As Vermeer left behind no drawings or preliminary studies, our information about his artistic process can only be gleaned from the paintings themselves. In this regard, The Art of Painting is particularly valuable for it depicts an artist at work. It demonstrates that an artist sat rather than stood at his easel, and also shows that he used his mahlstick to steady his hand while painting. Having already covered his canvas with a light gray ground and indicated his composition with white lines, the artist applies flat, unmodulated strokes of color as the underlying tones. At a later stage a variety of glazes and small highlights would model the form.
Technical examinations of Vermeer’s paintings have shown that he often followed this procedure. Sometimes it appears that he changed his mind during the painting process and made adjustments even after he had blocked in compositional elements. Nevertheless, in this painting not a single compositional change has been discovered, either through microscopic analysis, infrared photography, or x-radiography. Such compositional assurance seems to indicate that Vermeer had worked out his composition beforehand.
Whether or not he was inspired by the optical and spatial effects of the camera obscura, he organized and structured his painting with careful attention to the laws of linear perspective. As seems to have been his standard process, he marked his vanishing point, just below the black finial of the pole weighting the map, with a pin. Strings would then have been attached to the pin to mark the orthogonals of the tiles and table edge. Despite these careful preparations, Vermeer adapted his perspective to enhance the dramatic impact of the scene. To emphasize the artist’s central importance within the allegory, Vermeer painted him at a disproportionately large scale: standing, the artist would tower over his model. Even though the artist’s face is not visible, the viewer senses both the forcefulness of his personality and the intensity of his gaze.
The artist at his easel is executed with broad strokes that match the boldness of the image. The patterns of the black jacket, red hose, white boot hose, and black slippers are almost abstract in their crisp renderings of light and shadow. At the rear of the room, however, Vermeer has described forms with more attention to light and textural effects. The nuances of light falling across Clio’s hands, face, and robe convey the softness of her skin, the smoothness of the leather-bound folio she holds, and the sheen of the blue fabric. Vermeer similarly recorded the worn surface of the wall map as light models its form and reflects its aged appearance. Finally, in one of the most striking passages found in any of his works, he captured the brilliance of sunlight reflecting off the polished surface of the brass chandelier. With sure strokes that range from thick impastos of lead-tin yellow in the highlights to darker and thinner strokes of ocher in the shadows, Vermeer created the illusion of an object that seems almost tangible.
Excerpt taken from: http://www.nga.gov/exhibitions/verm_5.shtm