[Not necessarily about this painting just notes on painting in general from Sargent.]
• Painting is an interpretation of tone.
• Keep the planes free and simple, drawing a full brush down the whole contour of a cheek.
• Always paint one thing into another and not side by side until they touch.
• The thicker your paint — the more your color flows.
• Simplify, omit all but the most essential elements — values, especially the values. You must clarify the values.
• The secret of painting is in the half tone of each plane, in economizing the accents and in the handling of the lights.
• You begin with the middle tones and work up from it .... so that you deal last with your lightest lights and darkest darks, you avoid false accents
• Paint in all the half tones and the generalized passages quite thick.
• It is impossible for a painter to try to repaint a head where the understructure was wrong.
— John Singer Sargent