176. Every stone intended for building purposes that does not come from some well-known quarry should be tested by chemical analysis, and the results compared with the analyses of well-known stones of the same kind, and if found to differ materially in constituents soluble in water or attached by sulphuric or carbonic acids, they should be rejected; the presence of iron pyrites should also lead to the rejection of the stone, if intended for external use. If the building is one of importance the architect should insist on the owners getting the opinion of some expert chemist or mineralogist on the durability and weathering qualities of the stone.
As a rule, however, most buildings are now built from stone taken from well-known quarries, whose weathering qualities have been proved, so that if the quality is equal to the best that the quarry will supply the stone will prove all that was expected of it. The fact, however, that certain quarries have furnished good material in the past is no guarantee of the future output of the entire quarry. This is especially true regarding rocks of sedimentary origin, as the sand and limestones, different beds of which will of ten vary widely in color, texture, composition and durability, though lying closely adjacent. In many quarries of calcareous rocks in Ohio, Iowa and neighboring States, the product is found to vary at different depths, all the way from a pure limestone to magnesian limestones and dolomite, and in many cases an equal variation exists in point of durability.*
The architect should, therefore, make a careful examination of the stone as it is delivered on the ground, or in the yard before it is cut, to see that the quality of the stone is up to the standard, and in large buildings in which a great quantity of stone will be required, it will be advisable to visit the quarry and determine from which part of the quarry the stone shall be taken.
The following rules and tests will enable one to judge if the stone is of a good quality and likely to prove durable:
* "Stones for Building and Decoration," p. 380.
Compactness. - As a general rule, 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 good building stone should also give out a clear, ringing sound when struck with a hammer.
Fracture. - A fresh fracture, when examined through a powerful magnifying glass, should be bright, clean and sharp, with the grains well cemented together. A dull, earthy appearance indicates a stone likely to decay.
One of the most important tests for the durability of stone is that of the porosity, or degree with which the stone absorbs moisture, since, other things being equal, the less moisture a stone absorbs the more durable it will be.
To determine the absorptive power the specimen should be thoroughly dried at about 100° F. and carefully weighed; it must then be soaked for at least twenty-four hours in pure water; when removed from the water, the surface allowed to dry in the air and then weighed. The increase in weight will be the amount of water absorbed, and will stand, although not absolutely correct, as an expression of the stone's absorptive power. This test is extremely simple, and when done with care should give very practical results.
Any stone which will absorb 10 per cent, of its weight of water during twenty-four hours should be looked upon with suspicion until, by actual experiment, it has shown itself capable of withstanding, without harm, the different effects of the weather for several years. Half of this amount may be considered as too large when the stone contains any appreciable amount of lime or clayey matter.*
The porosity of a stone also has influence upon its appearance when in the building.
A non-absorbent stone is washed clean by each heavy rain, and its original beauty is retained, while a porous stone soon fills with dirt and smoke and looks little better than a wall plastered with cement. Even in stones for interior decoration absorption should not be overlooked, as ink, oils or drugs may ruin expensive furnishings if the stone is porous.
178. Acid Test.† - 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.
* "Stones for Building and Decoration," p. 371.
† "Notes on Building Construction," Part III., p. II.
Test for Solution. - The following simple test is useful for determining whether a stone contains much earthy or mineral matter easy of solution :
Pulverize a small piece of the stone with a hammer, put the pulverized stone into a glass about one-third full of clear water, and let the particles remain undisturbed at least half an hour. Then agitate the water and broken stone 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 specimen contains uncrystallized earthy powder, the water will present a turbid or milky appearance in proportion to the quantity of loose matter contained in the stone.