Re: Direct Carbon Potential.
Part of the reason Direct Carbon may not seem so hot today may be a change in photo practice. In the 1920s, for instance, most camera formats were large enough so the original camera negs could be used to print. Then -- what we take for granted -- "miniature" cameras became the norm. I'm supposing of course that most direct carbon requires a contact neg (tho as I recall, the Fresson lab was using enlargers?). So maybe Nadeau & company found the extra trouble -- and complications -- of making enlarged neatives took some of the joy -- and profit -- from the operation?
I also think that today's improvements in inkjet printing -- colors, versatility, ease, economy and archivality -- will obviate the need for direct carbon (except for what I've called the wish/need for labor-intensive art).