Acid Rain and Our Nation's Capital - A Guide to Effects on Buildings and Monuments

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Title: Acid Rain and Our Nation's Capital - A Guide to Effects on Buildings and Monuments
Author: McGee, Elaine
Language: English
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*** Start of this LibraryBlog Digital Book "Acid Rain and Our Nation's Capital - A Guide to Effects on Buildings and Monuments" ***

Acid Rain and
Our Nation’s Capital

_A Guide to Effects on Buildings and Monuments_

_by_ Elaine McGee

For sale by the U.S. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
ISBN 0-16-048068-X

[Illustration: Marble surfaces exposed to rain develop a rough
“sugary” texture because the calcite grains are loosened as the
edges dissolve in the rain water. Column capital volute, Jefferson
Memorial, Washington, D.C.]

[Illustration: A summer rain storm in Washington, D. C. (Memorial
Continental Hall)]

When polluted air mixes with rain, snow, and fog, acid precipitation
forms. This acidity has caused people to worry about the environment;
some reports show that acid rain has affected lakes, trees, and fish
populations in the Northeastern United States and Canada. Another
concern is its effect on historic buildings and monuments.

The booklet focuses on acid rain and its impact on our Nation’s capital.
Rain in Washington, D. C., has an average acidity of 4.2, about as acid
as a carbonated drink and more than ten times as acid as clean,
unpolluted rain. This booklet will define acid rain, explain what
effects it has on marble and limestone buildings, and show, on a walking
tour, some of the places in our Nation’s capital where you can see the
impact of acid precipitation.

[Illustration: The pH scale: pH = 7 is neutral, neither acid or
alkaline; smaller pH values are acid, larger pH values are alkaline.
A liquid with a pH of 3 is ten times as acid as one with a pH of 4.]

1 Battery Acid
2.8 Vinegar
4 Adult fish die
<5.5 ACID RAIN
5.2-6.5 Normal range of precipitation
6-8 Normal range of stream pH
<7 Acid
7 Neutral
>7 Alkaline
8.6 Baking soda and sea water
13 Lye

What is acid rain?

The term “acid rain” is commonly used to mean the deposition of acidic
components in rain, snow, fog, dew, or dry particles. The more accurate
term is “acid precipitation.” Distilled water, which contains no carbon
dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acid,
and those with a pH greater than 7 are alkaline (or basic). “Clean” or
unpolluted rain has a slightly acidic pH of 5.6, because carbon dioxide
and water in the air react together to form carbonic acid, a weak acid.
Around Washington, D.C., however, the average rain pH is between 4.2 and
4.4.

The extra acidity in rain comes from the reaction of air pollutants,
primarily sulfur oxides and nitrogen oxides, with water in the air to
form strong acids (like sulfuric and nitric acid). The main sources of
these pollutants are vehicles and industrial and power-generating
plants. In Washington, the main local sources are cars, trucks, and
buses.

Acidity in rain is measured by collecting samples of rain and measuring
its pH. To find the distribution of rain acidity, weather conditions are
monitored and rain samples are collected at sites all over the country.
The areas of greatest acidity (lowest pH values) are located in the
Northeastern United States. This pattern of high acidity is caused by
the large number of cities, the dense population, and the concentration
of power and industrial plants in the Northeast. In addition, the
prevailing wind direction brings storms and pollution to the Northeast
from the Midwest, and dust from the soil and rocks in the Northeastern
United States is less likely to neutralize acidity in the rain.

[Illustration: Wet and dry bucket collector, used to collect samples
for measuring rainfall acidity.]

[Illustration: 1992 annual precipitation-weighted mean hydrogen ion
concentrations as pH

A pH distribution map shows areas in the continental United States
of greatest acidity in the rain.]

When you hear or read in the media about the effects of acid rain, you
are usually told about the lakes, fish, and trees in New England and
Canada. However, we are becoming aware of an additional concern: many of
our historic buildings and monuments are located in the areas of highest
acidity. In Europe, where buildings are much older and pollution levels
have been ten times greater than in the United States, there is a
growing awareness that pollution and acid rain are accelerating the
deterioration of buildings and monuments.

Stone weathers (deteriorates) as part of the normal geologic cycle
through natural chemical, physical, and biological processes when it is
exposed to the environment. This weathering process, over hundreds of
millions of years, turned the Appalachian Mountains from towering peaks
as high as the Rockies to the rounded knobs we see today. Our concern is
that air pollution, particularly in urban areas, may be accelerating the
normal, natural rate of stone deterioration, so that we may prematurely
lose buildings and sculptures of historic or cultural value.

What about buildings?

