Hi Simon,

Thank you for contacting Mearto with your appraisal inquiry.
I've found only a few other figural sketches by Hartley (most are landscapes) and none in charcoal, at least not according to AskArt.
Based on the photos and information provided, and subject to examination, this is:

Marsden Hartley (American, 1877-1943)
Sketch of a nude
charcoal on paper
signed bottom left
sheet 17 by 12 inches
PROVENANCE: Acquired from a dealer in Long Beach, Calif.
$5,000-8,000*
*represents a fair-market value for auction purposes; retail or asking price may vary.

Please let us know if you have additional items to appraise, and thank you again for using Mearto.