When a stone is built into the walls of a building in such a way that the natural layers of the stone are vertical, or on edge, the water penetrating the stone and freezing causes the surface of the stone to exfoliate or peel off much quicker and to a greater extent than it would if the stone had been laid with its natural bed horizontal.
Stones that are so situated in a building that the rain will strike and wash over them, such as sills, belt courses, etc., also decay sooner than the ashlar forming the face of the wall, and should be of the most durable material.
The chemical action of the gases of the atmosphere, when brought by rain in contact with the surface of certain stones, seriously affects their durability. The most important changes produced by these agencies are oxidation and solution.
* " Stones for Building and Decoration," p. 353.
Oxidation. - The process of oxidation is, as a rule, confined to those stones which contain some form of iron, and particularly that known as pyrite. If the iron exists in the latter shape it generally combines with the oxygen of the air, forming the various oxides and carbonates of iron, such as are popularly known as "rust."
"If the sulphide occurs scattered in small particles throughout a sandstone the oxide is disseminated more evenly through the mass of the rock, and aside from a slight yellowing or mellowing of the color, as in certain Ohio sandstones, it does no harm. Indeed, it may result in positive good, by supplying a cement to the individual grains and thus increasing the tenacity of the stone."*
If the pyrite exists in pieces of any size, however, it is almost sure to oxidize and stain the stone so as to ruin its appearance, especially if it is of a light color.
In all other than sandstones the presence of any pyrite is a very serious defect, as it is almost sure to rust the stone and may also render it porous and more liable to the destructive effects of frost.
Solution. - The worst effect of the action of the gases of the atmosphere in connection with rain is in dissolving certain constituents of stones, thereby causing their decomposition. Pure water alone is practically without effect on all stones used for building, but in large cities, and particularly those in which a great deal of coal is consumed, the rain absorbs appreciable quantities of sulphuric, carbonic and other acids from the air and conveys them into the pores of the stone, where they very soon destroy those stones whose constituents are liable to be decomposed by such acids.
Carbonate of lime and carbonate of magnesia, the principal constituents of ordinary marbles, limestones and dolomites, are particularly susceptible to the solvent action of these acids, even when they are present only in very minute quantities, and on this account these stones are extremely perishable in large cities and manufacturing towns. Of course in dry climates the acids are not conveyed into the stone to any great extent, and the stones last much longer than in a damp climate. The less absorbent a stone is the less will be the solvent action of the acids, and the longer the stone will last. Dolomites are in this respect more durable than limestones.
Sandstones, whose cementing material is composed largely of iron or lime, are also subject to rapid decay through the solvent action of the acidulated rains. The feldspars of granites and other rocks are also susceptible to the same influence, though in a less degree.
* " Stones for Building and Decoration," p. 360.