"Binder" is the technical word for a substance that enables pigments to be fixed onto the ground. Lenbach's opinion was that "one can paint with anything that sticks." This remark is typical of the indifference of the later nineteenth century to technique. Lenbach was at that moment thinking only of those binders which work as adhesives, and, mixed with pigments, become "colors," that is to say, ready-prepared paints.
No one can paint with pigment powder; it must be made suitable for application with either a brush or pen or made into a pencil. The kind of binder chosen depends on the painting technique in view. The terms used for ready-prepared paints, like "oil colors" or "watercolors," give some indication of the binder, but closer examination shows that it is not quite as simple as it seems.
Colors which are petrified onto the painting ground by some chemico-mineralogical process, as in fresco, or are fixed by some material sprayed after their application, such as mineral paints, do not contain a binder. Pastel colors, which are bound by the application of a fixative, also contain no binder.
Adhesive binders are basically of two sorts. There are those which have a more or less constant natural consistency and have to be dissolved and liquified for painting purposes, hardening again after the evaporation of the solvent. All kinds of glue, resin, and wax belong to this group. The others are naturally liquid and do not harden by the evaporation of some constituent but by the absorption of acid from the air.
Certain oils have this property - but not all! If even the smallest trace of olive or lubricating oil (neither of which oxidizes to a hard substance) is mixed with a hardening oil it will prevent that oil from oxidizing.
The oil layer becomes more voluminous when hardening, but after long periods of time it shrinks again. This is due to the oil giving off carbon dioxide, which is in fact a sort of very slow burning. This can be seen on any very old oil painting. The surface is cracked and split and shows what might be called "late crack formation. "
There is also an "early crack formation" which may arise even a few hours after the paint has been applied. This is due to mistakes in technique. Forgers cause them intentionally to imitate late crack formation and make the picture look old. The crackle of ceramic glazes is due to similar "mistakes."
Since late crack formation has been found to be an inevitable evil of pure oil painting, attention has been turned again to tempera, which is a much older binder. True tempera consists of bringing together two substances which generally repel each other: water-soluble glue and oil. This is done in such a way that the two substances .form a completely new, inseparable liquid to which, within limits, either water or oil can be added. The general technical term for this true mixing of substances is "emulsion."
Milk is a well-known example of a natural emulsion, mayonnaise an artificial one. If the reader is versed in the art of making this delicacy he will know that it can succeed only if an egg yolk is stirred constantly while oil and vinegar (the watery element in this case) are added to it alternately in small drops.
The egg yolk functions as the emulsifying agent. Egg yolk is itself an emulsion of egg white and egg oil. Pharmacists seem to have the special gift of making emulsions without an agent. If you want to use tempera frequently you should enlist the kind aid of a member of this esoteric profession to see that your emulsion comes off! When it crackles and clicks in the mixing bowl, the magic has worked.
The word "tempera" comes from the latin temperatio, meaning a proper compound, as against mixtura, which means any ordinary mixture. Unfortunately, color merchants are not so precise in their use of the term "tempera colors," as you will soon discover! The technical advantage of genuine emulsions is that neither the glue nor the oil can form a hard film as it dries.
A honeycomb of each substance contains minute droplets of the other, although these are too small to be seen with an ordinary microscope. This structural parceling of each constituent enables additions of one or the other to be made without their showing when the emulsion is spread as paint. The surfaces are multiplied; thus, both evaporation and oxidation occur much more quickly.