The color of a pigment is not one of its definite, inherent properties, it is rather the effect on the eye produced by that particular substance under certain circumstances. Many conditions can alter the color effect of material, and two examples of the same pigment will not match each other exactly unless they are seen under exactly the same conditions.
When a dry pigment is mixed with a liquid its color is changed to a darker or deeper tone. This is an optical effect, which may be explained in the following manner.
The materials used as pigments differ widely in certain properties from the liquids used as mediums. One of these properties is the amount of light a substance reflects and absorbs. All solids and liquids vary from each other in this respect and each one has been measured and tagged with a number which is called its refractive index.
A sheet of glass is a transparent substance; when a ray of light strikes it at an angle, there is a varying amount of surface or mirror-like reflection, depending on conditions; however, the greater part of the light passes through its continuous, uniform structure and emerges refracted or bent, at an angle different from that at which it entered. The refractive index is computed from this change in angle, which depends in each case on the substance’s power to impede light rays.
When two substances of varying refractive indices meet, the greater the difference in their refractive indices, the greater will be the proportion of light reflected at the point where they meet. When a pigment with a refractive index of 2.00 is dry, each particle is surrounded by air, the index of which is 1.00, and a certain amount of white light is reflected. When the pigment is moistened with linseed oil, which has a refractive index of 1.48, much less light is reflected, more is absorbed, and the pigment appears darker or more intense in hue.
When the transparent glass is pulverized, the powder appears white. Water in the form of ice is transparent; in the form of snow, it is white and opaque. The reason for this is that while light rays are easily transmitted through the uniform, continuous mediums of the ice and the sheet of glass when they strike the powdered glass and the snow they are reflected in all directions from the myriads of tiny facets of the particles surrounded by air and are bent from one tiny particle to another until they become entirely diffused.
When such broken planes and irregular facets exist only on the surface, as when a sheet of glass has been rubbed with an abrasive to produce a ground-glass or non-transparent effect, the light is broken up and reflected on the surface, creating a whitish or frosted appearance. However, since the light-dispersing particles lie only on the surface in a thin layer, the rays are not entirely impeded and continue on through the glass, which is now translucent instead of transparent.
A Hat or mat effect on a paint or varnish film is always due to the fact that the surface consists of a thin layer of such irregular construction. When such a surface is moistened its opacity is temporarily diminished. In the same way, alumina hydrate, a white, opaque powder when dry, will become colorless and transparent when wet with benzol because the particles are then surrounded by a medium that has a refractive index very close to their own. The effect of liquids upon the color and opacity of pigments varies greatly in each case, depending upon the difference between the two refractive indices concerned.
If a pigment that appears transparent or translucent in a thinly applied layer is piled-up or applied to a surface in a thick layer, it appears more opaque because the light then travels through a great number of separate particles, each one of which impedes its progress by refracting it, also because there is more reflection of light from the points where the pigment particles and their surrounding medium meet, and because more particles absorb more light. The intensity of color also decreases the transparency or increases the hiding power of a pigment.
Pigments vary in transparency in direct ratio to their refractive indices, but all of them are transparent to some extent. If the proper medium is selected, laboratory tests can be made which will show that the most opaque colors, even flake white, appear transparent under certain conditions.
The liquids used in such laboratory experiments are not suitable for paint medium purposes, but the effect produced clearly demonstrates that hiding power or opacity in a paint film can be lost through a change in the conditions which surround the pigment that has been used. In an actual oil painting, this does not occur at once; but in galleries, one may frequently find a picture in which, by reason of changes wrought by time, oxidation, etc., the refractive index of the oil film has changed, and a thin coat of paint which originally sufficed to form an opaque film has become sufficiently transparent to allow underpainting or drawing to show through. The effect is called pentimento. (See Glossary, page 647.) Though all opaque pigments have this property, the whites possess it to a greater degree than do the others; pentimento in old pictures usually appears where white or colors reduced with much white have been used. The fact that all dried films of oil paints tend to become more transparent with age is well established.
It will be seen from the foregoing that when light impinges upon an object it is either transmitted, reflected, or absorbed, depending on the nature of the object; in the case of most paints it performs all three of these actions, in varying degrees.
The practical lesson that oil and tempera painters of the present day have learned from this is that the ground and underpainting always affect the final painting, even when it is not apparent, and that pictures must be built up carefully with this point in mind.
