Читать книгу The Mixing of Colours and Paints - F. N. Vanderwalker - Страница 8

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The chief chemical colors commonly used are Prussian blue, ultramarine blue, cobalt blue, black oxide of iron, black lead sulphide, graphite, manganese black, chrome green and chrome yellow. Chrome yellow is made in orange, medium and lemon. The vermilions—American, Chinese and English—are chemical colors, as are also Venetian red and Indian red.

Such chemical colors as Prussian blue, chrome yellow and green are manufactured by mixing together certain chemical solutions. The reaction which then occurs causes a precipitation, or throwing down, of a very fine colored pigment. When the chemical action has spent itself the water is drawn off the top of the tubs and the wet pigment is put into filter presses which squeeze out the balance of the water.

These chemically pure colors are very strong and it would be wasteful, as well as expensive, to use them in the pure state; so they are ground in linseed oil, water (distemper) or Japan and at the same time inert white pigments are added as extenders to make the paste tinting colors such as the painting trade uses. These colors, like the earth pigments, are sold also in the dry powder form, principally for mixing calcimine.

Lampblack.—A paint pigment of rather ancient origin and well known to all. It has great opacity and is an excellent pigment both for solid color and tinting purposes. A slow drying color which requires the addition of more than the usual amount of Japan drier.

Lampblack is made in many ways and from many materials which will burn with a slow, smoky flame. It is made from dead oil resulting from coal tar distillation, also from rosin and tallow.

The smoke from burning these oils is collected in chambers or bags attached to flues. The burning must be controlled by the amount of air admitted. Too much air makes grayish blacks and not enough air adds oil or volatile acid to the lampblack. In fact, much skill is required in the manufacture of lampblacks. Time must be allowed; if the burning is forced too rapidly an inferior black is produced which contains both oil and acid sufficient to cause spontaneous combustion. Thus a dangerous dry pigment to handle results and one which is slow to dry and likely to corrode metal because of the acid content. Such a black paint destroys metal surfaces it is supposed to protect. Lampblack made of rosin is especially likely to do this and it is difficult to detect from other blacks.

Gas and carbon blacks often added to high quality carbon black with the intention of giving greater tinting strength are really detrimental because they cause jet blacks to take on a brownish tone and also cause the pigment to separate from oil with which it is ground or mixed—uneven, streaky black paint and muddy gray tints are so produced.

A high quality lampblack, then, is clear, jet black, of good tinting strength with white bases, one free from dangerous oils and acids and a black which will dry within a reasonable time without using an excess of drier.

Drop Black.—The name comes from the fact that when first marketed this pigment was sold in the form of small drops or lumps. It is no longer marketed that way.

Drop black consisted of mixtures of animal and vegetable blacks ground to a very fine pigment in water, oil or Japan. The bulk of this black manufactured is used by sign, carriage and automobile painters.

As now made drop black is a very fine quality of pigment resulting from burning animal bones to make charcoal, which is then ground, first in water, and then reground in oil, Japan or water and glue or gums for binders.

For the manufacture of the finest ivory drop black only the hardest animal bones are used such as teeth and shinbones. The bones are crushed and roasted in crucibles to make the charcoal. Soft bones make blacks which have a brownish tone and which lack clearness. Such blacks are, obviously, much less valuable.

Cheap bone blacks, called sugarhouse blacks, are made from the bone charcoal used in sugar refineries to bleach the sugar. When this charcoal has become saturated with color matter it is re-burned and ground for a cheap paint pigment. It lacks the beauty and clearness of ivory drop black.

High quality drop black makes a beautiful pigment both for solid colors and tints and shades with a white base. Pearl grays, rich warm olives and bronze greens of pure and lively tone are mixed with it.

Drop black takes less oil than lampblack and is a slow drying pigment. It has greater density and opacity than lampblack.

Ivory Black.—While a limited amount of ivory black is made from ivory chips and turnings, the bulk of this pigment is made from selected hard animal bones, the choice bones which are also valuable for making buttons and knife handles.

This is a very clear, jet black particularly valuable as a solid color. It is not so strong for tinting purposes as some other blacks.

Ivory Black is made, like drop black, from bone charcoal and is also called ivory drop black.

