LINSEED OIL 331 



adheres well to the metal, and is durable." The pigment should be 

 inert to the metal to which it is applied and also to the oil with which 

 it is mixed. Linseed oil is commonly used as the varnish or vehicle 

 in oil paints, and is unsurpassed in durability by any other drying oil. 

 Pure linseed oil will, when applied to a metal surface, form a trans- 

 parent coating that offers considerable protection for a time, but is soon 

 destroyed by abrasion and the action of the elements. To make the 

 coating thicker, harder and more dense, a pigment is added to the oil. 

 An oil paint is analogous to concrete, the linseed oil and pigment in the 

 paint corresponding to the 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 flax- 

 seed. 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 be- 

 ing 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 tem- 

 perature of 400 to 500 degrees Fahr., 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 



