemicals might also be very complex and unpredictable in the long
run.]

Some matte coatings are almost entirely silica particles.

Some silica coatings might be slightly acidic. (44)

Laminates and Coatings (45)

It is generally recommended not to laminate or coat valuable prints, since
the danger of unwanted interaction between the laminate and the image
forming dyes is present and as yet widely untested.  Coatings are
practically non-reversible.

Varnishes are vegetable oil formulations that air-dry.  Lacquers are
solvent-based and dry to a high gloss, but they often lack good adhesion and
flexibility.

Lamination of Longevity?

Lamination at one time was used to preserve papers, but has fallen out of
favor.  Although lamination was controversial for the beginning, later
critics mainly only attacked the use of insufficient deacidification rather
than the lamination itself. (46)

Some documents had become brittle inside of the lamination, perhaps a result
of cellulose acetate [the film used] deterioration.

Lamination of Ink Jet Prints

For advertising banners, etc., the benefits of laminates or coatings are
well suited to the purpose -- short term protection.

A fin art hardcopy that will either be exhibited in controlled lighting or
be carefully housed will not necessarily need the brute protection of a
laminate.  In order to avoid any as yet unforeseen complications that might
arise with the long term intimate contact between a plastic and the ink, not
to mention the largely irreversible character of the physical fusion, it
might be wise to avoid laminates or coatings altogether.

If protection against physical damage is needed, then it might be
appropriate.

Preservation Issues (48)

The main concern in long term stability of ink jet prints is their
relatively weak light-fastness.

Light and UV-Fastness

Dyes will not only fade due to UV, but also due to the effects of visible
light. (52)

UV radiation in the 200 - 400 nm range is primarily what causes
photochemical reactions and bond ruptures in organic compounds.

Reactions between the colorants and the substances in the coatings are
typically of reductive and oxidative nature.

Colorant Fading

The photolytic deterioration of organic colorants is the main factor.

If a double bond is broken, the conjugation length of the molecule is
reduced, and the color absorption capabilities are disturbed, either leading
to a loss of its absorptive and reflective capabilities which is perceived
as fading, or leading to a shift in wavelength absorption which results in a
color shift.  The complete rupture of a bond (cleavage), such as  is
possible with azo groups (-N=N-) leads to the division of the molecule into
two smaller ones, which will each have shorter conjugation lengths. (53)

[This color shift may be part of the warming we see with most dyes and
pirments used in B&W pritning.]

Magenta dyes have continued to be the least light stable. [My recent
UltraChrome and MIS color pigment test seems consistent with this.]

Magenta and black dyes have long posed stability problems.

The size of the pigment stacks assist in dissipating the energy.   Also,
already faded surface molecules can act as light barriers for the internal
molecules of the pigment crystal. (54)  [Perhaps this is part of the reason
there seems to be faster fading of pigments at first.]

Pigments vary in their lightfastness depending on the internal structure of
their particles.  Large crystals free from defects will probably have a
higher lightfastness.

Substrates and Coatings

When the presence of the ink receptor coating with its complex mixture of
binders and additives is taken into account, the prospect of permanence
testing and research becomes daunting.  It is partly for this reason that it
is largely recommended to print documents that are to be valued in the long
term on thorough tested, well known, high quality, and uncoated substrates
such as 100% rag content artists' paper.

An alkaline environment might increase the lightfastness of a print, but it
could also reduce the color gamut.  As the image quality is the current
driving force in sales, manufacturers are reluctant to trade off color for
permanence.  [So, it might be that we would be better off with a more
alkaline surface -- contrary to the color printers.]

Little is known of interactions between the coating and the substrate in
environments of high humidity and temperature.  Problems have arisen with
the structural stability of coated vinyl substrates; due to the smooth
surface of the vinyl and the tendency for integrated plasticisers to migrate
to the film surface, the coating can partially fail to adhere, which can
lead to flaking. (54-55)

Laminates

For advertising, laminates will greatly enhance weatherability.

Water vapor and oxygen will diffuse through the plastic in small amounts
over a long period.

Lamination can only be recommended for prints that have no high monetary or
artistic value.

The overall understanding is that, thought the protective coatings may
enhance the durability in the short run and in extreme environments, the
long term consequences cannot be completely predicted.

One-sided coating can be problematic in that substances access the reverse
side and diffuse through the substrate to form stains that are visible on
the front, but inaccessible due to the plastic coating.

If a print is to be sealed, be sure it is completely dry -- 24 hours.

De-lamination has been successful. (56)

Spray-on lacquer used in photography consists of cellulose nitrate and
introduces all the aging related problems associated with that chemical,
including yellowing and production of nitric acid gases.

UV absorbers integrated into the lacquers and the cellulose nitrate were
also responsible for these reactions.

Recommendations (57)

Inkjet hardcopies can be extremely complex objects.

As inks become more and more light stable, the generation of hardcopies from
the last ten years will become one know to be condemned to rapid fading, but
it would seem that this is a phase that well eventually come to an end.
[But will any of us live long enough to see it?]

Paul
http://www.PaulRoark.com