Paint is a physical material, composed of pigment, binder and (sometimes) an extender. It is often combined with dilutent (e.g. linseed oil). Additionally, the manner in which paint behaves is, in part, dependent on the surface it is painted upon, as in the case of using the primer or underpainting to supply the white values in a painting. The painting can also be treated after it has dried, with varnishes or glazes. The many ways that paint can be applied are almost without bound and all are dependent upon these physical qualities.
A few of these application methods are presented below:
To apply ‘dry’ paint (ie paint that has not been diluted with thinners). This is usually done using a stiff brush.
In this detail of a painting by Monet (from this painting) you can see that this dark green paint is sitting on top of the lighter paint. This dependence upon the surface qualities of the painting is a feature of the scumble. Different to glazing, scumbling can be done with light paint as well as dark paint. Scumbling is usually only done on top of dry paint.
To lightly coat the surface of the painting with paint that has been thinned with oil or another thinner. This is physically similar to holding a sheet of darkened glass in front of your eyes. A glaze can only darken the painting. Glazing is done only on top of dry paint.
In this detail of a painting by Rembrant (from this painting) the left hand side of the detail has been darkened with a glaze. A glaze offers a large amount of control over the number and quality of the tones in a painting.
Rubbing back to the underpainting is, effectually, painting in reverse. Instead of adding paint to get the effect, the painter is actively removing it.
In this detail below of a painting by Durer (from this painting) the skin tones have been handled by a combination of applied lightness (with paint) and rubbed back lightness. The combination between these two different forms of lightness (one warm, one cool) is what gives the face its glowing quality.
In this detail below of a painting by Tinteretto (from this painting) the effect is, perhaps, more clear. The impasto lights are a dull green, whilst the rubbed back lights are a dull orange.
Impasto is a form of Italian pasta. Not. Actually, it is no more than paint that has been applied thickly. This can be applied to the entirety of the painting (Van Gogh was very much an impasto painter) or it can be applied locally. In the example below, from this painting by Renoir, the white of a reflection has been applied impasto. Generally, whites and very light values are best applied impasto.
Impasto can be seen very clearly when the light is coming in from the side (i.e. ‘raking’ light) as can be seen in the image pair below. Notice the impasto on the white cloth.
Some paints are better for glazing than other paints, some paints are better at rubbing back than other paints etc. This is because of the physical properties of their constituant pigments. A key physical property is transparency. In the examples below Prussian Blue and Alizarin Crimson are shown next to paints similar in hue but very different in the manner that their transparency manifests. The neat thing about these paints is that they can give the impression of painting with two colors simultaneously.
Tonking got its name from Sir Professor Henry Tonks. In the process of making a painting, it is common to find that one has overloaded the painting surface with too much paint. Wiping it off with a rag can spoil the surface, leaving behind a bunch of muddy and smudged paint. The alternative is to get a sheet of newspaper, lay it flat over the painting support and pat it down firmly onto the surface. Upon removal it will have taken with it most of the excess paint, but will not have smudged the surface greatly and also left the image largely intact. Sir Professor Henry Tonks invented this technique. Respect.