Paint Technology and Tests

by Henry A. Gardner

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 Oxide		Calc. Sulph. (CaSO4)	Alumina (Al2O3)	Insoluble (Silica and Silicates)	Color
		FeO	Fe2O3
		%	%	%	%	%
0	Bright Red	0.71	96.52	—	—	.30	Bright Scarlet
1	Bright Red	.71	95.92	—	—	.30	Scarlet Tone
2	Indian Red	.57	96.00	.78	1.40	.90	Indian Red, Medium Shade
3	Indian Red	0.29	97.82	.85	—	.52	Indian Red, Dark Shade
4	Indian Red	0.28	95.72	1.21	1.26	.58	Indian Red, Light Shade
5	Persian Gulf Mix	4.53	62.25	1.75	—	27.64	Rich, Medium Red
7	Native Red Oxide	0.85	89.00	—	0.91	6.09	Medium Red, Brownish Tone
8	Special Red	0.57	43.87	50.88	2.03	1.30	Scarlet Tone
10	Red Oxide	1.44	60.25	.78	5.41	15.78	Brownish-Red
11	Venetian Red	.30	34.08	52.60	2.20	3.39	Bright Red-Brown
12	B. Oxide	0.58	67.68	—	2.48	1.97	Dark Red Brown
13	Venetian Red	0.29	25.92	58.62	2.16	1.42	Medium Red Tone
14	Venetian Red	0.57	35.36	.99	12.06	47.97	Brown
15	Metallic Brown	2.59	64.00	.63	5.82	23.42	Rich Brown
16	Crimson Oxide	0.57	66.24	1.77	3.60	25.63	Rich Dark Red
17	Red Oxide	2.30	80.39	.37	.03	9.63	Medium Brown
18	Red Oxide	0.57	61.28	.97	2.68	15.94	Light Chocolate Brown
20	Red Oxide	7.78	46.72	1.70	7.64	20.38	Dark Reddish Brown
23	Special French Oxide	0.58	72.48	—	8.80	4.48	Deep Chocolate Brown
24	Micaceous Black Oxide	2.02	86.27	—	2.04	9.50	Dark Gray Tone
25	Black Oxide	33.12	57.12	—	1.44	—	Jet Black
26	Red Oxide	0.57	84.16	5.00	2.00	.63	Deep Red
27	Special Red	0.57	38.40	55.62	2.12	1.53	Medium 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.