Pigment

“ To each sample is added an equal quantity, say 15 gr., of vegetable black or of very finely ground barytes, according as the pigment is a light or a dark-tinted kind. The ingredients of each sample are most intimately and completely mixed, and the tints of the two mixtures are compared by observation with the naked eye. The pigment which most nearly retains its own colour, possesses the greatest body or covering power. ”

“ In pigments these effects are imitated by the mixture of white with a color to produce tints, and black for shades, or more generally instead of black some dark natural pigment approaching the hue of the color, may be preferred because a black pigment will too often impart an unexpected and undesirable hue to the color. As for example, in making shades of red some natural brown pigment is better than black, and so various dark browns and grays are used for different colors. ”

“ Nature, Source, And Manufacture Of Pigments.—A pigment is a dry earthy or clayey substance that, when mixed with oil, water, etc., forms a paint. Most pigments are of mineral origin, but there are vegetable pigments, as logwood, and animal pigments, as cochineal. In modern practice the colors are produced mainly by dyeing certain clays, which excel in a large percentage of silicic acid, with aniline dyestuffs. ”

“ The Palette.—You try to attain nature's effects of light with pigment. Pigment is less pure than light. You cannot have the same scale, the same range, but you must do the best you can, and the arrangement of your palette will help you. As you have not a perfect blue, a perfect red, and a perfect yellow, you must have two colors for one. ”

“ It is convenient to distinguish between pigments and paints, the latter being prepared from the former by the addition of a vehicle or medium. Nor are pigments and dyes identical, although there are cases in which the same colouring matter which yields a dye or stain may give rise to a pigment. A pigment is, in fact, a substance which is insoluble in the vehicle with which it is mixed to make a paint, while a dye is soluble. ”

“ Freeman’s.—This pigment, when ground with oil in the customary way, forms a paint equal in body and covering power to the best white lead, while it is superior in colour, permanence, and density, and is free from odour and noxious qualities.
It is produced by grinding together “zinc white” (either oxide or sulphide) , lead sulphate and barium sulphate, in certain proportions, in the dry state, in an edge-runner mill. By thus grinding the several pigments together, their particles become intimately incorporated and undergo changes in character. ”

“ When we use these as pigments, we must remember that the pigment particles have a certain degree of transparency so that light partly penetrates them, certain rays being then reflected and certain absorbed according to the hue. A blue pigment, for example, absorbs all constituent rays of white light except the blue and the hues which border on blue in the spectrum, it being impossible to procure pigments which are so pure that they do not let some other hues besides their own characteristic one pass through them. ”

“ To Determine The Covering Power Of Pigments.—To determine the covering power of white lead, or any other pigment, take equal quantities of several varieties of white lead and mix them with a darker pigment, black, blue, etc., the latter also in equal proportions. The white lead which retains the lightest color is naturally the most opaque. In a similar manner, on the other hand, the mixing power of the dark pigments can be ascertained. ”

“ Repeatedly have colours to be coaxed to behave themselves as pigments, coaxed not to 'run,' to work well, to dry well, &c.; and in the humouring of their likes and dislikes the skill and patience of the artist-colourman are sometimes severely taxed. Given a colour, it might puzzle most chemists to convert it into a pigment; luckily Commerce lends her aid. Lasting success, it is true, does not always follow, and oils will rise to the surface now and then, giving green hues to blues, orange hues to reds, and buff hues to yellows. ”

“ To determine the hue of a pigment from the analysis of the light it reflects is a problem in physiological optics, the solution of which varies with the character of the observer's eye. The three primary color-curves of the eye must be determined, and the total intensity of each wave-length of the spectrum of the pigment must be divided in the proportions indicated by the color-curves into three components—red, green, and violet. ”

“ There are some which exhibit all their colour at a glance: there are others that the more they are looked into the more colour they are found to have—containing, as they do, an amount of latent colour, not immediately apparent. Apart from the beauty which a wealth of colour imparts, those pigments imbued with it are, as a rule, the most permanent. And not unnaturally so, for the more colour there is present, the longer it takes to be affected, either by exposure or impure air. Colour within colour, therefore, not only lends charm to a pigment, but contributes to its safety. ”

“ For such, the introduction of a new permanent pigment is of little interest, unless its colour be primary; so wedded are they to that passion for compounding which the chemist views with dismay. With dismay, because he knows that the rules of mixture are severe, and cannot with impunity be altered; that, although disguised in oil or gum, each pigment is a chemical compound, with more or less of affinity and power, more or less likely to act or be acted upon. Because he knows that, except with the most experienced artists, compounding leads to confusion ”

“ Possessing such properties, it is curious that the red has never been—in this country at least—introduced as an artistic pigment, the more especially as seventeen years have elapsed since its discovery. ”

“ As a rule the colouring matter is beaten up into a smooth paste with the necessary quantity of a strong solution of albumen and then reduced to its proper strength by the addition of tragacanth mucilage or starch paste. The main factor in the successful working of pigment colours is their fineness of division; the finer they are the better they print and the more beautiful is their quality of colour. ”

“ It is not as a rule necessary to make an analysis of the pigment; the above tests serve to distinguish it from other pigments.
An assay of ultramarine should include the following points:—1st, colour or tint; 2nd, covering power or body; 3rd, acid resisting properties: this can be tested by making a very weak solution of sulphuric acid—about 4 oz. in 1000 oz. of water—and adding a little of this to the pigment contained in a glass, and noting how long it takes to bring about decolorisation; 4th, the power of resistance to the action of alum. ”

“ When it is considered that only one color is employed, the diversity of the results is wonderful. In many cases this is effected by apparently varying the application of the pigment, and laying it on more thickly in some places than in others. ”

