506 IOWA DEPARTMENT OF AGRICULTURE 



either turpentine or benzin, and are Ivuown as japan or japan driers. As 

 before stated, wliile the use of a drier is necessary in a great many paints, 

 the amount used should be small. It is a rather astonishing fact that 

 many driers, if used in small proportions, will very materially hasten the 

 drying of the linseed oil; whereas if a large amount of drier is added, the 

 drying of the oil is retarded. There is another objection to the use of a 

 large amount of drier, and that is that the tilm produced is not so durable 

 as one produced by raw linseed oil alone or by the use of a raw oil con- 

 taining the proper amount of drier. There are a number of other oils 

 which have the property of drying like linseed oil, but none of them is 

 the equal of linseed oil for a paint vehicle. 



Bearing in mind these facts, it is seen that an oil paint would consist 

 of the pigment mixed with a drying oil, preferably linseed oil, and gen- 

 erally with the addition of a drier. Some pigments, however, have the 

 property of hastening the drying of linseed oil, and when they are used 

 (red lead, for example) it is unnecessary to add any other drier. The 

 varnish-like film left by linseed oil is for practical purposes insoluble in 

 water. It is not, however, impervious to water. If a bright piece of iron 

 covered with a coating of linseed oil, and afterwards thoroughly dried, is 

 exposed to moisture it will be found that while the iron will not rust so 

 fast as uncoated iron, the rusting will take place to a considerable extent. 

 Other experiments can be performed which will demonstrate that moisture 

 passes through this film with comparative ease. But, if an oil paint is 

 employed — that is, a mixture of pigment and linseed oil — it will be found 

 that the water does not penetrate through the film so rapidly as it does 

 through the linsecd-oil film alone. Also the paint film is more resistant 

 to mechanical abrasion. While there is some difference of opinion among 

 experts as to the amount of pigment which should be used in a paint, it 

 is generally considered that the greater the amount of pigment the more 

 resistant the paint film is, provided all the particles of pigment are thor- 

 oughly covered with the oil. It would appear, therefore, that a film of oil, 

 while it may seem to be homogeneous even if examined under a high- 

 power microscope, is really porous, and by mixture of the oil with the 

 pigment the pores are more or less completely filled, thus making a more 

 impervious film. 



In addition to the linseed oil and drier, paints frequently contain 

 volatile substances, such as turpentine and benzine. The addition of 

 these is largely for the purpose of thinning the paint to a better work- 

 ing consistency, so that it can be spread in thin layers more easily. These 

 volatile substances evaporate almost completely and do not remain behind 

 in the dried film. The only substance remaining which binds the solid 

 particles of the pigment together is the oil. 



PREPARATION OF SURFACES FOR PAINTING. 



All surfaces should be clean and as dry as possible before the applica- 

 tion of an oil paint. Much new wood is very difficult to paint. The 

 resins in such woods as yellow pine and spruce tend to destroy any paint 

 that Is laid over them. When possible, it is well to allow a new house 



