192 RICHMOND. 



for use. However, it has been found in commercial practice that natural 

 resins will not dissolve in raw drying oils. In some instances, a fairly 

 clear solution is produced by a little heat, but almost invariably the 

 dissolved resin ,will either separate from. the oil upon cooling, or it will 

 be precipitated upon the addition of turpentine; hence, the practice of 

 subjecting the oil and the resin, both before and after mixing, to elevated 

 temperatures is almost universal. 



A brief description of the present practice of making oleoresinous 

 varnishes seems necessary. The following is the method according to 

 Sabin : 14 



Melting the resin. — The American practice is to heat about 125 pounds of resin 

 at a time in a copper kettle over a coke fire. When all the lumps are melted and 

 the resin is quite liquid, i. e., when it drops from the stirring rod, it is removed 

 from the fire. By this time, which requires about half an hour, from 10 to 20 

 per cent of the resin is lost in the form of pungent, irritating, highly inflammable 

 vapors. A thermometer is not commonly used, for the temperature is not so 

 essential as the melted feel imparted through the stirring rod. The temperature 

 is seldom below 343° C. when the melting is completed. The temperature and 

 the percentage of loss vary greatly with different resins. 



Adding the oil. — When the resin is all melted and removed from the fire, the 

 linseed oil, made ready in another kettle, is slowly added with constant stirring. 

 The oil is previously heated to about 260° C. Of course, if only a little oil is 

 to be added the temperature does not have much effect on the mass, but it is 

 common to have the oil hot. 



Cooking the varnish. — As soon as the oil is added the kettle is put back on 

 the fire. Although the mixture appears to be a complete solution, it is not really 

 so -at this stage, for if a drop of it be removed and allowed to cool the resin will 

 separate, making the drop cloudy, and the common rule is to withdraw the 

 stirring rod from time to time and let a drop of the mixture fall on a piece of 

 glass, where it cools at once and shows' by its cloudiness that combination has not 

 or, by its clearness, that it has taken place. A more approved practice is to keep 

 a thermometer in the liquid and heat to a certain temperature for a certain length 

 of time, previously determined as the best for the particular varnish which is 

 being made. This temperature, roughly speaking, is not very far from 260°. 



Thinning down with turpentine. — When the oil and resin have been properly 

 cooked, the kettle is withdrawn from the fire and allowed to cool somewhat and 

 the liquid diluted with turpentine to standard consistency. 



It will be seen from the above description of American varnish manu- 

 facture, which in its main features holds true elsewhere, that the art of 

 making oleoresinous varnishes rests entirely upon an empirical basis; 

 and that the results of practical experience, in the art of treating and 

 blending the unmixed ingredients have been handed down through 

 generations to the present time without particular regard for the un- 

 derlying causes of the effects thus produced. 



There is a steadily increasing demand for resins of the type known 

 as copal, which when dissolved in drying oils produce varnishes distin- 

 guished by leaving a hard, lustrous, transparent coating when applied to 



"Technology of Paint and Varnish. New York (1907), 71-81. 





