The Cone Carbon Sepia (now called Carbon K7) is the only full carbon inkset produced by Jon, AFAIK. As such, it's incredibly lightfast and this is well demonstrated in the AaI&A database. But this Carbon K7 longevity performance is in a class by itself and thus not to be confused with all the other monochrome Piezography shades that must add colored pigments to neutralize the warm full carbon hue. These tints are most likely incorporating the Cone magenta pigment now showing weakness in the full Color set. Magenta colorant dropping out first leaving full carbon backbone in the print explains the typical initial green shift we see in the testing of the other Piezography ink sets followed by a slowing down and stabilization in the overall fade rate after that (in other words, a classic fade rate bifurcation due to mixing of a less stable colorant with a more stable one to create the ink's desired initial hue and chroma values).
That's what I suspected. In other words, the coloured pigments quickly fade away, leaving the pure carbon print behind as a permanent print that essentially never fades. One may as well have just printed with the pure carbon inks in the first place.
The thing is, carbon nanoparticles, like all other nanoparticles, can produce different hues, depending on the exact size of the particle. By grinding the carbon to different sizes, you could produce a range of pure carbon inks varying in hue from very warm to very cool, and they'd all be as stable as each other. Similarly, using nanoparticles of gold, silver sulfides and selenides, and other chemically-unreactive and UV-resistant materials, it's possible to produce every shade and hue from bright red, to greens and blues, all the way to deep violet - and even infrared and ultraviolet 'colours', if you wanted to.
I'd say that, for archival and ultra-stable inks at least, nanoparticle 'pigments' relying on the quantum properties of light and interactions with small particles of chemically- and UV-inert, non-catalysing substances, are the future, rather than traditional chemical pigments.
Hence, the more neutral piezography sets are only moderately light fast, not a disaster by any means,
I guess it depends what standards of lightfastness you're using. Certainly, they'll quickly change colour, to that of a pure carbon ink. But, once there, it will stay that way forever - the final image isn't going anywhere.
but one is nevertheless better off with HP, Canon, and Epson OEM black inks as far as light fade resistance is concerned when trying to print neutral or near neutral B&W. Even Epson ABW mode with much magenta and yellow added to create a heavy sepia image tint still outperforms the majority of the Piezography ink sets except the full carbon K7. Please note, I'm remarking on light fade performance. Point is already well taken and duly noted that the 6 and 7 channel Piezography image quality is a very good incentive in its own right for discerning printmakers to want to use the Piezography ink systems, but the printmaker must nevertheless weigh the perceived nuances in image quality against the moderate but not great light fade resistance of many of the Piezography blends.
As far as I am aware, the problem is the lightfastness of the coloured pigment component in the neutral and cool inks. The black carbon pigment component of the inks is more durable than anything else out there. So, instead of using HP/Canon/Epson black inks, would it not be more sensible to continue using the ultra-permanent carbon Cone inks, toning them to a more neutral colour using the ultra-stable HP inks, rather than using the OEM inks alone? That way, the colour would still slowly shift to the warm black of pure carbon (but much more slowly than the Cone non-pure-carbon inks), but, unlike with the OEM inks, once they turned warm black, they would stay that way forever without fading.
Maybe, for neutral or cool prints, the answer isn't so much in changing the ink as it is in changing the paper. A custom paper, made with 100% cotton plus titanium dioxide or baryta particles incorporated into the paper itself (not just as a fragile layer on the surface) would have a much cooler white tone than standard non-OBA papers, and would give the overall image a cooler or more neutral tone, even using the same pure-carbon pigments.
The other reason for trying to put HP inks through an Epson print head is that, although the Vivera inks are demonstrably better than the Epson Ultrachrome inks, the thermal inkjet print process is quite inferior to Epson's piezo-inkjet system. The piezo system gives much finer control of ink droplet volume, allowing Epson heads to produce as many as seven different dot sizes (in Mimaki and Roland printers), allowing for smooth tonality, expanded gamut (particularly in the light tones) and true 1440dpi output all the way into the highlights - something that just isn't possible using thermal inkjet technology. Not only would putting Vivera inks through a piezo head, if it were possible, give you greater permanence than Epson inks, but, in theory at least, it should significantly improve the colour gamut of HP inks.