Indian Red.—One of the original colors extensively used and at first it was strictly an earth pigment. It was an especially bright oxide of iron earth found as a natural deposit near the Persian Gulf.

As manufactured today Indian Red is really classed with the chemical color group. In steel mills certain acid liquors are used to remove scale from iron and steel. After such use these liquors were dumped as waste in years past and were very destructive of fish in the lakes and streams into which this waste was run.

Today this waste acid containing iron scale is evaporated and from it sulphate of iron (copperas) is crystallized out. When this copperas is roasted a pure oxide of iron powder is recovered and the acid is again used to remove more scale.

This oxide of iron is Indian red. Its quality, shade and strength vary according to the care and ability shown by its manufacturers in roasting the copperas, in freeing it from acid and in grinding it. If the acid is not eliminated completely it will start an iron or tin roof to rusting, instead of protecting such surfaces as a paint should. Consequently, this Indian red must be made by able chemists and manufacturers.

High quality Indian red, then, must be pure oxide of iron, free from corrosive acids, have a deep, rich red color with a purple tinge to it, have a non-fading quality and possess good tinting strength.

Venetian Red.—Originally an earth pigment, venetian red is made by substantially the same process as Indian red and should therefore be classed with the group of chemical colors.

Venetian red is an oxide of iron red made by partially neutralizing the acid liquors used in removing mill scale from iron and steel as described under the section on Indian Red.

In making venetian red the acid liquors are mixed with lime. What is precipitated is sulphate of lime and hydrated oxide of iron. When this precipitate is roasted the acid is eliminated at a lower temperature than is needed in recovering Indian red and so venetian red is lighter and brighter in color.

Some of the cheap venetian reds used for box cars and rough barn work are recovered by crude processes and on a coarse, cheap base. While they are useful for some rough work, they do not compare favorably in value with venetian red made by highly efficient processes and better materials.

The cheaper venetian reds are not suitable for use as tinting colors and are not as good even for solid red paints. High class venetian red produces bright, lively tints and shades and is clear enough for delicate pink.

Carefully selected venetian red ground to a fine pigment in linseed oil makes about the most durable red paint known today.

Ultramarine Blue.—A most pleasing and valuable color made originally from a precious stone called Lapis Lazuli. It is a deep sky blue to a greenish blue in color. Made by a chemical process of burning in crucibles, such substances as China clay, carbonate of soda, sulphate of soda, sulphur, quartz, infusorial earth, charcoal and rosin.

It is interesting to note that in this process, discovered in 1828 by Guimet, a French chemist, the hot mass changes first to a beautiful rich brown which takes fire and burns in defiance of many efforts to hold it as a color pigment. Next the mass turns green and this color also disappears in flames on exposure to the air. Blue appears next, but if heated too long, it turns to violet, then to red and finally to white. After the mass cools off, if the fire is extinguished at the right time, the top layer is a clear bright blue. The bottom layer is a greenish blue of a lower grade.

Ultramarine blue is a combination of silica, alumina, sulphur and soda. The sulphur content of this color makes it an unsatisfactory blue to use with white lead, since sulphur turns white lead carbonate to lead sulphide, which is black. Traces of sulphur and sulphide in ultramarine blue discolor many pigments but not zinc oxide. It is not safe to use this blue with white lead.

The deep, rich color of ultramarine blue with its purple tinge is far more pleasing than Prussian blue, which has a greenish cast to it. Ultramarine blue is an excellent tinting color and glazing color; it is permanent in light (except with white lead) and durable on exposure to weather.

Ultramarine blue may be used on new plaster or cement walls, since lime, soda and alkali do not affect this blue. Fading and spotting occur when Prussian blue is used on such surfaces.

Cobalt Blue.—This is a color which is substantially the same as ultramarine blue—it is the purest and lightest blue so made, having neither the purple tone of most ultramarine from the top of the crucible nor the greenish cast of the bottom layer.

Cobalt is a most beautiful color pigment deserving of wider use by painters and decorators for delicate azure tints, using zinc oxide as the white base. White lead should not be used, unless in small proportions with zinc, since the sulphur content of cobalt blue may discolor the white lead, changing it to lead sulphide.

