Glazing is a technique employed by painters since the invention of oil painting. Although in theory it is very simple, in practice glazing can be a very complex undertaking. In the simplest terms, glazing consists of applying a transparent layer of paint over another thoroughly dried layer of opaque paint, usually with a wide, soft-bristled brush. The underpainting, as the dried layer below is called, is generally done is a single color but it may also contain some color. The upper and lower layers of paint are not physically but optically mixed. Glazing is similar to placing a sheet of colored acetate over a monochrome photograph. The paint used to glaze must be modified by an oil medium to achieve the correct fluidity for brushing. Glazing creates a unique "shine through," stained-glass effect that is not obtainable by direct mixture of paint.
Glazes cannot be used indiscriminately and were always restricted to specific passages of the composition. To begin with, there is nothing to be gained by glazing with neutral grays or dull colors. Grays are far more effective when they are created with opaque or semi-opaque earth colors or black admixed with white. The light gray background walls in Vermeer’s works were never achieved by glazing. Nor were the light base tones of flesh produced by glazing transparent layers of pink over a white underpainting, as if it were a watercolor, although thin red glazes may have been utilized to obtain the warmer variation in the cheeks and the lips. There is no line which divides a glaze from a semi-transparent layer of color, but in general only inherently transparent pigments are adapted for glazing. Modern art historian are apt to abuse the term and describe any layer of paint that is not completely opaque as a glaze. Bright colored drapery were often glazed.
Glazing was utilized for two reasons. One, artists of the past had very few of the brilliant colors that are available today. For example, strong purples, greens and oranges were either rare and unstable or could be mixed with available pigments. For example, purple was approximated by glazing blue over a reddish underpainting or vice a versa. Two, glazing creates, as we have said, an extraordinarily luminosity impossible to achieve otherwise. Only inherently transparent pigments, called lakes, are suited for glazing. The principal pigments used for glazing were madder lake, carmine, natural ultramarine, verdigris, various organic yellow lakes and indigo. For further information on these pigments see Palette.
The Sacrifice of Isaac
Andrea del Sarto
Oil on canvas, 208 x 171 cm.
Cleveland Museum of Art, Cleveland, Ohio
This detail of an unfinished painting by the Italian painter Andrea de Sarto shows the beginning of the working-up stage. The red drapery, which has been modeled with flat tones of vermilion and black, would have been successively glazed with madder lake. The sleeves have been modeled with light yellow and dull green and most probably would have been be glazed with verdigris, a deep and lustrous green pigment.
Glazing, however, has more than one drawback. It is difficult to anticipate the final chromatic effect of the glazed area with respect to the overall harmony of the finished work. Due to its transparency a glaze produces an optical depth that attracts the viewer's eye more than the surrounding layers of opaque paint that usually cover the great part of painted surface of the canvas. Furthermore, it is insufficient to know how a glaze is to be applied. One has to determine with the utmost precision how thick or thin the glaze-paint should be: a little too scanty or a trifle too lavish an application can alter a paint layer's color or tonal value to an important degree. The same holds true for the underpainting, which is usually brought to its final degree of detail since once glazed it is difficult to correct. For these reasons glazing was not used for other than specific areas of the painting.
Today, there are various informative studies which make reference to glazing in Vermeer's painting. However, art historians tend to overstate Vermeer's use of glazing and do not distinguish between glazing used as a corrective measure—very light glazes meant to alter only slightly the underlying paint layer which for one reason or another had not come up to the painter's original expectations—and true glazing which, instead, aims to create by plan a specific and otherwise unachievable pictorial effect. This difference might not seem fundamental but the idea that Vermeer built up his paintings in a series of successive glazes is incorrect and creates a distorted perception of Vermeer's painting methods. An oil painting cannot be created by a series of successive glazes as if they were water color washes. The bulk of painting in the 17th century was executed with opaque and semi-opaque layers of paint. Glazes also attract dust due to their high oil content. Dutch painters like Vermeer, used glazing very selectively and according to well-known formulas.
Reconstruction of Vermeer's Girl with a Red Hat by Jonathan Janson, author of Essential Vermeer.
A superbly conserved example of glazing can be found in Vermeer's Girl with a Red Hat. In the reconstruction to the left, various stages of the seventeenth-century multi-stage painting process can be observed. The red hat, according to common practice for painting bright red objects, is first modeled with shades of pure vermilion and black. Subsequently, the lighter areas will be glazed with a thin layer of pure madder lake while the shadowed areas would be deepened with a thicker glaze of madder lake and, perhaps, some black or natural ultramarine.
The background tapestry is briskly executed wet-in -wet using various earth colors and natural ultramarine. The blue satin garment, still in the underpainting stage, is modeled with raw umber and white in the highlights.