27 TOWERS AND TANKS FOR WATER-WORKS. 



tain any large amounts of volatile matter, so as to chill the surface 

 by evaporation. 



" DRYING. It does not seem possible without further ex- 

 perimentation to reach a final conclusion on this point. Whether 

 the paint coats shall dry in six or twenty-four hours is a matter 

 to be determined by the contingencies of the case. In general 

 this recommended that as much time as possible be allowed 

 between coats. It is, however, considered practicable to have 

 an efficient metal coating dry in eight hours, 



" SUCCESSIVE COATINGS. The under coatings must not be 

 softened or acted upon by the subsequent coats of paint. 



" PROTECTIVE POWER. This is the keystone of the whole 

 subject. The coating must protect. To accomplish this it is 

 recommended that the coating must have the maximum imper- 

 meability to moisture, air, and carbon dioxide. Iron and steel 

 will not rust in dry air or in water free from air and carbon dioxide. 

 The best protection will, therefore, be obtained from the most 

 impervious coating. To this end the pigment should be as finely 

 ground as possible; and, finally, it is recommended that the 

 vehicle or pigment, or both, be water repellent. Whether iluV. 

 last characteristic is to be obtained by a pigment such as lamp- 

 black, or by the use of some non-drying oi , must be the subject 

 for further investigation. 



" DURABILITY. It is the opinion of the committee that coat- 

 ings should be efficient under ordinary conditions for at 1 

 five years. The durability measures the life of the coatinir; 

 it should therefore adhere to the metal through all ranges of eon- 

 traction and expansion without peeling or cracking. 



" Neither the pigment nor the vehicle, nor compounds resulting 

 from a reaction of the two, should cause a disintegration of the 

 coating. 



" It is further recommended that the coating should not be 

 affected by products necessary for the maintenance, equipment, 

 or use of the structure protected. This applies especially to the 



