352 REPORT ON NATIONAL MUSEUM, 1880. 



K. METHODS OF PROTECTION AND PRESERVATION. 



(1) PRECAUTIONARY METHODS. 



Position in wall. — All authorities agree that stratified stone should 

 be placed in the walls with the bedding horizontal, or at right angles 

 to the direction of greatest pressure. Not only are they as a rule 

 strongest in this position, but as they will absorb less water they arc 

 correspondingly less liable to suffer from the effects of frost. This 

 fact has already been sufficiently dwelt upon. The denser and harder 

 stones should as a rule be used in the lower courses; the lighter ones 

 in the superstructure. The non-absorbent stones should be used in the 

 ground and in plinths, sills, strings, courses, and weather beds of cor- 

 nices, etc.; the softer and more absorbent ones may be used for plain 

 walling.* 



The necessity of laying non-absorbent stones in the ground becomes 

 apparent when we consider that in this position they are in contact with 

 more or less moisture, which, when absorbed, is liable to cause discolora- 

 tion and damp, unhealthy walls. If from necessity porous stone are 

 used, a coating of water-proof material, as asphalt, should be interposed 

 between those courses that are in contact with the ground and those of 

 the superstructure.! 



In laying the lower courses of Lee dolomite in the walls of the Capi- 

 tol at Washington, the stone was observed to show a brownish discol- 

 oration, due to the absorption of unclean water from the mortar. This 

 was finally remedied by coating the lower surfaces of the stones where 

 they came in contact with the mortar with a thin layer of asphalt which 

 prevented such absorption and thus removed the difficulty.^ 



No one who has given the subject any attention can have failed to 

 remark how, in town and city houses constructed of the Connecticut or 

 New Jersey brown sandstones, the blocks in the lower courses — those in 

 close proximity to the sidewalks — almost invariably scale after an ex- 

 posure of but a few years, while those in the courses above remain in- 

 tact for a much longer period. This is due to the fact that these lower 

 courses are kept almost constantly wet, receiving not only the water 

 that falls as rain upon the walls above, but also that which splashes 

 from the walk or is absorbed from the ground. As noted by Chateau 

 (op. cit., p. 352), it is not those portions of a wall that receive the water 

 from rains direct that are most and earliest liable to decomposition, but 

 the under and partially protected portions, as those under the cornices, 



* Cyclopedia of Arts and Sciences, Vol. vn, p. 839. 



1 T. Eggleston, Am. Arcli., Sept. 5, lb85. This authority states further, that in Ihe 

 exterior walls of Trinity Church, New York, ihe stone for the first 60 or 70 feet in 

 height is more decomposed than above this point. This is accounted for in part on 

 the supposition that the atmosphere near the ground contains a larger proportion of 

 acid gases than at higher altitudes. 



t Sill. Jour., xxii, 1856, p. 36. 



