In such methods, two classes may be distinguished, the natural and the
artificial.
The former embrace, first, the examination of quarry outcrops, where
the exposure of the surface of the rock during ages may give some
indication of its power of resistance to decomposition, _e. g._, the
dolomitic marbles of New York and Westchester counties, some of which
present a surface crumbling into sand; and, secondly, the examination
of old masonry. Few old buildings have survived the changes in our
restless city, but many observations were presented in regard to the
condition of many materials, usually after an exposure of less than
half a century.
Another source of information, in this regard, was found in the
study of the stones erected in our oldest cemeteries, _e. g._, that
of Trinity Church. There could hardly be devised a superior method
for thoroughly testing by natural means the durability of the stone
than by its erection in this way, with partial insertion in the moist
earth, complete exposure to the winds, rain, and sun on every side,
its bedding lamination standing on edge, and several of its surfaces
smoothed and polished and sharply incised with dates, inscriptions, and
carvings, by which to detect and to measure the character and extent of
its decay. In Trinity Churchyard, the stones are vertical, and stand
facing the east. The most common material is a red sandstone, probably
from Little Falls, N. J., whose erection dates back as far as 1681, and
which remains, in most cases, in very fair condition. Its dark color,
however, has led to a frequent tendency to splitting on the western
side of the slabs, _i. e._, that which faces the afternoon sun. Other
materials studied consisted of bluestone, probably from the Catskills,
black slate, gray slate, green hydromicaceous schist, and white oolitic
limestone, all in good condition, and white marble, in a decided state
of decay.
The artificial methods of trial of stone, now occasionally in vogue,
whenever some extraordinary pressure is brought upon architects to pay
a little attention to the durability of the material they propose to
employ, are, from their obsolete antiquity, imperfection, or absolute
inaccuracy, unworthy of the age and of so honorable a profession.
They usually consist of trials of solubility in acids, of absorptive
power for water, of resistance to frost, tested by the efflorescence
of sodium-sulphate, and of resistance to crushing. The latter may have
the remotest relationship to the elements of durability in many rocks,
and yet is one on which much reliance of the architectural world is now
placed. Sooner or later a wide departure will take place from these
incomplete and antique methods, in the light of modern discovery.
Reference was made to certain experiments by Professor J. C. Draper on
the brownstone and Nova Scotia stone used in this city, by Dr. Page, on
a series of the building stones, and by Professors J. Henry and W. R.
Johnson on American marbles, in some cases with conflicting results,
which were probably due to the limited number and methods of the
experiments.