Vermeer's technical evolution can be roughly divided into three periods. His first works are characterized by evident brushwork and rich impasto application of paint. The Italian term "impasto," indicates a thick, light-toned opaque layer of paint that leaves observable brush strokes. Passages painted with impasto acquire a vigor and forcefulness not achievable with flat layers of paint.
In the works of Vermeer's middle period, such as the Woman in Blue Reading a Letter, pictorial substance is hidden as much as possible so as to not destroy delicate chiaroscural transitions which define form and volume via optical, rather than tactile description. The face of the young woman in the Woman in Blue provides perfect example of the suffused contours and refined modeling which are characteristic of works of this period. In order to obtain such delicately transitions, Vermeer may have used a so-called badger brush.
The badger brush was standard equipment in the seventeenth-century workshop. In the 18th and 19th centuries it was widely used by portrait painter and were known as "sweeteners" or "softeners. In the 20th century, the badger brush has greatly diminished among artists since painters, who began to favor painterly styles, no longer found smoothness a desirable aesthetic quality.
The badger brush was used principally for two reasons. One, adjacent areas of wet paint could be blended creating impalpable transitions unattainable with rounded or fine tipped brushes. Two, glazes, or transparent layers of paint, could be applied uniformly over a monochrome underpainting without leaving even the slightest trace of the brush.
The badger brush was originally made with badger hairs which are softer than hog hair but not as flexible as sable. Badger hair comes from various parts of the world and is more readily available than most animal hair, although the quality varies greatly. Badger hair is thickest at the point, and relatively thin at the root, so it has a distinctive "bushy" appearance. Badger hairs are disposed in the metal ferrule of the brush in such a manner as to create a flat fan-like form.
Curiously, the badger brush is not actually used to apply paint. Two different shades of paint are roughly applied to the canvas with a normal brush. While the paint is still wet, the badger brush is delicately maneuvered over two tones with a light, sweeping back and forth motion blending and removing all traces of paint relief. Due to its feather-like thinness, the badger brush does not really move appreciable quantities of paint. Once the brush has picked up too much paint on its tip, it must be cleaned with a solvent and dried thoroughly before it can be used again.
The badger brush may also used to spread out thin transparent layers of paint, called glazes, which are applied of over a dry, monochromatic underpainting. The glaze paint is usually composed of an inherently transparent paint with the addition of the highly viscous Stand Oil or Venetian turpentine and a bit of gum turpentine to improve flow. The superimposed glaze functions only as a coloring agent, similar to a thin sheet of transparent colored acetate placed over a monochrome photograph.