332 ANNUAL. REPORT OF THE Off. Doc. 



lu addition to durability, a good paint sliould possess body; that 

 is, tlie power to conceal tliorouglily underlying layers to which the 

 paint coat is applied; and also covering power; that is, the property 

 of being capable of spreading uniformly from a brush over a large 

 surface. 



Oft-times the painter desires to increase the covering or spread- 

 ing quality of a paint, and for that purpose mixes with the paint 

 volatile liquids known as thinners, which rapidly evaporate from 

 the thin layer of the paint-coat leaving the drying oil exposed to 

 the air. It is probable that the formation of varnish from the 

 drying oil is not so simple a chemical process when the oil is thor- 

 oughly mixed with certain pigments; such, for example, as white 

 lead. Experience shows further that where thinners are employed, 

 the physical quality of the paint coat is not so good, that is, not 

 so tough, durable and glossy as where the oil hardens in the ab- 

 sence of thinners. The principal thinning agent which is in use 

 by painters upon first-class jobs is spirit of turpentine, and even 

 this thinner must not be employed in too large proportion. 



Prominent adulterants of the liquid portion of paints are semi- 

 drying and non-drying oils such as corn or maize oil and cottonseed 

 oil, which are used as partial substitutes for linseed oil, and largely 

 diminish its varnish-forming power. Benzine from petroleum and 

 water are the other principal adulterants of the liquid portion of 

 oil paints. Benzine is used as a thinner, being a substitute for 

 spirits of turpentine. It volatilizes more rapidly than the turpen- 

 tine and leaves a fair inferior coating. Benzine is much cheaper 

 than spirits of turpentine, and water is simply a make-weight which 

 seriously reduces the gloss and life of the paint coating. Water 

 has the quality of giving to mixtures of some pigments, such as 

 white lead, and linseed oil, the appearance of a thicker consistence, 

 sometimes termed "a livery condition." Paints in this condition 

 do not spread uniformly from the brush. 



One other ingredient sometimes used in the liquid portion of the 

 ready-mixed paints is w^hat is called a drier. For this purpose either 

 boiled oil or a japan is used. Both of these preparations are made 

 by heating linseed oil in contact with oxid of lead (litharge) or 

 other metallic compound. In these processes the liquid takes up a 

 little of the metal which seems to act as a carrier of oxygen, so 

 that these driers possess, under certain conditions of use, the prop- 

 erty of forming varnish very rapidly, and when mixed with an oil 

 compound, of securing a rapid drying and hardening of the coating 

 which is often desirable in painting exterior surfaces in showery 

 weather, or the interiors of rooms that must be quickly prepared 

 for use. It is generally known, however, that excessive use of driers 

 is not advantageous and that where used in any considerable quan- 

 tity, the paint coat suffers in gloss and durability. 



It would be impossible in this brief paper to discuss in detail 

 the numerous pigments employed in the manufacture of paints. In 

 order that the paints may have both body and a large covering 

 power, it is clear that the pigments must be distributed through 

 the paint in a very finely divided condition. This is generally 

 secured by grinding them in oil as the final step in the process of 

 manufacture. With the exception of reds and yellows, the princi- 

 pal paints used for the exterior of dwellings, barns, etc., are made 



