[Illustration: Calcium Carbonate (Whiting)]
[Illustration: Calcium Carbonate. By transmitted light
(_The Pigment shows black_)]
[Illustration: Calcium Sulphate. By transmitted light
(_The Pigment shows black_)]
[Illustration: Calcium Sulfate]
[Illustration: Calcium Sulphate (Gypsum)]
[Illustration: Silica (Silex)]
[Illustration: Silex. Mag. 250 Diam.
(_The Pigment shows white_)]
[Illustration: China Clay. By transmitted light
(_The Pigment shows black_)]
=Barium Sulphate (Barytes).= By grinding the crude ore, treating with acid to remove the iron, and finally washing, floating, and drying, there is produced the commercial form of this valuable pigment. It is used in large quantity as a base upon which to precipitate colors, and also together with other white pigments in the manufacture of ready-mixed paints. It renders the paint coating more resistant to abrasion, and gives to the paint certain very important brushing qualities. It is a very stable pigment, not being materially affected by either acid or alkali, and can be used with the most delicate colors. In oil it is transparent and must be mixed with opaque pigments when used in ready-mixed paints. It is generally used with lighter pigments, such as asbestine, in order to prevent settling. Under the microscope, both by polarized and transmitted light, the sharp angles of the particles appear distinctly, with no tendency to mass into a compact form.
Although transparent in oil, it is valuable in moderate percentage in a ready-mixed paint.
=Barium Sulphate (Blanc Fixe).= Blanc fixe is the precipitated form of barium sulphate, resulting from the action of soluble barium salts on soluble sulphates. The specific gravity (4.2) of this compound is lower than that of barytes. Possessing greater opacity in oil, it is of more value as a paint pigment for some purposes. It comes in for its greatest use as a base on which to precipitate lake colors. The very fine particles show a slight tendency to agglomerate.
=Calcium Carbonate (Whiting).= The natural form of calcium carbonate, prepared from chalk, has a much higher specific gravity (2.74) than that of the artificial form (2.5) prepared by the precipitation of calcium carbonate. The latter, however, possesses greater hiding properties.
Both grades find a wide use in distemper work and in the manufacture of putty. It is often used in small percentage in many ready-mixed paints.
The photomicrograph of the pigment shows the presence of many large particles.
=Calcium Sulphate (Gypsum).= The mineral gypsum, consisting of calcium sulphate and about 21% of water of combination, is sometimes used as a paint pigment after grinding and dehydration. Being slightly soluble in water it has a tendency to pass into solution when exposed to atmospheric agencies. It lacks hiding power in oil. Its specific gravity is 2.3. As in the case of all pigments prepared directly from mineral substances, the many-sized and shaped particles appear clearly when enlarged. Partially and wholly dehydrated forms of gypsum are also used in paint.
=Silica (Silex.)= This white pigment possesses great tooth and spreading properties. It is of use as a wood filler and as a constituent in combination paints. It wears especially well when used in combination with zinc oxide and white lead. Its purity often approaches 97%. The particles when enlarged are seen to have sharp angles and are not uniform in size, which accounts for its marked tooth and properties.
[Illustration: Aluminum Silicate (China Clay)]
[Illustration: Ochre]
[Illustration: Raw
Burnt
Sienna]
[Illustration: Raw
Burnt
Umber]
=Aluminum Silicate (China Clay).= China clay, or aluminum silicate, is a permanent and valuable white pigment showing very little hiding power in oil. It is found widely distributed in granitic formations. It is very stable, with a gravity of 2.6. Particles are found in many shapes and sizes, showing sharp and definite angles.
=Ochre.= Ochre is a hydrated ferric oxide permeating a clay base, largely used as a tinting material. It has a specific gravity of about 3.5, and a decidedly golden yellow color. A good quality should contain 20% or over of iron oxide. The particles of this pigment are flocculent and very uniform in appearance.
=Sienna.= Sienna, like umber, is essentially a silicate of iron and alumina, containing manganic oxide. It contains, however, a lower percentage of the latter than in the case of umbers. The photomicrograph of the burnt variety shows clearly the fine condition of the pigment, while large particles are shown in the raw variety.
=Umber.= Umber, another naturally occurring pigment, consists of iron and aluminum silicates, containing varying proportions of manganic oxide, its color and tone varying according to the percentage of the latter. The raw variety is drab in color, which in burning changes to reddish brown. A marked percentage of large-sized particles exist in this pigment.
=Indian Red.= Indian red is the term applied to natural hematite ore pigments and to those produced by the roasting of copperas (iron sulphate). They generally contain 95% or more of iron oxide, with varying percentages of silica. The pigment is heavier (specific gravity 5.2) than that of Metallic Brown. The crystalline, mineral-like structure of the particles differ greatly from the amorphous particles of Metallic Brown.
=Metallic Brown.= The natural hydrated iron oxide or carbonate as mined largely in Pennsylvania, yields, when roasted, a sesquioxide of iron known as Metallic Brown. It contains a high percentage of alumina and silica, and has a characteristic brown color with a gravity of 3.1. It finds wide application as a pigment for protective purposes. The particles when enlarged show the usual appearance of a natural compound which has been roasted and ground.