Many buildings and monuments are made of stone, and many buildings use
stone for decorative trim. Granite is now the most widely used stone for
buildings, monuments, and bridges. Limestone is the second most used
building stone. It was widely used before Portland cement became
available in the early 19th century because of its uniform color and
texture and because it could be easily carved. Sandstone from local
sources was commonly used in the Northeastern United States, especially
before 1900. Nationwide, marble is used much less often than the other
stone types, but it has been used for many buildings and monuments of
historical significance. Because of their composition, some stones are
more likely to be damaged by acidic deposition than others. Granite is
primarily composed of silicate minerals, like feldspar and quartz, which
are resistant to acid attack. Sandstone is also primarily composed of
silica and is thus resistant. A few sandstones are less resistant
because they contain a carbonate cement that dissolves readily in weak
acid. Limestone and marble are primarily composed of the mineral calcite
(calcium carbonate), which dissolves readily in weak acid; in fact, this
characteristic is often used to identify the mineral calcite. Because
buildings and monuments made of limestone and marble are more likely to
be damaged by acid precipitation, they are the main focus of this
booklet.

[Illustration: Memorial Bridge in Washington, D.C., is made of
granite, the most widely used stone type.]

[Illustration: Marble used as a trim on the First Bank in
Philadelphia, Pennsylvania.]

How do you recognize limestone and marble?

The main difference between limestone and marble is that limestone is a
sedimentary rock, typically composed of calcium carbonate fossils, and
marble is a metamorphic rock. Limestone forms when shells, sand, and mud
are deposited at the bottom of oceans and lakes and over time solidify
into rock. Marble forms when sedimentary limestone is heated and
squeezed by natural rock-forming processes so that the grains
recrystallize. If you look closely at a limestone, you can usually see
fossil fragments (for example, bits of shell) held together by a calcite
matrix. Limestone is more porous than marble, because there are small
openings between the fossil fragments. Marble is usually light colored
and is composed of crystals of calcite locked together like pieces of a
jigsaw puzzle. Marble may contain colored streaks that are inclusions of
non-calcite minerals.

[Illustration: Limestone is made of fossil fragments, held together
with calcite; the shell near the center is about 1 cm across.
Botanic Gardens building, Washington, D.C.]

[Illustration: Marble is made of calcite crystals (white) and some
colored grains of mica inclusions; the grains in a marble are locked
together like jigsaw puzzle pieces.]

How does acid precipitation affect marble and limestone buildings?

Acid precipitation affects stone primarily in two ways: _dissolution_
and _alteration_. When sulfurous, sulfuric, and nitric acids in polluted
air react with the calcite in marble and limestone, the calcite
dissolves. In exposed areas of buildings and statues, we see roughened
surfaces, removal of material, and loss of carved details. Stone surface
material may be lost all over or only in spots that are more reactive.

You might expect that sheltered areas of stone buildings and monuments
would not be affected by acid precipitation. However, sheltered areas on
limestone and marble buildings and monuments show blackened crusts that
have spalled (peeled) off in some places, revealing crumbling stone
beneath. This black crust is primarily composed of gypsum, a mineral
that forms from the reaction between calcite, water, and sulfuric acid.
Gypsum is soluble in water; although it can form anywhere on carbonate
stone surfaces that are exposed to sulfur dioxide gas (SO₂), it is
usually washed away. It remains only on protected surfaces that are not
directly washed by the rain. Gypsum is white, but the crystals form
networks that trap particles of dirt and pollutants, so the crust looks
black. Eventually the black crusts blister and spall off, revealing
crumbling stone.

[Illustration: When marble is exposed to acidic rain, sharp edges
and carving details gradually become rounded. Antefixes, roof of the
Philadelphia Merchants’ Exchange (built in 1832).]

[Illustration: Blackened crusts on sheltered portions of the
limestone Chicago Tribune Building, Chicago, Illinois.]

[Illustration: Formed as a result of air pollution, gypsum
alteration crusts have blackened, blistered, and spalled from a
marble baluster at the Organization of American States building,
Washington, D.C.]

[Illustration: Scanning electron microscope photograph of gypsum
crystals with dirt and pollution particles trapped by the network of
crystals. The scale bar is 10 micrometers long.]

[Illustration: A marble column at the Merchants’ Exchange in
Philadelphia shows loss of material where the stone is exposed to
rain and blackening of the stone surface where the stone is
sheltered from rain.]

Where can we see the effects of acid precipitation?

Washington’s buildings and monuments use many different stone types.
Marble and limestone buildings are the most likely to show damage,
because they are more affected by acidic precipitation and urban
pollution. As you follow the tour described in this book, see how
granite and sandstone buildings compare with the marble and limestone in
the same environment.

This guide will help you recognize some geologic features of buildings,
in addition to their historical and architectural aspects, wherever you
travel. However, remember one important point when examining buildings
and monuments for deterioration: stone deterioration has many causes.
Although acid precipitation and urban pollution can accelerate stone
deterioration, people, pigeons, and other organisms may also harm our
stone structures. In addition, the process of weathering has been going
on since the Earth first had an atmosphere. Although we can observe
deterioration of the stone, it is hard to determine how much of the
deterioration is from acid precipitation and how much is from other
causes.