“ Mineral and Pigment Dyestuffs.—These colours are insoluble [Pg 83] in water and alcohol. They are either fixed on the fibre by mechanical means or by precipitation. For example, you use blacklead or plumbago to colour or darken your hats, and you work on this pigment colour by mechanical means. I will show you by experiment how to fix a coloured insoluble pigment in the fibre. I take a solution of acetate of lead (sugar of lead) , and to it I add some solution of bichrome (potassium bichromate) . ”

“ The material colour should be called “pigment” or “dyestuff” in the raw state, and paint when compounded with other substances for application in the form of a coating.
The sense of colour is due to light. In the absence of light there is no colour, only blackness; and black is really no colour, but an absence of colour. Very many conditions combine to cause different colour sensations, some of which are understood, while others we are not able to explain.
For instance, take the action of heat upon a solution of chloride of cobalt. ”

“ Little specks of coloring matter, or pigment, which lie in the outer layer of the skin. Even white skins contain a little pigment, they are not a pure white. A Chinaman’s skin has a little more of this pigment, so that it looks yellow; an Indian’s has still more; and a negro’s has most of all, making him black.
Sunlight can increase the amount of pigment in the skin. The people who live in the torrid zone have much darker skins than those who live where the days are short and cold. ”

“ The pigments for painting on glass are powders, being the oxides of various minerals, chiefly iron. There are others; [Pg 57] but take it thus—that the iron oxide is a red pigment, and the others are introduced, mainly, to modify this. The red pigment is the best to use, and goes off less in the firing; but, alas! it is a detestably ugly colour, like red lead; and, do what you will, you cannot use it on white glass. Against clear sky it looks pretty well in some lights, but get it in a sidelight, or at an angle, and the whole window looks like red brick ”

“ The skin rested, as did the brain, so that it seemed probable that central activity, thought, will, or whatever name is given, has some effect in the change of color. The probability is that, as they become pale, the pigment does not leave the skin, but that it is collected in spheres too small to affect our retina, which will be impressed by the same quantity of pigment when more extended. ”

“ Mineral black is a pigment made by grinding a black form of slate. It contains a comparatively low percentage of carbon and consequently has low tinting value. It finds use as an inert pigment in compounded paints, especially for machine fillers. The pigment has a flocculent appearance, the particles showing a strong tendency to mass. ”

“ It is non-granular, covers well, mixes with water or oil, and is a good artists’ colour for sky effects. It is permanent in strong sunlight and impure atmosphere, and resists acids and alkalies at normal temperatures. Hot hydrochloric acid dissolves it, and addition of water to the pale blue solution produces a violet red; evaporation to dryness restores the original pigment. The green tint of a nitric acid solution is due to iron and nickel impurites. ”

“ One may be noticed here as it is claimed for it that the pigment is gifted with greater covering power or body, the limited degree of which is the only drawback to zinc oxide whites. The plan consists in this, that the oxide is allowed to collect in the condensing chambers till it is of such a depth that a man entering stands waist deep in the pigment. ”

“ Hence pure intermediate tints in single, permanent, original pigments, are to be preferred to pigments compounded, often to the dilution and injury of their colours. Cadmium Orange, for instance, which is naturally an orange pigment and not composed of red and yellow, is superior to many mixtures of those colours in a chemical sense, and to all such mixtures in an artistic sense. At the same time, it is quite possible for the artist to multiply his pigments unnecessarily. ”

“ For instance, artists, and other individuals who require it, may easily manufacture a black pigment in the following manner, with a certainty of its being genuine, which can scarcely be placed in the article sold at most shops. ”

“ A pigment filler is quite as necessary as a liquid one. And pure white lead, Dutch process, if it please my readers—observe its properties in this respect: It is of great density, body, permanent to a rare degree, of impalpable smoothness when properly ground, and chemically unites with oil, forming a kind of varnish, which makes it a filler and pore sealer of great value. In addition, it absorbs more oil, solidifies it, and remains elastic for a greater length of time, except red lead perhaps, than any other pigment so far discovered. ”

“ It were folly, therefore, and a silly conservatism on the part of the artist to limit himself to such pigments only as were employed by his forefathers, especially as their merits were often more than doubtful. New colours, it is true, have to be learnt, for each pigment has its own peculiar habitudes, chemical, physical, artistic; but if they be good and durable, no amount of time and study spent upon them is thrown away. To think less of the quality of one's materials than of the effects which can be produced with them is mistaken policy ”

“ They took feathers of the English rook, which contain one per cent. of pigment, and having cut the vanes from the central rib, cleaned them from fat by treatment with alcoholic ammonia. Warm dilute sulphuric acid was then applied, until it was no longer colored, and the residue was treated with dilute hydrochloric acid and boiling alcohol and ether. Black pigment is usually found in black, brown, and dark red hair, but in the latter it is associated with a brown pigment that is soluble in dilute sulphuric acid. ”

“ Ultramarine Blue. This bright blue pigment is prepared by burning silica, china clay, soda ash and sulphur in pots or furnaces. It has a specific gravity of 2.4. It is of little value as a paint pigment on account of its sulphur content, which causes darkening when mixed with lead pigments, and corrosion when applied to iron or steel. The darkness of the photograph is due to the massing of the pigment particles.
Chrome Green. Chrome green is prepared as a paint pigment from nitrate of lead, Chinese blue, and bichromate of soda. ”

“ As colour, according to the regular scale descending from white, ceases properly with the last of the tertiaries, olive, in theory the neutral black would here form a fitting conclusion. Practically, however, every coloured pigment, of every class or tribe, combines with black as it exists in pigments—not simply being deepened or lowered in tone thereby, but likewise defiled in colour, or changed in class. ”