==========+=====+===========+==========+=============+=======+========= No. Name Iron Calc. Alumina Insoluble Color Oxide Sulph. (Silica +-----+-----------+(CaSO_{4}) (Al_{2}O_{3}) and FeO Fe_{2}O_{3} Sili- cates) ----------+-----+-----------+----------+-------------+-------+--------- % % % % % 0 Bright 0.71 96.52 -- -- .30 Bright Red Scarlet 1 Bright .71 95.92 -- -- .30 Scarlet Red Tone 2 Indian .57 96.00 .78 1.40 .90 Indian Red Red, Medium Shade 3 Indian 0.29 97.82 .85 -- .52 Indian Red Red, Dark Shade 4 Indian 0.28 95.72 1.21 1.26 .58 Indian Red Red, Light Shade 5 Persian 4.53 62.25 1.75 -- 27.64 Rich, Gulf Medium Mix Red 7 Native 0.85 89.00 -- 0.91 6.09 Medium Red Red, Oxide Brownish Tone 8 Special 0.57 43.87 50.88 2.03 1.30 Scarlet Red Tone 10 Red 1.44 60.25 .78 5.41 15.78 Brownish- Oxide Red 11 Vene- .30 34.08 52.60 2.20 3.39 Bright tian Red- Red Brown 12 B. 0.58 67.68 -- 2.48 1.97 Dark Red Oxide Brown 13 Vene- 0.29 25.92 58.62 2.16 1.42 Medium tian Red Red Tone 14 Vene- 0.57 35.36 .99 12.06 47.97 Brown tian Red 15 Metal- 2.59 64.00 .63 5.82 23.42 Rich lic Brown Brown 16 Crimson 0.57 66.24 1.77 3.60 25.63 Rich Oxide Dark Red 17 Red 2.30 80.39 .37 .03 9.63 Medium Oxide Brown 18 Red 0.57 61.28 .97 2.68 15.94 Light Oxide Choco- late Brown 20 Red 7.78 46.72 1.70 7.64 20.38 Dark Oxide Reddish Brown 23 Special 0.58 72.48 -- 8.80 4.48 Deep French Choco- Oxide late Brown 24 Mica- 2.02 86.27 -- 2.04 9.50 Dark ceous Gray Black Tone Oxide 25 Black 33.12 57.12 -- 1.44 -- Jet Oxide Black 26 Red 0.57 84.16 5.00 2.00 .63 Deep Oxide Red 27 Special 0.57 38.40 55.62 2.12 1.53 Medium Red Red 28 Oxide C -- 30.40 .94 13.60 42.30 Brown ==========+=====+===========+==========+=============+=======+=========
=Analysis of Iron Oxide Pigments.= Because of the great consideration now being given to iron oxide paints, the writer secured a series of oxides widely used in this country, and has determined the most important constituents of each.
=Basic Lead Chromate (American Vermilion).= By boiling white lead with chromate of soda and subsequently treating with small quantities of sulphuric acid, American vermilion, or basic lead chromate, is prepared.
It contains 98% of lead compounds, frequently free chromates, and has a gravity of 6.8. The particles appear granular and large, frequently assuming a square structure.
=Red Lead.= By the continued oxidation of litharge in reverberatory furnaces, red lead is produced as a brilliant red pigment with a specific gravity of 8.7. The pigment particles appear to be of many sizes, showing a slight tendency to form a compact mass.
=Paranitraniline Red.= Paranitraniline red, a very bright red material largely used in tinting paints, is prepared by diazotizing paranitraniline in hydrochloric acid by means of sodium nitrite in the cold. This compound is rendered insoluble when precipitated directly on barytes, by acting on it with an alkaline solution of beta naphthol. It is the most stable and permanent bright red organic pigment which the paint manufacturer uses. The particles of this pigment appear in various sizes, due, no doubt, to a massing of the particles in the precipitation process.
=Chrome Yellow.= The neutral chromate of lead, made from either the nitrate or acetate of lead and chromate of soda, finds wide use as a tinting pigment. When precipitated on a white pigment base, various trade names are given to it. The microscope shows clearly the physical character of this pigment.
=Zinc Chromate.= This pigment is made either from zinc salts and bichromate of potash or zinc oxide heated with chrome salts, frequently in the presence of acid. Like the rest of the chromate pigments, it is a very slow-drying material, often requiring over a week to set up, unless considerable drier is added. In spite of the impurities which it carries, it has shown itself to be one of the most inhibitive pigments known and has demonstrated its value in even small percentages in paints for iron and steel. It dries to a hard adherent film that tends to protect metal from corrosion.
[Illustration: Indian Red]
[Illustration: Metallic Brown]
[Illustration: Basic Lead Chromate (American Vermilion)]
[Illustration: Red Lead]
[Illustration: Paranitraniline]
[Illustration: Chrome Yellow]
=Prussian Blue.= On oxidizing the precipitate resulting from the interaction of solutions of prussiate of potash and copperas (iron sulphate), Prussian blue as used in the paint trade is prepared. It has a specific gravity of 1.9. The pigment shows an amorphous structure, the particles varying greatly in size.
=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. It has a gravity of 4 and is liable to contain slight traces of lead salts. The particles when magnified appear very fine and flocculent. This color is often precipitated on pigments, such as barytes, which do not reduce its tone.
=Bone Black.= By grinding the carbonaceous matter resulting from the charring of bones, in iron retorts, the pigment bone black is prepared.
It contains about 15% of carbon and 85% of calcium phosphate. It has a gravity of 2.7. Comparatively large particles of charred bone can be seen scattered throughout the mass, resulting from the difficulty of grinding to a uniform size.