Mineral browns, or, as they are more familiarly known, metallic browns, are pigments containing more or less sesquioxide of iron with a portion of inert materials as they occur with the iron oxide in nature, such as alumina, silica, with traces of lime and manganese. These ores are found in their best composition in the Lehigh Valley region of Pennsylvania, but also in Virginia, Tennessee and Alabama. The ores found in Pennsylvania are in two varieties, one being natural hydrated oxide or iron, while the other form is carbonate of iron (siderite). In either case the ore is calcined for several hours at red heat, which process changes the hydrated oxide as well as the carbonate to the sesquioxide. The highest percentage of sesquioxide produced from the siderite form of ore does not exceed 45 per cent while it mostly averages 38 per cent sesquioxide of iron (Fe203) only. The metallic brown prepared from the natural hydrated ore reaches as high as 72 per cent of Fe203, but many prefer the former grade, especially for making brown roof paints in the liquid form, its specific gravity being much lighter, keeping in better suspension in the paint. As is well known, metallic or mineral brown paint is specified by a number of railroad corporations as well as by the Army and Navy Departments and the various U. S. Lighthouse Districts, and the Isthmian Canal Commission have purchased millions of pounds by contract. While most of this brown is used for painting wooden surfaces, such as freight cars and freight stations, way houses, bridges, etc., it is a fact long established that a well-prepared metallic brown, no matter what it may contain in its composition as to the percentage of sesquioxide of iron, and if free from any appreciable traces of sulphur and ground in and thinned with pure linseed oil and a minimum of good drier, makes a good protective paint for iron or steel, as well as for wood. The government service specifications are especially emphatic in barring out any metallic brown deliveries that show in the pigment sulphur in any form that would equal more than 2 per cent of sulphuric anhydride. Mineral or metallic brown should be ground very fine in the dry state, especially when the paste or semi-paste is to stand the test for fineness prescribed by government and railroad specifications, otherwise the grinding will not only become expensive, but it is difficult to maintain the standard shade on account of the heat generated in the mills, and by running the paste through very close stones it is liable to become gummy and will not break up so readily on mixing, a point on which much stress is laid.
Iron or steel mills are not adapted for grinding mineral browns nor can roller mills be recommended. Buhr stone mills of 24 to 30-inch diameter, with hoppers of good capacity and a good grinding surface, are best for these pigments. For a heavy paste 78 per cent dry pigment and 22 per cent raw linseed oil is required to grind the metallic brown of light gravity, while for the heavy gravity brown, that runs from 60 to 70 per cent in sesquioxide of iron, 20 per cent oil to 80 per cent pigment will be sufficient. A gallon of 231 cubic inches of the former will weigh 17 to 17 1/4 pounds, while of the latter grinding a gallon will weigh 18 pounds net. When it is specified that the brown shall not contain in the pigment over 40 per cent of sesquioxide of iron and that the consistency be that of a semi-paste, showing approximately 75 per cent of dry pigment and 25 per cent raw linseed oil, a gallon of this semi-paste should weigh 15 1/2 pounds net.
The practice of mixing the dry mineral or metallic brown by hand in the oil should be discouraged by the manufacturers of the dry pigment as well as by color grinders and their representatives, because many roofers simply use cheap mineral oil or even kerosene for mixing the pigment for painting tin roofs, thus causing the metal to corrode very rapidly, while if the pigment was properly enveloped in linseed oil the paint would give protection for years. There are offered to the paint market some earths as mineral or metallic browns that contain very little oxide of iron and are no better than ordinary colored clay or slag, the use of which should be carefully avoided by reputable manufacturers.