354 REPOKT ON NATIONAL MUSEUM, 1886. 



With the softer and more iibsorbent stoues, on tlie other hand, the 

 rock surface from its irregularity and rougimess is more susceptible to 

 the attacks of moisture and atmospheric acids, and hence would proba- 

 bly be found less durable, although from its roughness at the start any 

 disintegration is less noticeable than on finely finished work. With 

 such stones a smoothly sawu or polished surface seems best adapted to 

 our variable climate.* 



(2) PROTECTION BY MEANS OF SOLUTIONS. 



Many methods have been devised for checking or altogether prevent- 

 ing the unfavorable action of the weather upon building stone of va- 

 rious kinds, but none of them can be considered as really satisfactory. 

 The problem, as may readily be understood, consists in finding some 

 fluidal substance into which the stone may be dii)ped or which may be 

 applied with a brush to its outer surface in such a manner as to till its 

 pores and thus prevent all access of moisture. Whatever the sub- 

 stance, it must be of such a nature as in no way to discolor or disfigure 

 the stone. 



Paint. — This is one of the substances most generally used and which 

 has been em])loyed on the porous sandstone of the Capitol, White 

 ilouse, Patent Oflice, and other public buildings in W^ashington. It is 



proof is offered. A polished surface must naturally shed water more readily than a 

 sawn or tonl-dressed cue, and hence it would seem that it should ho more durahlo. 

 It is of course possible that, owing to the uianuor in which the smooth surface neces- 

 sary for polishing was ])roduced, the surface minerals were badly shattered, and hence 

 succumbed the more readily on exposure. 



* Professor Ilall, writing on the methods of dressing certain argillaceous limestones 

 (Kcp. on Building Stones, p. 3(3-37), says : "In the dressingof limestone the tool crushes 

 thestone to a certain depth, and leaves the surface with an interrupted layer of a lighter 

 color, in which the cohesion of the particles has been partially or entirely destroyed; 

 and in this condition the argillaceous seams are so covered and obscured as to be 

 scarcely or at all visible, but the weathering of one or two years usually shows their 

 presence. 



" The usual process of dressing limestone rather exaggerates the cause of dilapida- 

 tion from the shaly seams in the material. The clay being softer than the adjacent 

 Ktoue and the blow of the hammer or other tool breaks the limestone at the margin 

 of the seam and drives forward in the space little wedge-shaped bits of the harder 

 gtone. A careful examination of dressed surfaces will often show the limestone along 

 the seam to be fractured with uumeroua thin wedge-shaped slivers of the stone which 

 have been broken olf and are more or less driven forward into the softer parts. In 

 looking at similar surfaces which have been a long time exposed to the weather, it 

 will be seen that the stone adjacent to the seam presents an interrupted fractured 

 margin, the small fragments having dropped out in the process of weathering. Lime- 

 stones of this character are much better adapted to rough dressing, when the blows 

 are directed away from the surface instead of against it, and when the entire surface 

 shall be left of the natural fresh fracture. By this process the clay seams have not 

 been crushed, nor the limestone margining them broken, and the stone withstands 

 the weather much longer than otherwise. The attempt at «line hammer-dressing is 

 injurious to any stone, for the cohesion of the ixwticles is necessarily destroyed, and 

 a portion of the surface left in a condition to bo much more readily acted upon by the 

 weather." 



