614 ENGINEERING MATERIALS. CHAP. XV. 



cement and the aggregate in the concrete. The pigments used in making oil paints for protecting 

 metal may be divided into four groups as follows: (i) lead; (2) zinc; (3) iron; (4) carbon. 



Linseed Oil. Linseed oil is made by crushing and pressing flaxseed. The oil contains some 

 vegetable impurities when made, and should be allowed to stand for two or three months to purify 

 and settle before being used. In this form the oil is known as raw linseed oil, and is ready for use. 

 Raw linseed oil dries (oxidizes) very slowly and for that reason is not often used in a pure state for 

 structural iron paint. The rate of drying of raw linseed oil increases with age ; an old oil being 

 very much better for paint than that which has been but recently extracted. Raw linseed oil 

 can be made to dry more rapidly by the addition of a drier or by boiling. Linseed oil dries by 

 oxidation and not by evaporation, and therefore any material that will make it take up oxygen 

 more rapidly is a drier. A common method of making a drier for linseed oil is to put the linseed 

 oil in a kettle, heat it to a temperature of 400 to 500 degrees F., and stir in about four pounds of 

 red lead or litharge, or a mixture of the two, to each gallon of oil. This mixture is then thinned 

 down by adding enough linseed oil to make four gallons for each gallon of raw oil first put in the 

 kettle. The addition of four gallons of this drier to forty gallons of raw oil will reduce the time of 

 drying from about five days to twenty-four hours. A drier made in this way costs more than the 

 pure linseed oil, so that driers are very often made by mixing lead or manganese oxide with rosin 

 and turpentine, benzine, or rosin oil. These driers can be made for very much less than the price 

 of good linseed oil, and are used as adulterants; the more of the drier that is put into the paint, the 

 quicker it will dry and the poorer it becomes. Japan drier is often used with raw oil, and when this 

 or any other drier is added to raw oil in barrels, the oil is said to be " boiled through the bung hole." 



Boiled linseed oil is made by heating raw oil, to which a quantity of red lead, litharge, sugar of 

 lead, etc., has been added, to a temperature of 400 to 500 degrees F., or by passing a current of 

 heated air through the oil. Heating linseed oil to a temperature at which merely a few bubbles 

 rise to the surface makes it dry more rapidly than the unheated oil; however, if the boiling is con- 

 tinued for more than a few hours the rate of drying is decreased by the boiling. Boiled linseed 

 oil is darker in color than raw oil, and is much used for outside paints, It should dry in from 12 to 

 24 hours when spread out in a thin film on glass. Raw oil makes a stronger and better film than 

 boiled oil, but it dries so slowly that it is seldom used for outside work without the addition of a 

 drier. 



Lead. White Lead (hydrated carbonate of lead specific gravity 6.4) is used for interior and 

 exterior wood work. White lead forms an excellent pigment on account of its high adhesion and 

 covering power, but it is easily darkened by exposure to corrosive gases and rapidly disintegrates 

 under these conditions, requiring frequent renewal. It does not make a good bottom coat for 

 other paints, and if it is to be used at all for metal work it should be used over another paint. 



Red Lead (minium; lead tetroxide specific gravity 8.3) is a heavy, red powder approxi- 

 mating in shade to orange; is affected by acids, but when used as a paint is very stable in light and 

 under exposure to the weather. Red lead is seldom adulterated, about the only substance used 

 for the purpose being red oxide. Red lead is prepared by changing metallic lead into monoxide 

 litharge, and converting this product into minium in calcining ovens. Red lead intended for 

 paints must be free from metallic lead. One ounce of lampblack added to one pound of red lead 

 changes the color to a deep chocolate and increases the time of drying. This compound when 

 mixed in a thick paste will keep 30 days without hardening. 



Zinc. Zinc white (zinc oxide specific gravity 5.3) is a white loose powder, devoid of smell 

 or taste and has a good covering power. Zinc paint has a tendency to peel, and when exposed 

 there is a tendency to form a zinc soap with the oil which is easily washed off, and it therefore does 

 not make a good paint. However, when mixed with red oxide of lead in the proportions of i lead 

 to 3 zinc, or 2 lead to i zinc, and ground with linseed oil, it makes a very durable paint for metal 

 surfaces. This paint dries very slowly, the zinc acting to delay hardening about the same as 

 lampblack. 



Iron Oxide. Iron oxide (specific gravity 5) is composed of anhydrous sesquioxide (hematite) 

 and hydrated sesquioxide of iron (iron rust). The anhydrous oxide is the characteristic 

 ingredient of this pigment and very little of the hydrated oxide should be present. Hydrated 

 sesquioxide of iron is simply iron rust, and it probably acts as a carrier of oxygen and accele- 

 rates corrosion when it is present in considerable quantities. Mixed with the iron ore are 

 various other ingredients, such as clay, ocher and earthy materials, which often form 50 to 75 

 per cent of the mass. Brown and dark red colors indicate the anhydrous oxide and are considered 

 the best. Bright red, bright purple and maroon tints are characteristic of hydrated oxide and 

 make less durable paints than the darker tints. Care should be used in buying iron oxide to 

 see that it is finely ground and is free from clay and ocher. 



Carbon. The most common forms of carbon in use for paints are lampblack and graphite. 

 Lampblack (specific gravity 2.6) is a great absorbent of linseed oil and makes an excellent pigment, 

 Graphite (black lead or plumbago specific gravity 2.4) is a more or less impure form of carbon, 

 and when pure is not affected by acids. Graphite does not absorb nor act chemically on linseed 



