Why Does the Mona Lisa Have No Outlines? Thirty Layers of Almost Nothing

Painting before this approach defined a form by drawing its edge and filling it, so a face met the background at a line, however carefully that line was softened. The technique removes the line entirely, so that a form emerges from shadow by gradual change of tone with no point at which one thing becomes another. Leonardo described the aim as working without lines or borders, in the manner of smoke, which is what the Italian word means. Applied to a face, it makes the corners of the mouth and eyes indeterminate, which is why expressions painted this way seem to change as the viewer looks and why the most famous example has been argued about for five centuries.

The method is a matter of patience rather than of any secret:

• •Build tone in translucent glazes rather than opaque paint
• •Make each layer so thin it barely alters what is beneath
• •Let each dry fully before applying the next, which takes days
• •Accumulate depth over dozens of layers rather than one
• •Blend while wet with a soft brush or, reportedly, fingers
• •Avoid any stroke that reads as a separate mark

Modern examination has measured what was previously guessed at, and the numbers are striking. A study published in 2010 used a technique that fires X-rays at the surface and reads what comes back, allowing layers to be counted without taking a sample, and found glazes on the faces of several Leonardo paintings that are between one and two micrometres thick, with total layer structures of around thirty to forty micrometres. A human hair is roughly seventy micrometres across. The analysis also showed that the glazes contain very little pigment in a great deal of medium, which is what makes them transparent, and that the composition varies between works, indicating continued experiment rather than a fixed recipe.

The term is used loosely enough that it is worth separating from three things it is often confused with. It is not the same as aerial perspective, which is the practice of painting distant things paler and bluer to suggest depth, though Leonardo used both and they reinforce each other. It is not simply blending, since a soft edge produced by working wet paint together in one sitting looks different and reads as a smear rather than as atmosphere. And it is not chiaroscuro, which concerns the arrangement of strong light and dark across a composition and can be perfectly hard-edged. The technique here concerns the transition itself, not the tonal scheme it sits inside.

A technique this effective might be expected to be universal, and it never was, for reasons that are mostly practical. It is extraordinarily slow, since each layer must dry before the next, so a single head can take months of intermittent work, which suits an artist who was notoriously unable to finish anything and suits almost nobody paid by the commission. It requires oil paint and a slow-drying medium, so it was unavailable in fresco or tempera. It produces a restricted range of effects, being suited to soft flesh and atmosphere and useless for crisp drapery or bright surfaces. And painters with a different temperament wanted visible handling, which the method forbids entirely.

Building tone in dozens of transparent glazes, each a micrometre or two thick, lets a form emerge from shadow with no edge anywhere, which is why faces painted this way have expressions that refuse to settle. X-ray analysis published in 2010 measured the layers without sampling and found total structures thinner than half a human hair. The method never became standard because it is desperately slow and suits only a narrow range of effects.