Digital BW, The Print

Peter Nelson wrote:
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Paul Roark" 
> <paul.roark@v...> wrote:
> 
>>>>...Ironically, the smaller the particle, the
>>>>larger the surface area to volume ratio is.  This larger
>>>>surface-area-to-volume causes faster oxidation, which is the 
>>>>cause of most fading.
> 
> 
> Oxidation is a chemical process, not a physical one, so why would 
> the surface area to volume ratio matter?    For a pigment particle 
> to fade only the molecules at the surface need to change color.   
> The fact that a faded pigment molecule on a small particle might 
> have a zillion unoxidized molecules underneath it and one on a big 
> particle might have TEN zillion unoxidized molecules underneath it 
> doesn't seem all that significant since it's only the one on top 
> that we see, unless you're implying that the molecules have 
> motility.  (are you?)

I agree with most of what you write but the assumption that the 
pigment particles in inkjet inks are opaque, inkjet pigment inks 
are opaque and inkjet pigment ink layers are opaque isn't 
correct.  If you accept that they are all more or less 
transparent then it isn't just the top of the ink layer that does 
the coloring, it isn't just the outer molecule layer of the 
particle that does the coloring but also the mass of the pigment 
particles. Then the total surface of the particles to the total 
volume of the particles plays a role in the fading properties. 
The larger total surface of the smaller particles will react 
faster to any possible reagent and so more molecules of the mass 
will loose their unique coloring power faster and become 
something else, it isn't that important if the shift occurs in 
tone, hue or saturation, it is a shift.

Surface area to volume area matters a lot in a chemical process. 
Adding fresh aluminium powder to firework certainly gives more 
reactive effect than adding an aluminium slab.

The color pigments of inkjet inks are much more related to the 
dyes that are also used in inkjet inks than to the opaque 
"grinded rock" mineral pigments.

A bit off topic in this thread but I have not seen a reference so 
far: the opaqueness of layers is easier to achieve with non 
uniform particles (different size, form) than with totally 
uniform particles. That's observed with paints, photographic 
emulsions, printed layers etc.  I wonder how uniform the carbon 
pigment particles are as used in quad inks and where the limit is 
where no extra density is achieved (where the reflectance of the 
pigment is the only thing measured and no white substrate plays a 
role in the reflectance).  This is quite theoretical and doesn't 
have much influence on the range of 0-95% but may play a role in 
the darkest shadows. The more uniform the particle size is the 
more transparent the ink will be (not like a dye though). For 
quad printing transparency can't be bad, opaqueness doesn't mean 
a higher density just that the substrate isn't taking part in the 
reflectance anymore, a transparent black ink can achieve more 
density than an opaque grey ink.



Ernst