13. Weight Of Stone

Weight Of Stone. The following are average weights of stone of the class mentioned, per cubic foot:

Marble, in blocks.................

. 170 pounds.

Limestone, in blocks..............

. 158 pounds.

Granite, in blocks...................

. 167 pounds.

Sandstone, in blocks..............

. 139 pounds.

Slate, in blocks..................

. 174 pounds.

The New York, Boston, and Chicago building laws give the weight of building stone of all kinds, when laid in the wall, at 165 pounds per cubic foot, which is near enough for most computations.

14. Color

Color. In rural districts and places where little or no soft coal is consumed, light-colored stones may be used, with little liability of their becoming dirty or disFigured, while in very smoky cities they will become very dark in a few years. In such cases, the red or brown silicious, or flinty, sandstones are the most durable; and next in value are the granites. The stone which retains its native color best is the most desirable to use; but, when it does change, the one most to be preferred is that in which the alteration is as little as possible, and uniform throughout.

15. Durability

Durability. The durability of stonework is of prime importance. If the stone will last only a comparatively short time, it is practically throwing away money to use such a poor kind. It is evident, therefore, that buildings of importance should be constructed of the most durable materials to be had.

The following table, from the census of 1880, gives the length of time that the several varieties of stone named have lasted in New York City without material deterioration:

Brownstone, coarse...................

5

to

15 years.

Brownstone, fine laminated.....

20

to

50 years.

Brownstone, compact...........

100

to

200 years.

Bluestone (blue shale)..........

100

to

200 years.

Sandstone, Nova Scotia.................

50

to

100 years.

Limestone, Ohio, best silicious.....

100

to

200 years.

Limestone, coarse fossiliferous. .......

20

to

40 years.

Limestone, oolitic..............

30

to

40 years.

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

40

to

50 years.

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

50

to

100 years.

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

75

to

200 years.

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

50

to

200 years.

16. Probably ordinary variations of temperature test building stones most severely. Stones consist of particles cohering more or less closely, and an increase in temperature causes each particle to expand, tending to force apart those surrounding it; while with a lowering of temperature a corresponding contraction occurs. As the temperature is ever varying, there is a continual motion of the particles, which, although very small, will, in course of time, produce cracks and result in the slow' and gradual destruction of the stone. Such changes are among the most potent causes of the disintegration of stone.

The effect of frost on stones saturated with moisture is always disastrous. The expansive force exerted by water in solidifying is nearly 140 tons per square foot; hence, a stone of open texture, which is exposed to heavy rains, and then to the action of frost, must suffer deterioration in course of time. Sandstones are the most porous, and granites the least. For this reason, granite is best adapted for use in wet places, as in foundations, etc.

Stone should always be laid on its natural bed wherever possible. If placed so that the layers are vertical, water penetrates between them much more easily, and, on freezing, will very quickly split the stone. Stones, such as sill and belt courses, so placed that rains wash over them, will deteriorate much more rapidly than the rest of the masonry, and on this account should always be the most durable kinds.

17. Atmospheric gases, when brought by rains into contact with the exposed surfaces of some kinds of stone, often affect their durability. The changes are the results of oxidation and solution. When iron exists in stone in the form of pyrites, it becomes combined with the oxygen in the air, producing discoloration, known as rust. When very minute, these particles of iron pyrites are not injurious, and the only effect of the rust is to give the stone a yellowish tinge. But if the pieces are of considerable size, the oxidation will discolor the stone unevenly. Some authorities say that the presence of pyrites in small quantities is beneficial to the stone, increasing the tenacity by its cementing qualities.

18. Pure water has practically no effect on building stone. Rain, however, contains traces of nitric, sulphuric, and other acids, absorbed from the smoke, etc. in the air. These, when brought in contact with stones, tond to dissolve such portions as are soluble. Lime and magnesia in the form of carbonates (as in all marbles and limestones) are, in particular, easily acted on. Sandstones containing iron or lime suffer from the same causes, while granites are the least affected.

19. Heavy pounding or hammering tends to destroy the cohesion of the grains, thus rendering the stone more readily acted on by the atmosphere. Only granites and the hardest sandstones should be peen or bush hammered. The most durable surface for granite is rock faced, as the crystalline facets, being but little disturbed in the dressing, shed moisture readily. For other stones, however, a smooth surface is usually the best in a changeable climate. Quarrying by explosives often causes cracks in the stone, so small as to be unseen until the application of the load increases them enough to make them visible. The fracture of stones in buildings is often due more to imperfect setting than to any lack of strength in the stone.

For some purposes, as for steps, door sills, paving, etc., the hardness of a stone is of importance, in which case granites and other hard stones are the most suitable.

20. In selecting building stone, it is often a point of much importance that it should have good fire-resisting properties. The fine-grained compact sandstones endure fire the best; while the exposed surfaces of limestones and marbles become converted into lime by intense heat. Granites are more affected than sandstones, but less than limestones.