Correct procedure calls for keeping the ground and underpainting as white or pale as possible, superimposing darks over lights (so far as the nature of the work allows), and, whenever possible, scraping down or removing any substantial thick or dark areas when these are to be over-painted or obliterated for the purpose of making corrections.
If two pictures were identical except for the fact that one was painted upon a brilliant white ground and one upon a black ground, the difference between them would be apparent; any departure from the pure white ground or from strictly necessary underpainting will produce an effect which tends toward that of the black ground. It is especially important to use the whitest sort of ground and pale underpaintings when glazes and veils are employed.
There are few activities other than the use of artists’ paints where opaque and transparent color effects are manipulated and where their differences are so significant.
There are many aspects of color and light, each of which is a study or field in itself, such as color harmony, color classification or notation, and color considered from aesthetic, psychological, or scientific viewpoints. The present account is simply an outline of the physical and optical considerations which are relevant to the technology of painting materials and methods.
List of Transparent and Opaque Oil Pigments
| T – Transparent | ST – Semi Transparent | SO – Semi Opaque | O – Opaque |
! = Caution Hazardous material, colors should be treated with extra caution
| Pigment Name | Opacity | Pigment Name | Opacity |
|---|---|---|---|
| Alizarin Crimson | T | Payne’s Grey | ST |
| Aureolin | T | Permanent Green | T |
| Bismuth Yellow | O | Permanent Green Deep | ST |
| Blue Black | SO | Permanent Green Light | ST |
| Bright Red | T | Permanent Magenta | T |
| Brown Madder | T | Phthalo Blue | T |
| Bronze | SO | Phthalo Turquoise | T |
| Burnt Sienna | T | Prussian Blue | T |
| Burnt Umber | T | Purple Lake | T |
| Charcoal Grey | ST | Purple Madder | T |
| Chrome Green Deep Hue | O | Prussian Green | T |
| Chrome Yellow Hue | SO | Permanent Alizarin Crimson | T |
| Cerulean Blue | SO | Permanent Mauve | ST |
| Cobalt Blue Deep | ST | Permanent Sap Green | T |
| Cobalt Turquoise | O | Permanent Rose | T |
| Carmine | T | Pewter | SO |
| Cobalt Green | SO | Purple Madder | T |
| Cobalt Green Deep | O | Rose Doré | T |
| Cobalt Violet | ST | Rose Madder Deep | T |
| Cadmium Green | O ! | Rose Madder Genuine | T |
| Cadmium Green Pale | O ! | Renaissance Gold | SO |
| Cadmium Lemon | O ! | Raw Sienna | ST |
| Cadmium Orange | O ! | Raw Umber | ST |
| Cadmium Red | O ! | Sap Green | T |
| Cadmium Red Deep | O ! | Scarlet Lake | SO |
| Cadmium Scarlet | O ! | Silver | SO |
| Cadmium Yellow | O ! | Transparent Yellow | T |
| Cadmium Yellow Pale | O ! | Transparent Gold Ochre | T |
| Cadmium Yellow Deep | O ! | Terra Rosa | O |
| Copper | SO | Terre Verte | T |
| Cobalt Blue | ST | Ultramarine Green Shade | T |
| Davy’s Grey | ST | Ultramarine Violet | T |
| Flesh (no one is this color) Tint | O | Vandyke Brown | T |
| French Ultramarine ( Ultramarine Blue ) | T | Venetian Red | O |
| Gold | SO | Viridian | T |
| Gold Ochre | O | Vermilion Hue | O ! |
| Indian Red | O | Winsor Blue (Green Shade) | T |
| Indian Yellow | T | Winsor Blue (Red Shade) | T |
| Indigo | T | Winsor Emerald | O |
| Indanthrene Blue | T | Winsor Green (phthalo) | T |
| Ivory Black | SO | Winsor Green (Yellow Shade) | T |
| Lamp Black | O | Winsor Lemon | ST |
| Light Red | O | Winsor Orange | SO |
| Lemon Yellow Hue (Nickel Titanate) | O | Winsor Red | SO |
| Manganese Blue Hue | T | Winsor Red Deep | ST |
| Mauve (Blue Shade) | T | Winsor Violet (Dioxazine) | T |
| Mars Black | O | Winsor Yellow Deep | ST |
| Mars Violet Deep | O | Winsor Yellow | ST |
| Naples Yellow Light | O | Yellow Ochre Pale | O |
| Naples Yellow | O ! | Yellow Ochre | ST |
| Olive Green | T | Yellow Ochre Light | ST |
| Oxide of Chromium | O |