It's done with a soft, fairly long-haired brush (squirrel hair?), one which can spread to do broad fluid strokes, but also be brought to a fine point to draw more precise lines.
It uses brown ink with a little white body colour (it's there on the sitter's arm and her face, just making the dark ink a little less intense so that it isn't the same value as the arm of the couch, thus avoiding the image jumping around too many marks of the same colour). There's a mixture of marks, in that some are wet and fluid (around the eyes and under the chest, the arm of the couch), but some are dry and scumbled.
In the background the ink has been thinned to make it paler so that it can be moved over the paper in a wash. Some body colour has also been applied on top of the wash to knock the background right into the back, make the darkness of the figure punchier.
It's quickly done with confident, loose, expressive, almost calligraphic strokes. It's almost impressionistic, as there isn't a lot of detail, and yet there's all the detail that you need.
It's very intimate. The figure is obviously someone the artist knows well, not just a model. There's an air of concern about the figure - not only as if she's got something on her mind, but also as if the artist has concern for her. The background hangs over her head like a dark cloud. So it has an emotional punch, something very human about it.
What has always fascinated me about this little piece is that so few strokes can be used and yet it says everything it needs to about volume, the weight and the different textures, but also goes beyond mere representation. All that in a few licks of ink.
So who's the modern impressionistic genius? Answer tomorrow!