According to Vasari, Leonardo made clay figures which he covered with a soft linen dipped in wet clay. Using black and white pigment applied with the point of a brush he would execute studies such as this on fine linen with “great patience”. Strictly speaking, this is not a preparatory study, but an exercise in modelling in light and shade. In its sculptural effect, Leonardo has achieved a timeless perfection that remains unsurpassed to this day.

Realising the importance of light and shade in painting, Leonardo planned to write a treatise on the subject, and outlined its proposed contents in his notes. Amongst other issues it would deal with the categorisation of shadows, an examination of their effects on colour and the effects of the colour on shadows. It would also examine how shadows are conditioned by distance from a given light source.

Given the unimaginable complexities of the subject it is not surprising that Leonardo never did complete the treatise. Nonetheless, he continued to observe light in nature throughout his lifetime and filled hundreds of pages of his notebooks with experiments and observations on the subject.In

Leonardo's words
Shadows appear to me to be of supreme importance in perspective, because without them opaque and solid bodies will be ill-defined; that which is contained within their outlines and their boundaries themselves will be ill understood unless they are shown against a background of a different tone from themselves.