The question of polluted atmospheres worldwide has occupied the minds of people interested in the quality of the environment for several hundreds of years. Only within a recent time span has the effect of deleterious airborne materials, or "acid rain" to use the common and accepted if somewhat inaccurate term, concerned architects, builders and their clients.
We need look only at newly cleaned masonry buildings in such cities as London and Paris to see the results of centuries of discoloration by wood and coal fires. Walls blackened with crusts, in some cases inches thick, were revealed in original colors seen for the first time in living memory.
This condition, that of accumulated dirt and scale, proved in the case of some materials to be a cover under which degradation had proceeded unobserved. Scholarly treatises on the subject offered case histories of twelfth- and thirteenth-century cathedral statuary where features had been completely eroded, apparently within the last three to four decades. The architectural press published graphic evidence of deterioration in all manner of materials, in every kind of environment. Paints appeared to be particularly delicate; aluminum, especially millfinished, became pitted and corroded. Anodized finishes too seemed to show streaking and pitting. Coatings on glass flaked and bubbled.
Some masonry materials appeared to fare better than others in the same atmospheres. Certain classes of stone, predicted by common knowledge to be immune to attack, showed severe deterioration. While some carbonate rocks exhibited great degradation, others showed little if any change. Some quartz and feldspar rocks escaped damage entirely; others crumbled.
The broadest of generalizations, based on reports from Europe, formed the basis of materials usage recommendations here in the Western Hemisphere, and claims and counterclaims about weatherresistant properties in materials became as important as aesthetics. Important voices were heard to question the very existence of pollution.
Still, in the New England states, something was causing massive fish-kills. Tree-covered mountains were denuded. Acrimonious political exchanges flew across the border between the U.S. and Canada. Coal-burning generating plants installed massively expensive scrubbers to remove sulphurous fumes from their stacks. U.S. car makers, after long and angry debate, installed expensive catalytic converters in the exhaust systems of their automobiles to reduce sulphurous emissions.
The smoke began, both literally and metaphorically, to clear. Air quality did improve notably in places, and it became obvious that the dramatic conclusions reached by groups and spokesmen on both sides of the question were far less dramatic than had been originally imagined, or were, simply, wrong.
As concern for integrity in construction materials increased, Indiana Limestone Institute (ILI) and its member companies began an intensive study of limestone performance. That effort, involving both visual inspection and physical testing of Indiana Limestone recovered from older buildings, is the subject of the following technote.
During October, 1986, ILI personnel with disinterested observers examined Indiana Limestone buildings of various ages in Vermont, one of the New England states worst hit by acid precipitation. The object was to determine the extent of damage to the stone. The results of that trip are expressed in this excerpt from a report to the U.S. Department of the Interior by Indiana State
Geologist Emeritus, Dr. John B. Patton: " . . . acid precipitation or acid atmosphere, despite whatever damage to [flora and fauna] is attributable to it, does not seem to have affected the limestone . . . noteably or measurably within the last several decades . . . We saw . . . structures dating from the first quarter of the 19th century that still had most of their original mortar in place, even though it was burned lime at that time, rather than portland [cement] mortar. Tool marks were still clear in the form of drafted margins and scabbling, and arrises were crisp. . . . I am not suggesting that acid rain . . . is to be taken lightly, but I doubt that it is a major factor in the durability of stone."The Soldiers and Sailors Monument in Indianapolis
These comments are supported by the appearance of stones from the several Vermont buildings pictured below. Shown also are details of the Soldiers and Sailors Monument, in Indianapolis, completed in 1902. In the years since its completion, it has been cleaned three times, once by unknown methods, once by sandblast, and most recently by a proprietary acid-based chemical. Sand-blasting and acidic chemicals are known to roughen surfaces of calcareous stones. Even so, and after nearly ninety years of exposure, the surface features of carvings on that monument are still nearly as crisp as they were originally. In protected areas, where little cleaning was necessary, original tool marks are sharp. Tool marks are still evident on most corners of the monument's shaft.
||One of eight large eagle carvings at the 200 foot level, Soldiers and Sailors Monument, Indianapolis, in place since 1900. This bird has lost one talon in 90 years.||
||Detail of carving, showing original details still crisp.|
||Detail of shaft corner at 100 foot level, showing drafted margin still clear after 90 years.||
||Detail of column capital, Memorial Free Library, Vergennes, VT, ca. 1925. Ornament still sharp and clear.|
Waist-level sill, athletic field ticket booth, Norwich College, Northfield, VT, ca. 1940. No evidence of chemical attack.
