Speaking generally, in comparing stones of the same class, the least porous, most dense, and strongest, will be the most durable in atmospheres which have no special tendency to attack the constituents of the stone.
A recent fracture, when examined through a powerful magnifying glass, should be bright, clean, and sharp, with the grains well cemented together. A dull, earthy appearance betokens a stone likely to decay.
This is not a very important test, for the reasons given at page 6, but some authorities consider that it affords an idea of the powers of the stone with regard to resisting frost.
A more important guide to the relative qualities of different stones is obtained by immersing them for twenty-four hours, and noting the weight of water they absorb. The best stones, as a rule, absorb the smallest amount of water.
The Table at p. 83 shows the amount of water absorbed in twenty-four hours by several of the most important English stones, some known to be durable, and others the reverse. This will afford a useful guide in judging of the quality of any new stone after ascertaining its powers of absorption.
The salt crystallises in the pores of the stone, sometimes forcing off bits from the corners and arrises, and occasionally detaching larger fragments.
The stone is weighed before and after submitting it to the test. The difference of weight gives the amount detached by disintegration. The greater this is, the worse is the quality of the stone.
The action of the salt was supposed at one time to be similar to that of frost, but Mr. C. H. Smith has pointed out that it is essentially different, inasmuch as water expands in the pores as it freezes, but the salt does not expand as it crystallises.
Simply soaking a stone for some days in dilute solutions containing 1 per cent of sulphuric acid and of hydrochloric acid, will afford a rough idea as to whether it will stand a town atmosphere.
A drop or two of acid on the surface of the stone will create an intense effervescence if there is a large proportion present of carbonate of lime or carbonate of magnesia. Mr. G. H. Smith's Test was proposed for magnesian limestone, but is useful for any stone in determining whether it contains much earthy or mineral matter easy of solution.
"Break off a few chippings about the size of a shilling with a chisel and a smart blow from a hammer; put them into a glass about one-third full of clear water; let them remain undisturbed at least half an hour. The water and specimens together should then be agitated by giving the glass a circular motion with the hand. If the stone be highly crystalline, and the particles well cemented together, the water will remain clear and transparent, but if the specimens contain uncrystallised earthy powder, the water will present a turbid or milky appearance in proportion to the quantity of loose matter contained in the stone. The stone should be damp, almost wet, when the fragments are chipped off."
The best way of carrying out this test is to pulverise the stone and then treat it as above described. The heavy particles will sink to the bottom and the earthy turbid matter will settle more slowly.
The durability of a stone to be obtained from an old established quarry may generally be ascertained by examining buildings in the neighbourhood of the quarry in which the stone has been used.
If the stone has good weathering qualities, the faces of the blocks, even in very old buildings, will exhibit no signs of decay; but, on the contrary, the marks of the tools with which they were worked should be distinctly visible.
Exposed cliffs or portions of old quarries, or detached stones from the quarry, which may be lying close at hand, should also be examined, to see how the stone has weathered.
In both cases care should be taken to ascertain from what stratum or bed in the quarry, the stones have been obtained.
This is too large a subject to be entered upon in these Notes.
Care should be taken to cut the blocks so that they can be placed in the work for which they are intended with their natural beds at right angles to the pressure that will come upon them.
If this is not attended to, the blocks will be built in in a wrong position, or great waste will be incurred by converting them.
The different kinds of stone used for building and engineering works are sometimes divided into three classes: - 1. The Siliceous. 2. The Argillaceous. 3. The Calcareous; according as flint (silica), clay (formerly called "argile"), or carbonate of lime,1 forms the base or principal constituent.
In describing the physical characteristics of stones, for practical purposes it will be better to classify them as follows: -
1. Granites and other igneous rocks.
2. Slates and Schists.
1 Calcium Carbonate.