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These were done for a figure drawing class my freshman year of undergrad at UPenn. They were usually done on 18x24" paper with charcoal and conte crayon. We had one or two three hour class sessions to complete these from observation.
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The first image above was done with graphite on paper in an hour or so from observation. The second image is of a Native American woman from the Pequot tribe my father worked for. I did that one in an hour or so from a photograph for reference.