[Illustration: Pigeons sitting on the statue heads have created
distinctive deterioration on this building.]

[Illustration: Flowers and grasses have grown in the cracks between
stones on this church.]

[Illustration: This limestone column in the Lincoln Memorial is
darkened and dirty from people’s hands touching the stone.]

[Illustration: Microorganisms have caused this stain to appear on a
marble column at the Jefferson Memorial.]

What are we doing about acid rain?

Scientists from many disciplines are studying acid precipitation and its
impact. The National Acid Precipitation Assessment Program (NAPAP), a
Federal program involving representatives from more than a dozen Federal
agencies, has sponsored studies on how acid rain forms and how it
affects lakes, crops, forests, and materials. Because buildings and
monuments cannot adapt to changes in the environment, as plants and
animals can, historic structures may be particularly affected by acid
precipitation. Scientists are studying effective control technologies to
limit the emissions from power plants and automobiles that cause acid
rain. The impact and usefulness of regulations that would require limits
on air pollution are also being studied. Finally, scientists are
examining the processes of deterioration to find effective ways to
protect and repair our historic buildings and monuments. Agencies like
the National Park Service, which are charged with protecting and
preserving our national heritage, are particularly concerned not only
about the impact of acid rain but also about making the best choices for
maintaining and preserving our historic buildings and monuments.

[Illustration: Beginning in 1984, the National Acid Precipitation
Assessment Program sponsored exposure site studies of limestone and
marble, to examine the contribution to stone deterioration that
comes from acid precipitation.]

A field guide to buildings in our Nation’s capital

Washington, D.C., has many buildings of historic and cultural
significance, and many of them are made of marble and limestone. This
self-guided tour will point out damage to buildings and monuments in our
Nation’s capital that may have been caused by acid precipitation.
Similar effects may be found in other cities as well.

Places to visit have been divided into several areas, so the trip can be
done either in segments or all in one day. A suggested tour route is
described within each area. A car provides the most efficient transport
between areas, but parking may be hard to find. The Metro subway system
can easily be used to visit all areas except the Jefferson and Lincoln
Memorials. The closest Metro stations in each area are shown on the map.
You will need comfortable walking shoes, and you may want to bring along
a camera, a hand lens (about 10× magnification) for observing details of
minerals and weathering, and a pair of binoculars for closer examination
of inaccessible areas.

The area around the National Capitol

This area includes the Capitol building, the Peace Monument, the Grant
Memorial, and the Botanic Gardens. We begin the tour at the southeast
corner of the Capitol, and go clockwise around the Capitol (along the
south, west, and then north sides). We then follow a walkway heading
west, from the northeast corner of the Capitol, to see the Peace
Monument at the intersection of First Street and Pennsylvania Avenue,
NW. We continue south along First Street to the Grant Memorial and
then south again to the Botanic Gardens. Total distance is about one
kilometer, or about three-quarters of a mile.

The Capitol Building—Site 1

The Capitol was built in stages; the cornerstone of the main building
was laid in 1793, the north wing was completed in 1800, and the south
wing was completed in 1807. Both wings were burned by the British in
1814. The capitol was then rebuilt, and it has been modified several
times throughout the years. A major program of cleaning, replacement,
and repair was begun in the late 1980’s. The center building of the
Capitol is painted sandstone, but the north and south wings, housing the
Senate and the House chambers, are marble. Around the Capitol we will
observe various examples of dissolution and blackened alteration,
especially on the marble balustrade that surrounds the south, west, and
north sides of the building.

Beginning at the southeast corner of the building, by using binoculars
we can see some areas of blackened alteration in the Corinthian column
capitals. A more accessible example is found under the overhang of the
large square ends of the marble balustrade at the southeast corner of
the building. The black crust is made of gypsum plus dirt that
accumulates in sheltered areas. No black crust is present along the
cracks between the stones; rain water probably flows in these areas,
dissolving the gypsum and preventing accumulation of a crust. Not all
black areas on this baluster are gypsum; in some places near the bushes,
you can see greenish-black moss growing on the stone. The top surfaces
of the marble balustrade are coarse and rough, because of dissolution
between grains, compared to areas that are protected from running or
washing water where the black alteration crust forms.

[Illustration: The United States Capitol building.]

[Illustration: Blackened alteration has accumulated under the
overhanging edge of this marble balustrade corner. Southeast corner,
U.S. Capitol Building.]

[Illustration: Pock marks in marble columns, south side of U.S.
Capitol building. Silicate mineral inclusions in the marble loosen
and fall out when the calcite around them is dissolved by acid
rain.]

