A Glaze or Scumbling is a thin transparent or semi-transparent layer on a painting which modifies the appearance of the underlying paint layer. Glazes can change the chroma, value, hue and texture of a surface. Glazes consist of a great amount of binding medium in relation to a very small amount of pigment. Drying time will depend on the amount and type of paint medium used in the glaze. The medium, base, or vehicle is the mixture to which the dry pigment is added. Different media can increase or decrease the rate at which oil paints dry.

Scumble is a technique similar to glazing, except that the coating is opaque, and is just painted on very thinly to allow bits of the paint below to shine through. Scumbling works by a principle similar to that used by pointillists, mixing colors optically. While most painters glaze with dark colors, scumbling is more popularly used for lighter colors; especially atmospheric effects when rendering fog or clouds.

The glaze consists in superimposing, at the end of the work, one or more layers of transparent paint, which by an optical phenomenon of reflection of the surface, increases the effect of depth of the pattern and the colors. Containing more binder than pigment, the glaze forms a veil (“velature”) that creates an optical mixture with the previous layer.

Often, because a paint is too opaque, painters will add special media or a lot of medium to the paint to make them more transparent for the purposes of glazing. While these media are usually liquids, there are solid and semi-solid media used in the making of paints as well. For example, many classical oil painters have also been known to use ground glass and semi-solid resins to increase the translucency of their paint.

History:
The glaze is an old process, born from the desire to better render, to tempera and then to the oil, the effect of transparency of the tissues or the vibration of the flesh.

Developed since Antiquity, this practice is visible on the murals of Pompeii. But it is with the advent of oil painting that this practice has become widespread. It has led to the development of many resin-based mediums and drying oils, raw or cooked, for different effects.

We can locate his beginnings in the Renaissance, with the Flemish Primitives first, who sought a certain illusionism, then with the Venetians who began to play with the dough (impasto, touch, glaze). In Flemish painting, there is often the superimposition of many layers of glaze, allowing to create refined nuances by the superposition of colors and thus reinforce the realism of the representations, the traces of the brush becoming almost imperceptible. The sfumato practiced by Leonardo da Vinci is possible thanks to the superposition of glazes. The practice of chiaroscuro, from Leonard to Rembrandt through Caravaggio and La Tour, has also brought the glacis to a high degree of refinement.

According to Jacques Lacombe in his 1752 book, it is an artifice very difficult to practice and can only be learned by studying the paintings of the great masters who put it into use.

In oil painting, the simplest form of a glaze is a thin, oily, transparent layer of paint spread over the top of an opaque passage that has been given some time to dry. Light travels through the glaze and is reflected back off of the opaque layer below. This can cause a glowing effect similar to looking at a brightly lit white wall behind a film of colored cellophane. The thin oily layers of a glaze can facilitate the rendering of details that would be more difficult with opaque paints—e.g. the complexities of skin tones.

When multiple layers of glazes are used, the colors in all visible layers can appear combined. However, the pigments are not physically mixed, since the paint is left to dry before each successive glaze is applied. The artist may apply several layers of paint with increasing amounts of oil added to each successive layer. This process of applying the fat layers (more oil in the painter’s medium) over the lean layers (less oil) can minimize cracking; this is the “fat over lean” principle.

Many painters juxtapose glazes and opaque, thick or textured types of paint application (that appear to push forward) as a means to increase surface variety, which some painters feel increases a painting’s drama, brightness and depth.

Wall glazing: When the technique is used for wall glazing, the entire surface is covered, often showing traces of texture (French brush, parchment, striae, rag rolling). Either oil-based or water-based materials are used for glazing walls, depending upon the desired effect. Kerosene or linseed oil may be used to extend the “open” or working time of oil-based glazes. Water-based glazes are sometimes thinned with glycerin or another wetting agent to extend the working time. In general, water glazes are best suited to rougher textures where overlaps of color are acceptable.

Glaze is also used in cabinet, furniture, and faux finishing.

Implementation:
Glazing is mainly practiced in oil painting because the brilliance of the oil is the best to create this effect of depth.

A glaze settles dry. It suffices to mix a very small quantity of the desired color with a large quantity of binder (oil, acrylic binder) or medium to be painted, or even medium glazing.

In oil painting, the most beautiful glazes are obtained by dilution of the color with oil standolie or amber.

The pigment of the color must also be transparent in nature and little covering. The lacquer pigments (alizarin) are therefore recommended The artist will refer to the degree of transparency indicated by the manufacturer on the color tube (mentioned by an empty square or a T).

This glaze can be located on a pattern or generalized over the entire canvas.

A localized glacis will allow to work the depth of a particular pattern: the distance in a landscape (sfumato), the modeling of a fruit in a still life. It can also act by optical mixing: a carmine glaze for example on a blue will produce a violet effect more subtle than the simple mixture on the pallet of carmine and blue.

A uniform glaze, placed on all the canvas, will allow to regulate the general harmony of the painting. Based on yellow for example, it will bring a warmer atmosphere to the whole. More blue, it will help cool the atmosphere. In watercolor, it may be a gray wash to put a veil of shadow on the landscape.