Painting Engineering Work

Manufacturer and builder 9, 1875

After questions of form, strength, constructive material, and similar matters have been duly settled in conncetion with any engineering work made of wood or motel, the engineer has to consider the best method of maintaining that work in good condition. Apart from working casualties, the material of which the particular work is constructed is exposed to atmospheric and chemical influences which tend more or less to modify and corrode its surface, and an artificiel surface is therefore formed by applying paint. Most of the paints used for ordinary work are composed of the coloring matter, then of a quantity of white lead, with which and a particular oil they are worked into a paste of the shade required, and are afterward trimmed down with oil and turpentine when used. The white lead which thus forms the basis of most paints, and is by itself a color, is the basic carbonate of lead, a heavy earthy powder, white when first made, but soon becoming of a grayish tint when exposed to the air, from the action of sulphureted hydrogen. It is insoluble in water, and effervesces with hydrochloric acid, dissolving when heated, as chlorid of lead, which crystallizes in needles on cooling. Dilute nitric acid easily dissolves white lead, with effervescence caused by the escape of carbonic acid gas. When hosted on a knife or a piece of glass it becomes yellow. It is not very generally known that white lead and oil combine with such energy that if linseed oil is poured upon a very large quantity of white lead, and the mass allowed to stand for a few hours, the temperature becomes so high that the oil is carbonized and colors the whole a black. We should carefully avoid mixing with white lead substances that may impair its brightness or depreciate its other qualities, and it should be kept in closed vessels, otherwise it will acquire a brownish shade. For good paint it should be pure and without foreign mixture; however, both manufacturers and painters add to it variable proportions of chalk, sulphate of lead, and the like, and it is often mixed with that sulphate of baryta which is called baryta white, and which is prepared from the native sulphate or from carbonate of baryta artificially treated with sulphuric acid. Baryta white is an adulteration which ceases to be objectionable when the manufacturer makes the composition known, as it is of a handsome white color, entirely innocuous, fast, and resisting most reagents, its great defect being that it possesses but little body or covering power. The manufacturers sell various qualities of white lead, sometimes in powder or in lumps, as genuine dry white lead, but the greater portion in A paste, holding from 7 to 9 per cent of oil. White lead paint is solid and durable, but the disagreeable vapors given off by the lead exercise a dangerous effect upon the health of the workmen who are engaged either in its manufacture or its use. Many substitutes have been tried to obviate the employment of white lead. Zino white in particular has received considerable attention; it has not such a bad effect upon the health, having no smell of itself, and does not impart any to the liquids with which it may be mixed, so that any place freshly painted with it may be at once inhabited without fear of its injuring the occupants. Zinc white is the oxid of zinc; it is insoluble in water, but dissolves in hydrochloric acid, usually effervescing slightly from the escape of carbonic acid, which oxid of zinc absorbs from the air. When heated, oxid of zinc becomes yellow, but resumes its white color on cooling. It is as brilliant, white, and fine as white lead, and on drying becomes so hard that it will she a bright polish; it does not alter under the destructive action of sulphurous vapors, or of equivalent gases; it covers a larger surface than carbonate of lead, but it is very dry under the brush, and therefore requires more labor in applying it, which in a great extent explains the disinclination to use it, in spite of all the efforts made in its favor. It also takes longer lo drying, and when adulterated it very liable to change color.

Red lead, so largely used by engineers, is an oxid of lead, usually in the form of a bright red powder, which is not affected by water, but evolves the smell of chlorin when boiled with hydrochloric acid, and is slowly converted into chlorid of lead. Dilute nitric acid only partly dissolves it, leaving a brown powder. On account of its durability, it is frequently used as the priming coat, often the only coat given on iron-work. Care should be taken that no salt is present, otherwise a chemical action commences, blisters are formed, and the lead is reduced to the metallic condition. It has been proposed to substitute for red lead a red obtained from a sulphid of antimony, termed antimony vermilion, which is sold in a state of very fine powder, without taste or smell, and which is insoluble in water, alcohol, or essential oils. It is but little acted on by acids, and when ground in oil acquires great intensity or brightness of color, has a good body, is unalterable by air or light, and may be freely mixed with white lead. Black paints made from the residual products obtained in distilling coal and shale oils are largely employed for rough work. They combine readily with drying oils, and give an intense and handsome black, which is at the same time very economical. Oxid of iron paints are most effective and durable paints to use on iron, as they have no tendency to change or affect the surface of the metal. An analysis of one of these paints gave, peroxid of iron, 68.95; aluminous earth (clay,) 1.48; burnt clay, 29.57: total, 100.

Under equal volumes iron paints cover more than those from lead; mixed with one-third of white lead, it forms an excellent mastic, similar to that made from red 1ead, and which becomes very hard after drying for some time. As the iron oxid paint resists strong heat, it is advantageously employed for painting parts of machines and boilers. The so called anti-corrosive paint is made of equal parts by weight of whiting and white lead, with half the quantity of very fine sand or road dust, with colors at pleasure. The mixture being made with water can he used as a water-color, but it is usually applied as an oil paint. The preparation of oil recommended for this purpose 12 parts by weight of linseed oil raw, 1 part of boiled linseed oil, and 2 parts of sulphate of lime, the whole well mixed. One gallon of oil thus prepared is used to 7 pounds of the paint. Paints containing silica have been used for both wood and metal; they give a hard surface which is very durable; it is stated that when mixed with proper oils they will resist the action of salt water or acids better than Iron or lead paint, that they cover well, and that in the ease of wood they form a considerable protection against fire. In addition to the pigments which are in themselves colors, various tints are produced by additions of ochres, earths naturally colored by iron; chromes or yellows, consisting of oxid of lead and chromic acid; blues, such as Prussian blue, from animal refuse burnt with potash and iron; smalts, from oxid of cobalt; ultramarine blue, from carbonate of soda, silica, alum, and sulphur; or greens, from oxid, carbonate, and arsenates of copper.

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