what is a "sanguine carbon print" ?

I'm just reading Margaret Harker's history of the Linked Ring, and a few of the plates are labelled Sanguine carbon print.

I can't find any reference to this as a separate process in either of my reference books (Farber, Crawford) or online, but I also know there were dozens of different carbon processes.

It ("sanguine") is a term applied to a reddish-brown colour, so I surmise that a "sanguine carbon" would be one made with a red-brown pigment rather than black.

Anyone know a definitive answer?

I think it refers to the colour of the print. Reddish.

for those of us, who went to art school, and HAD to use conte crayons for drawing class, will remember there was a choice of black, bister, and sanguine. sanguine was "used" for . . . to mimic the color or "hue" (if you were very "artsy") of Watteau drawings!!!

I think they call Michelangelo's and Da Vinci's reddish chalk , sanguine. Many used before and after them. May be Italy specific rock but very famous in art world.

I was wondering what the connection was (if any) to "being sanguine" or optimistic. Here it is from the OED:

sanguine |ˈsaNGgwin|
adjective
1 optimistic or positive, esp. in an apparently bad or difficult situation: he is sanguine about prospects for the global economy. the committee takes a more sanguine view.
• (in medieval science and medicine) of or having the constitution associated with the predominance of blood among the bodily humors, supposedly marked by a ruddy complexion and an optimistic disposition.
• archaic (of the complexion) florid; ruddy.
2 literary & Heraldry blood-red.
3 archaic bloody or bloodthirsty.
noun
a blood-red color.
• a deep red-brown crayon or pencil containing iron oxide.
• Heraldry a blood-red stain used in blazoning.
DERIVATIVES
sanguinely adverb,
sanguineness noun
ORIGIN Middle English: from Old French sanguin(e)‘blood-red,’ from Latin sanguineus ‘of blood,’ from sanguis, sanguin- ‘blood.’