48 NEW YORK STATE MUSEUM 



pressure exerted by the freezing of the included moisture that 

 causes the damage. As already stated, however, neither the porosity 

 nor the ratio of absorption can be regarded as an index of the 

 resistance to such action under all conditions, since the character of 

 the pore cavities exercises probably even more influence than their 

 relative proportion. 



Other things being equal, if the pores are sufficiently large and 

 connected to permit the fairly rapid escape of the absorbed water, 

 the stone will prove more resistant than one having an intimate 

 network of fine or capillary pores. 



The expansion of water in changing to solid ice amounts to one- 

 tenth of its volume. It is, therefore, necessary that the pores 

 should be filled to about nine-tenths of their capacity before the 

 frost begins to become effective; otherwise, there will be room for 

 the expansion to take place without exerting any pressure. In 

 nature, the condition of saturation in stone is very rarely approached 

 and it is difficultly attainable even with the methods employed in 

 the laboratory for determining porosity. It is, therefore, the degree 

 to which the pores of a stone can be filled under natural conditions 

 that determines the resistance to frost. The" experimental tests in 

 which complete saturation is established by long-continued soaking 

 or with the aid of a vacuum are too severe for practical use. 



Hirschwald found that pieces of sandstone and granite removed 

 from a building in Berlin at the end of December, about the begin- 

 ning of freezing weather, and after a rainfall of 80 mm in the 

 months of November and December, showed only a fraction of 

 the moisture they were capable of absorbing. The specimens were 

 taken from a height of 20 cm above and below the ground level. 

 The samples of sandstone contained from one twenty-fourth to 

 one twenty-eighth the amount of water they would hold after one 

 hour's immersion. The granite from above ground level held about 

 one-third and that from below the same quantity that the granite 

 would absorb in one hour. 



The quantity of water absorbed by stone under natural condi- 

 tions divided by the amount the same stone requires for the entire 

 filling of the pores is termed the saturation coefficient. The danger 

 point is reached when the coefficient is .9, as with more than that 

 proportion the water on freezing will expand and exert pressure 

 upon the cavity walls. According to Hirschwald, who bases his 

 conclusions on about twelve hundred tests of different stones, the 

 practical limit may be taken at .8. 



