Binders and Diluents- Materials List
[See Painting Methods and Techniques for
Use of Mediums and Thinners]

l. It should not change color as it ages.
2. It should retain its binding strength as it ages.
3. It should remain structurally stable with time, resisting cracking, peeling, etc.
4. After it has dried, it should not be easily dissolved by mild solvents used in normal cleaning operations.
5. It should not activate a radical color change in the pigment.
6. It should not be unusually toxic or flammable when handled with normal caution.

The diluent, or painting medium, should be employed only to facilitate manipulation of the paint. It should not act upon either the pigment or the binder in such a way as to lesson its conformity with the standards for each, listed here. [p. 32]

For Example - The Binding Medium for Oil Painting
The simplest way to paint in oils is to brush a single coat of oil color, just as it comes from the tube, onto a canvas. Such an uncomplicated method of painting is apt to cause fewer defects in the physical structure of the picture than will more complex procedures. However, the brushing quality of the paint as it comes from the tube today is not agreeable to most artists, who find that they prefer [p. 74] paints of greater fluidity with different setting characteristics. [They might try applying a thin coat of medium over the entire canvas before proceeding with the painting] Therefore almost all painters add some liquid to the paint to assist them in spreading and manipulating it. This liquid diluent or painting medium may be made up of varying proportions of drying oils [such as the linseed oils, poppyseed oil, or walnut oil], combined with varnishes [such as dammar], and diluted with a volatile solvent [such as turpentine or mineral spirits].

The painter should remember that such painting media should be used as sparingly as possible, for their oily content will increase the yellowing of the paint films, and their varnish content tends to make the films more brittle and more vulnerable to solvents. Painting media should be considered as necessary evils that must be employed with an awareness of the troubles inherent in excessive use.

In any single picture the composition of the painting medium should not be changed unsystematically through the successive layers of paint. Haphazard mixing of small quantities of oil, varnish, and turpentine in the palette cup may yield combinations that vary considerably from one working period to the next. It is much better to mix a half pint or so of medium in advance and keep it handy in a bottle labeled with the date and recipe of the mixture. In this way the medium may be kept uniform throughout the picture, and the various ingredients will always be thoroughly mixed. It is also possible to relate the results obtained in the particular painting to a specific known recipe rather than to a random mixture. The recipe may then be altered subtly and precisely when a slightly different rate of drying or quality of brushing is desired.

Although today the various color manufacturers still offer prepared painting media bottled under brand names, most painters buy the ingredients [oils, varnishes, and diluents] from the manufacturer and make up the painting medium to suit their individual needs. No single recipe is recognized as superior for all requirements, and painters should experiment to develop the medium most useful to them. The drying and handling properties of the various ingredients are mentioned in the preceding section.

Recipe for an Oil Medium
A typical recipe for a painting medium which can be used as a starting point for individual experimentation:

2 parts by volume linseed oil [cold-pressed, sun-thickened, or stand oil]

1 part by volume dammar varnish [5-pound cut]

6 parts by volume turpentine

These proportions can be varied, but the percentage of varnish should not be increased, since dry paint films, which include heavy amounts of soft resin like dammar, can be redissolved too easily in turpentine. Unless the paint has been ground with an excess of oily binder, it is not advisable to use turpentine alone as a painting medium, since it tends to spread the paint over too great an area, with the result that the binder can no longer form a continuous pigmented film. In cases of extreme overthinning, when the paint dries the pigment may powder off, or the film may crack. It will be found that even small additions of the various oils and varnishes will alter considerably the drying speed and brushing quality of the paint. In general, rather than an exotic concoction of many ingredients, the recipe for the painting medium should remain as simple as possible. [ pp. 74-75]

[Kay, Reed. The Painter's Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.]