This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
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. A stone may be subjected to various tests, some of which afford a certain amount of information as to its characteristics. An important guide to the relative qualities of different stones is obtained by immersing them for 24 hours, and noting the weight of water they absorb. The best stones, as a rule, absorb the smallest amount of water: good traps and granites, 1/10-1/2 per cent.; good sandstones, 8-10 per cent.; good limestones, 1 1/2-15 per cent.
Small pieces of the stone are immersed in a concentrated boiling solution of soda sulphate (Glauber's salts), and then hung up for a few days in the air. The salt crystallizes 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. This action is not similar to that of frost, inasmuch as water expands in the pores as it freezes, but the salt does not expand as it crystallizes.
Simply soaking a stone for some days in dilute solutions containing 1 per cent. sulphuric and hydrochloric acids, 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 effervescence if a large proportion of lime or magnesia carbonate is present.
Smith's test 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 1/3 full of clear water; let them remain undisturbed at least 1/2 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 uncrystallized 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 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 bo 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.
In quarrying stone for building purposes, there should be as little blasting as possible, as it shakes the stone, besides causing considerable waste. Care should be taken to cut the blocks so that they can be placed in the work 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 3 classes: .
(3) calcareous; according as flint (silica), clay (formerly called "argile"), or lime carbonate, forms the base or principal constituent. In describing the physical characteristics of stones, for practical purposes it is better to classify them as follows : .
(1) granites and other igneous rocks.
(2) slates and schists.