354 REPORT ON NATIONAL MUSEUM, 1886. 



With the softer and more absorbent stones, on the other hand, the 

 rock surface from its irregularity and roughness is more susceptible to 

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

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

 disintegration is less noticeable than on finely finished work. With 

 such stones a smoothly sawn 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 lindiDg some 

 tiuidal substance into which the stone may be dipped or which may be 

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

 pores and thus prevent all access of moisture. W r hatever 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 employed on the porous sandstone of the Capitol, White 

 House, Patent Office, and other public buildings in Washington. It is 



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

 sawn or tool-dressed one, and hence it would seem that it should ho more durahle. 

 It is of course possible that, owing to the manner in Which the smooth surface neces- 

 sary for polishing was produced, the surface minerals were hadly shattered, and hence 

 succumbed the more readily on exposure. 



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

 (Rep. on Building Stones, p. 36-37), says: "In the dressing of limestone the tool crushes 

 the stone 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 

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

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

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

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

 havo been broken off 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 «fine hammer-dressing is 

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

 a portion of the surface left in a condition to be much more readily aeted upon by the 

 weather." 



