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 marcasite
 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 removes
 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 solution
 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.