BUILDING STONES AND ROAD METALS 181 
A sandstone should be compact, for, if porous, the water 
attacks the cement, and weakens the stone on freezing. 
Hence the brown sandstone used in New York is rejected 
if it weigh less than 130 lb. a cubic foot, or absorbs over 
5 per cent. of water on 24 hours’ immersion, as either test 
indicates too high a porosity. 
Slate is often a durable building material. The main 
source of weakness is when its iron sulphide occurs as mar- 
casite instead of as the ordinary cubic pyrite. Slate might 
be more largely used, but for its dull colour. 
STONE PRESERVATION—The preservation of building stones 
depends mainly on preventing the entry of moisture. Paint 
is effective but expensive, as it has to be renewed, and re- 
moves the beauty of stonework. The exclusion of moisture 
is the purpose of numerous processes. Waterglass or silicate 
of soda fills the surface pores with silica. Sulphur dissolved 
in hot oil closes the pores with sulphur. The Szerelmy 
process uses silicate of soda containing a bituminous material. 
The drawback of any impermeable crust is that it flakes off 
owing to the expansion of the air within the stone. A 
coating of paraffin over the grains of the stone prevents the 
entrance of moisture by surface tension and yet allows the 
stone to breathe. The process is however too expensive for 
general application. A. P. Laurie has introduced the use 
of silicon ester (prepared by the action of alcohol on silicon 
tetrachloride) which deposits silica on the grains and not as 
a film; it leaves the pores open (¥. Soc. Chem. Ind.. xliv, 
1925, p. 91 T.)., 
The baryta method was designed by Church for the Chapter 
House at Westminster Abbey; ! it was built in the thirteenth 
century of Upper Greensand from Reigate in Surrey, which is 
a siliceous sandstone containing grains of glauconite and 
cemented by from IO to 15 per cent. of carbonate of lime. 
The cement has been converted by sulphuric acid into gypsum 
(hydrous sulphate of lime). The building was sprayed—as 
the stone was too friable to withstand a brush—with a solu- 
tion of barium hydrate (3 per cent. of BaO in water), which 
converts the sulphate of lime into the insoluble barium 
sulphate : the calcium of the gypsum is left as hydrate, 
LA. H. Church, Memoranda Concerning Treatment of . . . Chapter 
House . . . Parl. Pap., 1904, Cd. 1889.