Listen to part of a lecture in an art history class. The class is discussing Leonardo da Vinci.(male professor) Last class, we discussed the symbolism found in the subject matter of Leonardo da Vinci’s paintings.Today, I wanna focus not on the content of his work, but on specific painting techniques that this famous Renaissance painter used.We\'ll start with a technique that Leonardo used to produce various gradation of shading——a technique called sfumato.Sfumato is an Italian word, meaning smoky or smoked.Sfumato is an attempt Leonardo da Vinci wrote in his notebooks to suddenly represent the soft smoky, infinite gradation from dark shadow to light by delicately and gently merging darkness and light.So, sfumato, according to Leonardo is an effect in which there are no lines or borders. In the same way as the smokers, no lines or borders.In other words, he totally avoided the use of sharp, harsh outlines or contours to show shading.To help him accomplish a subtle, smoky merging of darkness and light, he focused on eliminating signs of the act of painting itself, like visible brushstrokes.This was not a focus of all Italian Renaissance painters. For example, the great painter Titian, Titian’s loosen in exact brushstrokes are clearly visible, especially in his later work.He sometimes painted with thick paint, thick enough to leave impressions of his brushstrokes, painting with thick paint. That\'s the impasto technique, impasto meaning paste.Titian’s style of painting wasn\'t the ideal Renaissance style.Ideally, Renaissance painters sought to create a three-dimensional illusion, a precise reflection of the world, like a photograph can do these days.They sought to eliminate any signs that the painter had been there at all. And with sfumato, that\'s what Leonardo da Vinci was trying to do.So how did he actually achieve subtle gradation in shading without visible brushstrokes?Well, part of it is, unlike Titian, he used very thin layers of paint.Each layer was a nearly transparent glaze, a glaze with only a small amount of a single pigment in it.That is, he did not directly mix different pigments together, didn\'t blend colors together before applying them to the canvas.Instead, he applied a single pigment glaze, let it dry, then applied another, one on top of another, and so on and so on. The result is an image with remarkable depth.So, we\'ve known all that general information about Leonardo’s sfumato technique for a long time.But researchers wanted specifics. How many layers of glazes? Exactly how thin? What ingredients did he use in these glazes?Sometimes researchers take tiny samples right from a painting. Analysis of these samples can provide information on the thickness and composition of the layers.But can you imagine taking samples from the face of the Mona Lisa? From a priceless painting like that?The shadows on the faces of Leonardo’s subjects are the most remarkable examples of the use of sfumato.But, you know, that\'s really the last place you\'d ever consider removing a sample from.So, what can be done? Well recently researchers have developed a non-invasive technique.Instead of removing samples, they’re using something called X-Ray fluorescence spectroscopy, or XRF.XRF involves exposing a painting to X-Rays. Without getting too technical, it\'s a way of seeing beneath the surface without harming the painting.Researchers used XRF to analyze faces in 7 of Leonardo da Vinci’s paintings, including the Mona Lisa, and they found some answers to their questions.First of all, they found that Leonardo’s layers could be almost unbelievably thin, as thin as merely 2 micrometers, which is microscopically thin.He may have even used his fingertips to apply layers rather than a brush to help control the thickness. And he\'d lay on up to 30 layers to create the darkest areas.Now, remember that he had to let each of these layers dry before adding the next one.Several days, weeks or even months for each layer. No wonder Leonardo was a slow painter.The 7 paintings studied were from various stages of Leonardo da Vinci’s career, expanding over 40 years.The 2 earliest work show evidence that Leonardo was using mixed paints in the faces.I mean, the analysis reveals that the shading in the faces was accomplished by directly mixing various pigments.But with the later paintings, well, it appears that as time went on, Leonardo used single pigment glazes more and more.And what\'s the significance of that? Well, we can see that he kept experimenting with glazes with this sfumato technique.