168. Durability

Naturally the durability of a stone is of the first importance, for unless the stone will last a reasonable length of time, the money spent on the structure will be largely wasted, and all public buildings should be built of material that is practically imperishable.

The following table, taken from the Report of the Tenth Census, 1880, Vol. X., p. 391, gives the number of years that different stones have been found to last in New York City, without discoloration or disintegration to the extent of necessitating repairs:

Coarse brownstone.........................................

5 to 15

Fine laminated brownstone...................................

20 to 50

Compact brownstone.....................................

100 to 200

Bluestone (sandstone), untried...........................

Probably centuries

Nova Scotia sandstone, untried..............................

Perhaps 50 to 200

Ohio sandstone (best siliceous variety),

Perhaps from one to many centuries

Coarse fossiliferous limestone.................................

20 to 40

Fine oolitic (French) limestone...............................

30 to 40

Marble, coarse dolomite.............................

40

Marble, fine dolomite.............................

60 to 80

Marble, fine.........................

50 to 100

Granite...........................

75 to 200

Gneiss..................................

50 years to many centuries

There are many circumstances and conditions, aside from the quality of the stone, that affect the durability of exposed stonework, the more important of which are heat and cold, composition of the atmosphere, position of the stone in the building, and manner of dressing the stone.

169. Heat and Cold

The most trying conditions to which a building stone is subject are the ordinary changes of temperature which prevail in the Northern and Eastern States. "Stones, as a rule, possess but a low conducting power and slight elasticity. They are aggregates of minerals, more or less closely cohering, each of which possesses degrees of expansion and contraction of its own. As temperatures rise each and every constituent expands more or less, crowding with resistless force against its neighbor; as the temperatures decrease a corresponding contraction takes place. Since the temperatures are ever changing, often to a considerable degree, so, within the mass of the stone, there is continual movement among its particles. Slight as these movements may be they can but be conducive of one result, a slow and gradual weakening and disintegration." * This is supposed to be the chief cause of the disintegration of granites.

There are several examples of old stonework in New York City that have begun to decay on the south and west sides, where the sun shines the longest, but not on the north and east. The effects of moderate temperatures upon stones of ordinary dryness are, however, slight when compared with the effects of freezing upon stones saturated with moisture. The pressure exerted by water passing from a liquid to a solid state amounts to not less than 138 tons to the square foot; and it is, therefore, evident that any porous stone exposed to heavy rains and a temperature several degrees below the freezing point must be seriously damaged by a single season's exposure. It is also evident that the more porous a stone the greater will be the deterioration, and as sandstones are the most porous of all building stones they suffer the most from this cause and granites the least, hence granite is the best stone for a base course or underpinning. [For the effect of absorption on the durability of stones see Section 177.]