For the mixing of clear, light greens with zinc, or compounds where zinc predominates, cobalt is very fine. It is strong in tinting strength, durable and permanent in strong light. Hot lime and alkali spots in new plaster or cement walls do not spot and fade this blue as with Prussian blue.

Real cobalt blue is a combination of oxide of cobalt metal with alumina. It is so made for use as artists' water colors. It isn't so good as an oil color. The high cost of real cobalt blue prevents its general use in quantities.

Prussian Blue.—The best known and most extensively used of the blue pigments. Varying shades of Prussian blue are marketed under such names as Berlin, Chinese and Milori blue.

This is a chemical color discovered by accident. In the year 1700 a Berlin colormaker learned that when oxblood and wood were burned together, the ashes yielded a yellow solution which could be precipitated by iron as a brilliant blue color pigment. This yellow solution was yellow prussiate of potash or ferrocyanide of potassium.

The chemical process used in making Prussian blues now has, of course, been perfected far beyond its crude beginning. As done today yellow prussiate of potash is mixed with sulphate of iron (copperas) and the result is that a fine white pigment is precipitated. On being exposed to the air this white substance oxidizes into blue.

The blue color may have a purple, bronze or green cast to it or it may be quite a pure blue, depending upon the manipulation during the chemical process of formation.

Lime, soda, white wash, hot spots in new plaster and cement walls cause Prussian blue to fade and tints made with it to become spotty.

Prussian blues are rather fugitive in sunlight and are not used on exterior painting. They are among the strongest tinting colors and produce bright and clear tints on any white base. Prussian blue is used considerably as a glazing color.

Chrome Yellow.—Chromium is a metal remarkable for the beautiful colors it compounds. The precious stone called emerald has wonderous beauty because it contains chromium.

Chromium combined with lead produces a series of yellows which is most valuable. These yellows range from pale canary, citron or lemon yellows, through medium shades of yellows to orange chrome and finally to orange, red and scarlet.

These beautiful colors are made by the mixture of chemical solutions. Solutions of bichromate of potash, or soda, are mixed with solutions of nitrate or acetate of lead; from this a yellow pigment is precipitated. The water is drawn off, the pigment is put through a filter press to remove more moisture and is then ground in oil for the market.

Manipulations of the chemicals and other elements in the process enable the manufacturer to make the many yellows in this group: Canary, Lemon, Light Medium, Medium, Light Orange, Orange and red-orange chrome yellows.

Chrome yellows are bright, clear and opaque colors with great tinting strength. Because they are not transparent they are not suitable for glazing colors, stains or graining. They are very durable as protective coatings and quite permanent as to color in strong light. If not well made they are easily affected by gases of the atmosphere and strong light, which cause them to fade, get spotty and dingy.

Chrome Green.—The combination of Prussian blue and lemon chrome yellow makes chrome green. The combination is made intimately at the time the two color pigments are precipitated from the solutions.

This intimate mixture is very important; that is, the time when the blue and yellow are mixed. When each color is made separately (a yellow and a blue) and mixed later to make green a separation may occur and fading is pronounced. For this reason it is much better for painters to mix tints and shades of green by adding chrome green to white or a color than by adding blue and yellow to white or a color. When the medium chrome green is used the paint will not fade so soon as when blue and yellow are used to make green. In one case it is an intimate chemical mixture, while in the other purely a mechanical mixture, and so the two colors are more likely to separate and fade in the mechanical mixture.

Chrome greens, except the very dark colors, made largely of raw umber, are not permanent in sunlight. They are excellent tinting colors, and are bright and clear. They are used for glazing colors and may be used for stains.

Chinese and English Vermilions.—A chemical color, a sulphide of mercury, which is quite a permanent, brilliant red. English vermilion is practically the same as Chinese, but American vermilion is quite a different pigment. Though its color is brilliant, too, the latter is permanent in strong light.

The Chinese and English vermilions should not be used with white lead, chrome yellows, chrome greens, any of the copper colors or emerald green. Unfavorable chemical reactions result.

American Vermilion.—A basic chromate of lead, brilliant red, made by the same chemical process as are the chrome yellows. It is fine in texture, has a clear color, is a strong tinting color and has remarkable covering capacity.