In August of 1988, ILI obtained specimens of Indiana Limestone which were salvaged from the Illinois State Capitol in Springfield, where they had been placed just below the dome, exactly 100 years earlier.
The thickness of the stones was such that weathered, 2 1/4" thick specimens could be sliced off their top surfaces, exposing an unweathered layer which was sliced off, also 2 1/4" thick. The two layers, weathered and unweathered, were tested for modulus of rupture (bending), compression, and abrasion. Results of that test series, encompassing 36 specimens, showed that the weathered stone had gained strength as compared to the unweathered stone.
These results are the exact opposite of those predicted by conventional wisdom. They seem to confirm the opinion long held by stone producers, that Indiana Limestone weathers by developing a "skin" which is significantly harder and more durable than unweathered stone surfaces. Note that all strength data quoted by ILI is based on freshly quarried specimens. Details of the Illinois State Capitol test series are available on request.The National Acid Precipitation Assessment Program (NAPAP)
During the period of these investigations, an experiment by the U.S. Government was under way. Common building materials were chosen to determine their susceptibility to acidic atmospheres. Indiana Limestone was chosen for the study as one of these standard materials. Specimens collected from Indiana quarries in 1983 were placed in exposure panels in five locations around the U.S.
During its annual assessment in 1987, the NAPAP Executive Summary, in its section titled Effects on Materials, stated that "Measurements of carbonate dissolution based on gravimetric mass loss, surface recession and calcium measured in runoff solutions yield consistent estimates of minimum stone damage."The Test Wall at National Bureau of Standards
In 1948, ASTM Committee C-18 cooperated with the National Bureau of Standards in the construction of a test wall, using stone specimens donated by the National Museum. The stones had been collected by the Museum over the previous 60 years. When the Bureau moved from its original location on Wisconsin Avenue in Washington, D.C. to Gaithersburg, MD, it arranged to move the wall as well.
The wall is 40 feet long and 12 feet high. It contains more than 2,000 stone specimens from all over the world. Seventeen of those specimens are Indiana Limestone. (Interestingly, nine of that number were obtained from working quarries when the wall was constructed, to be used as coping stones, because of the acknowledged durability of Indiana Limestone.) In the years since the wall's erection, many of the stone specimens have begun to show wear; severe deterioration is obvious in some. But on the Indiana Limestone specimens, sharp arrises and corners are still apparent. Not a single Indiana Limestone specimen shows weathering distress other than minor surface fretting.Finally . . .
ILI takes no position on the major questions being discussed at seminars and symposia across the country regarding acid rain, its causes and its cures. Our official interest in the problem is limited to a single question: How does Indiana Limestone perform in polluted atmospheres? Cited here are test results, experiments, comments from knowledgeable observers, and evidence of long-term durability, all supporting the conclusion that Indiana Limestone coexists satisfactorily with pollution. Stated another way, if humans can live in a given environment, they will be pleased with the performance of Indiana Limestone there.
Indiana Limestone's performance record is, for practical purposes, unique among construction materials. Poor performance by other stones has caused concern for the durability of stones in general; indeed, accelerated weathering test programs may be necessary for untried materials. Many such programs are widely promoted; they tend to be costly and time-consuming.
When ILI procedures are followed, Indiana Limestone's performance record makes such programs redundant.
ILI recommends including the following paragraph in specifications where major testing programs are required:
Stones which have a satisfactory performance record in thicknesses, sizes and climates similar to those for this project may at the option of the architect be exempted from the specified testing requirements. The contractor may submit evidence as to such past performance and/or test results for the architect's review. The architect shall have sole authority to approve materials without further testing, or to modify test procedures required for approval.
Among ILI publications is The Indiana Limestone Specification Guide. It contains suggested standard and performance specifications for all Indiana Limestone products. Designers and specifiers will be interested in ILI publications such as the Indiana Limestone Handbook, How to Avoid Small Area Stains and Blemishes, The Finishing Touch, and our various Technotes. A publication list is free on request.