A crayon portrait is drawn with what we might call chalk, a monochrome drawing done in a range of shades from black to white, often on a gray sheet of paper. The chalk is smudged to obtain various gradations. Shadows might be added with black and highlights finally with white. Brown chalks were also commonly used for monochrome pictures.
The caption also tells us that the portrait was photographed from a picture belonging to Mrs. Bronson Alcott Pratt, probably Louise DeRevere Grant Pratt (1892-1984) wife of Bronson Alcott Pratt, Abba's great-grandson.
"A tinge of the incomprehensible lies softly around it, a field of atmosphere as if she had worked with down from an angel's wing rather than with a crayon as if the moonlight had cast a shadow on the lights."
She was not pleased with the "mantle draped about the shoulders," not in keeping with Bronson's "neat and poor" appearance. About her own portrait we find no comment. And no one but me complains about the shiny noses.