The tests described above reveal to us that the power, coloring and coating are not synonymous. A color can color strongly and be transparent, that is, it does not mask what is below it. It can happen vice versa. These are not defective, but particularities that determine the differences in use. From here comes the multitude of tones in appearance identical in the catalogs of shades of manufacturers. But we will know that we have transparency, when the pigment and the binder have the same refractive index, or otherwise, of opacity. So we have to compare the pigments except in the presence of the same binder.
We must also take into account the changes in drying. If it makes it grow, over time, the transparency of the colors with oil, the drying will opacize, as it occurs, the weak colors. These contain a lot of volatile diluent very refresh (water or essence), which by evaporation makes the air to penetrate the pores (Dry-Heaping). Thus we see how a roof or wet soil opens on color on measure what life is. Finally, we must take into account the relative proportions of the pigment and the binder, at least of its non -volatile part, oil or resin. Indeed, I saw when I talked about the medium how it, when we dilute the pigment more, makes any color more transparent, no matter how opaque it is. This fact allows us to get a deeper tone and to "keep the layers below", as the Mayerne said.
The tests will be made, therefore, without diluting, or, if obliged, we will dilute equally. Limiting us to oil painting, applying colors on a contrasting background will quickly prove to us that there are few rigorously opaque colors and few completely transparent colors. Most are staggered in intermediate positions. They are those whose refractive index differs, but quite a little, from the oil. More or less translucent, they are especially useful in demi-pastes, because they make the "underlying" underlying tones. So it would be very little according to the reality we establish a rigid classification, opposing only two groups. Because of this, some manufacturers prefer to mention in the initial color catalogs o (opaque) and t (transparent) to the extreme differences.
The three white. Even the white, so used for opacization, do not resemble the optical character, nor by behavior. Used alone, on average, as well as all the other colors of the palette, exert an essential influence on the preservation of art works and deserve our attention. The classic white silver, very opaque and pleasant to work, it keeps the cough, the easier it accepts the emulsion with water. He facilitates drying and, aging, maintains a remarkable suppleness. We must avoid it from its toxicity, the alterations caused by the introduction of sulfur elements, coming from some pigments or from the atmosphere. An investigation we did with Mrs. Madeleine Hours allowed us to know the methods of making lead white, currently used in Europe.
It is a big difference compared to the one that, from the plinius, consists in the maceration of lead blades under the action of vinegar vapors, from which a white in the form of scales, highly appreciated (additional note for page 100). As for the direct passage of water in oil, "flushing-process", practiced empirically, I said all the positive things I thought. Appeared in the last century, the white zinc is very pure white, but a little opaque. He will mask, therefore, stupid the yellowing of the oils. It is not blackened by the mixtures containing sulfur, nor by the impurities of the air. His reaction to the oily binder makes it with time, less covering and, unlike the silver white, aging, does not leave the matter. We can, therefore, be surprised that it is still so used, despite the accidents it causes.
However, the warnings regarding him, for more than a century now, did not miss, among others those of Delacroix and Cormon, which summarizes: "It covers, it dries badly, it causes cracks." To which is added the appreciation of Jacques Blockx who should do authority: "A capital defect forbids, in oil painting, the use of zinc white. This oxide, uscin, crack, especially in the second layer; it becomes very brittle and, therefore, peel. " If we use it in a low dose, its translucent and reactivity with the oil offers great advantages, without letting the shortcomings appear. A study presented at Conference C A in 1951 finds that a pigmented painting with zinc oxide will crack rather than a pigmented painting with titanium oxide. The modern white titanium is increasingly used.
Neutral, he practically does not exercise any action on binder or other colors. He supports all his mixtures, and at the same time he does not lose his remarkable opacity. Apart from this, it is not toxic. Although, by its nature, it beats very easily to yellow, the product does not pose problems if the manufacturer has taken care to ensure a correct drying. However, those who appreciate the remarkable ductility of the silver white, the ease with which they dry well in depth, will remain faithful, even if they are interested in using zinc white for the velat and what remains on the surface, so in Contact with the air. Zinc white is suitable when we want to create the sensation of distance or temper through a veil. True velin, with opalescence healing, requires more transparent and calm pigments.
Turquet de Mayerne quotes white bismuth or mower as "good just to glad" what we follow in this case. These are low products used in current painting, much focused on opacity effects; In fact, for this reason, very fine pigments are avoided, because they do not stop the light. The three white are compatible with each other, and their mixing allows one to correct one with another. Thus, some manufacturers, adding white zinc to rub, improve the behavior of titanium to oil and reduce the tendency of silver white to become brown in the sulfurous atmosphere. The silver white can be mixed on the palette with the titan who transmits his nervous cough. He will improve his drying, without sacrificing the opacity what would, in this case, the defect of an addition of medium. Diffusion resistance.
If a color is so soluble in the oil, it risks broadcasting and altering those that will overlap it. This is the case of so much red that "rely" migrating to the white with which they were covered. As for brown, it is not only the case of bitumen. The diffusion of the shadow earth is terrible. This resistance is checked by putting a little color on a sugative or a piece of newspaper. The oil aureole that forms around is not colored. The colors that stretch should not be used in the layers below, besides this, we risk broadcasting.
Finally, let's note that, in general, they are a little stable in light ("aniline" colors).
* Note:Havel, Marc -The technique of the painting, Meridiane Publishing House, 1980