Digital BW, The Print

On 01/20/2012 06:16 PM, Paul wrote:

>Keeping the particles separated is mostly done these days
> with a coating that gives the particles an electrostatic repulsion.
> I think this coating oxidizes.  I've seen old Eboni bottles pull in
> slightly, as if air is being sucked out of them.  I think it is.  The
> coatings (not the carbon) may be oxidizing.  This would mean the
> particles would not have the electrostatic repulsion needed to keep
> them from clumping.

Other mechanisms can pull your bottles in too. I used to make an ink for 
silkscreen masks that artists could paint with. Sieved medicinal carbon 
powder (a Norit quality) that only particles the size of approx 50 
micron were available. That size could just be resolved on silkscreen 
emulsions and fine mesh. Mixed that powder in PVA + alcohol. I called 
that ink liguid screen raster ink, it allowed tone variations when 
painted on PET film. Carbon particles like that have an enormous free 
surface compared to their size, the same stuff is used in gas masks to 
trap fumes, gases. The first days the fluid in the inks was absorbed 
completely by the carbon and I had to refill again and again to keep the 
mix fluid enough. Bottle pulled in too. You may have seen that effect if 
the pigment carbon particles have that characteristic too. Or the effect 
that polyethylene bottles are not as tight for certain solvents while 
air can not get through to compensate the reduced pressure in the 
bottle. Breathing on temperature changes.

Off topic sample of a silkscreen print detail:
http://www.pigment-print.com/Van%20Erven/target2.html

-- 
Met vriendelijke groeten,   Ernst
http://www.pigment-print.com/spectralplots/spectrumviz_1.htm

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