In past centuries the preparation of paint was a long and tedio.us process. The pigments came to the painter in large lumps or in coarse irregular powders. They had to be ground with pestle and mortar and then further rubbed down on stone or glass with a muller, while at the same time being mixed with the binder.
Nowadays the pigments are bought already ground so fine that no further grinding is possible, for every pigment has a definite fineness suitable to it. The figures, in thousandths of a millimeter, are as follows: ochre .5 to 30, iron reds 1 to 80, Prussian blue .3 to 10, white lead 2 to 5. If pigments are ground too fine their colors tend to go muddy, and in colloidal condition can change color entirely.
Grinding by hand can reach a colloidal state, as you know from Chinese ink. The lamp black "takes" indissolubly only when it becomes a colloid. Pigments are bought today fine enough to mix with the binder when stirred" with a stiff bristle brush. As little binder as possible is used, just enough to make a stiff paste. A few hours, or at most a day, give the binder time to penetrate through the powder and show whether the paste is too dry and needs more binder or is too soft and can take more pigment. Pigments vary in their reactions to binders; some repel water, some oil. Alcohol helps to make a uniform paste in these cases, as it combines equally well with water and oil.
In most ready-made colors the pigments are generally finer than the dry powder. The mechanical rollers between which pigments are mixed with binders grind the pigments still further. This is not always an advantage, as a coarser granulation prevents too smooth a film of oil from forming and reduces the danger of early cracking by making the surface larger.
The painting layer is also more solid if the granulation is coarser. For both these reasons the old painting instructions always warned against excessively long and fine grinding of the pigment with the binder.
While most materials are best bought in art shops, it is advisable to go to a druggist for some others, notably cold pressed linseed oil, balsams, double rectified turpentine and gum arabic. The druggist has to follow specifications established by the Federal government, so the products are fully reliable, and their methods of extraction guaranteed: The painting material industry, on the other hand, carefully guards its processes as a trade secret, which unfortunately does not help the acquisition of a solid painting technique.
Glues are the easiest binders to handle. They are dissolved in water, which evaporates sufficiently after the paint has been put on for the work to feel dry after only a few moments. The surface can be painted over immediately, if the artist is a fast worker, but it should not be gone over too often with the brush.
Some of the water takes a few days to evaporate, during which time the glue remains swollen and can easily dissolve again. The glue can also be dissolved again even after the paint has dried fully, but it takes longer. All glues which act like this are called "reversible," as distinct from the irreversible glues which cannot be dissolved once they have set.
The most important of these in painting is casein, a milk product. Glues can also be divided according to their animal or vegetable origin, but this is of no practical importance to the painter. Carpenter's glue, prepared from the boiling of skins, behaves no differently from plant glues like gum tragacanth or gum arabic.