1 68 ENGINEERING ON THE FARM 



sulphate and zinc sulphide are used to some extent for inte- 

 rior work where the exposure is less than on outside surfaces. 

 There is a number of mineral or metallic base substitutes 

 which cheapen the product in both price and quality. The 

 most common of these arc: gypsum, or plaster of Paris; 

 whiting, or chalk; ochre, a natural earth colored by iron 

 oxide and zinc oxide. The last is not a cheap substitute and 

 one which, if used in excess, produces a paint inferior to 

 that made of white lead. 



Vehicles. Linseed oil is the most extensively used vehicle. 

 It is used both raw and boiled. It is produced from flaxseed 

 through a process of grinding and pressing. Raw linseed oil 

 is better than the boiled for mixing the first or priming coat, 

 since the aim of this coat is to penetrate the wood as far as 

 possible. The boiled oil dries by producing a film over the 

 surface. This oxidizing of the surface is conducive to a 

 strong and lasting paint surface. The raw oil is slow to 

 oxidize. No real substitute has ever been found for linseed 

 oil, although there are numerous substitutes on the market, 

 some of which may be mixed with linseed oil in limited pro- 

 portions without materially affecting the life and appearance 

 of the paint. These are usually of animal or vegetable 

 nature . The most common vegetable oils are hemp and cotton- 

 seed. Either is quite difficult to detect. Fish oil is used 

 to some extent, and mineral oils when thinned with benzine. 



Solvents. A solvent is added to a paint to give a lighter 

 body or consistency. It is the intention that the solvent 

 shall evaporate. It is possible, therefore, to use more solvent 

 and produce a paint which will spread more easily, or which 

 may be spread in a thinner coat. Turpentine is the best 

 solvent known to the paint trade. It is a product distilled 

 from the yellow pine tree. It is colorless and has a distinc- 

 tive odor. It is adulterated by the use of mineral oils such 

 as gasoline, benzine, and naphtha. 



Driers. A drier is added to a paint to hasten oxidization 

 of the surface, thereby producing a quicker setting and drying 



