Digital BW, The Print

Here is an excerpt from a short article that mentions a type of 
carbon fading that wouldn't be detected by Wilhelm-type 
longevity testing:

"Sometime in the 1440's or 1450's Johannes Gensfleisch zur Laden, 
better known as Johannes Gutenberg, invented a method of printing 
from movable metal type. One component of this technological leap 
was an oil-based ink that, unlike earlier water-based preparations, 
would adhere to metal. Historians have long assumed that Gutenberg's 
ink was a mixture of linseed oil and lampblack, or soot, a formula 
known to have been common in the 16th century.

Richard N. Schwab and Thomas A. Cahill of the University of 
California, Davis have now shown the assumption to be wrong. 
Gutenberg employed a superior formula consisting largely of 
copper and lead. The workers made that discovery by analyzing 
most of the pages in one copy of Gutenberg's masterpiece, the 
42-line Bible. The analysis involved exposing the pages to a 
proton beam in a cyclotron. Energetic protons excite molecules 
in the ink, causing them to fluoresce in the X-ray range; each 
element emits a characteristic spectrum, and the intensity of the 
radiation is a measure of the abundance of the element.

The use of metals instead of soot as a pigment was probably 
inspired by oil painting. It helps to explain why Gutenberg's 
print is still glossy and black after five centuries, whereas many 
later works have faded badly. Copper and lead oxides are stable, but 
the carbon in lampblack oxidizes to carbon monoxide and so tends 
to evanesce."

Scientific American, February 1985, page 63.

See also: Schwab 1983 Schwab, Richard et al. "Cyclotron Analysis 
of the Ink in the 42-line Bible". /Papers of the Bibliographical 
Society of America/..Vol.77, No.2 (1983). pp. 285-315.