Another dissolution feature of marble is the pock-mark effect on the
square bases of the building columns. Silicate mineral inclusions in the
marble were loosened by the dissolution of the surrounding calcite,
causing the inclusions to fall out of the stone. A particularly good
example of this is found on the fourth column west from the southeast
corner of the Capitol building. The pock-mark dissolution is also found
at several other places on the building.

[Illustration: The marble balustrade on the west side of the Capitol
building shows both dissolution and alteration.]

[Illustration: A recently replaced marble baluster at the Capitol
has edges that are only beginning to round.]

[Illustration: Black alteration crusts under the west balustrade of
the Capitol have begun to spall (peel) off, revealing crumbling
white marble underneath.]

We will follow the marble balustrade around the building, noting
differences in deterioration. Some parts of the balustrade have
obviously been replaced, thus enabling us to observe various stages in
the stone deterioration. The edges of the balusters are sharp when new
and become rounded as they age. Blackened alteration crusts have
accumulated on the sheltered sides of the balusters and under the
overhanging top of the balustrade. In some spots under the rail the
blackened crust has spalled off, exposing fresh surfaces and more
vulnerable stone. Some carvings on the balustrade corners are worn,
whereas others have blackened alteration; this difference in weathering
may be due to local effects of wind and rain. Along the steps leading to
the terrace on the west side of the Capitol, gypsum has accumulated on
large areas of the wall. Gypsum can accumulate on any surface that is
not washed by water.

As you walk north along the west side of the Capitol, look at the
central part of the building. The walls here are painted sandstone.
Despite recent restoration of the building, you can see evidence of past
stone deterioration, including the accentuated lines from bedding in the
stone and the pock marks where rounded inclusions have disappeared. We
will see an example of this same sandstone that is not painted in the
buildings near 17th Street.

At the northeast corner of the Capitol building, the marble balustrade
ends in square blocks like the ones we first examined. Here you can see
an example of preferential dissolution where the silicate mineral
inclusions remain and the calcite around them has been dissolved away.
Also, on the north side of this block, examine the blackened grains on
the top surface with a hand lens. Not all of the black material you see
on stone is gypsum; some is of biological origin, probably algae or a
fungus.

[Illustration: A marble block that forms the northeast corner of the
Capitol balustrade shows preferential erosion of the calcite around
a silicate mineral inclusion.]

To continue the tour, follow the pathway that heads west along the
north side of the Capitol, towards First Street. As you approach First
Street you will see a sandstone, diabase (a dark igneous rock), and
granite fence with various carvings. Because these stone types are
resistant to acid attack, the carvings show little damage.

The Peace Monument—Site 2

[Illustration: The Peace Monument, dedicated in 1878, is made of
Italian marble.]

This monument, dedicated in 1878, is made of marble from Carrara, Italy.
The statue does not show much damage, but if you look closely, you can
see alteration crusts (some are light orange) in protected places and
graininess and roughness in places that are exposed to rain. Carved
statues present varied surfaces that direct rain washing and runoff.

[Illustration: MAP OF WASHINGTON]

Sites on the tour:
1. THE CAPITOL BUILDING
2. THE PEACE MONUMENT
3. THE GRANT MEMORIAL
4. BOTANIC GARDENS BUILDING
5. JEFFERSON MEMORIAL
6. LINCOLN MEMORIAL
7. CAPITOL GATEHOUSE
8. ORGANIZATION OF AMERICAN STATES BUILDING
9. DAR—CONSTITUTION HALL
10. DAR—MEMORIAL CONTINENTAL HALL
11. CORCORAN BUILDING
12. RENWICK GALLERY
13. FEDERAL TRIANGLE BUILDINGS
14. WASHINGTON MONUMENT

Continue south along First Street to the Grant Memorial on your right.

The Grant Memorial—Site 3

[Illustration: The Grant Memorial consists of bronze statues on
marble bases; although the statues were cleaned, the staining on the
marble remains.]

This memorial, dedicated in 1922, consists of a group of bronze
sculptures mounted on marble bases. Bronze weathers outdoors if it is
not cleaned and waxed regularly. Like stone, bronze dissolves where it
is exposed to rainfall, developing a green color and a pitted surface,
and it also alters in sheltered areas, with accumulation of a blackened
layer. The most notable stone deterioration visible here is the green
stain on the marble bases, caused by runoff from the weathered bronze.
The green stain does not damage the marble, but it is unattractive, and
there are no methods currently available to remove the stain without
damaging the marble.

[Illustration: The light-green color on the bronze statues at the
Grant Memorial is typical of the way bronze weathers when it is
exposed to acidic rain; the rain dissolves some of the metal and
causes staining of the white marble bases.]

Continue south, crossing Maryland Avenue, to the Botanic Gardens.

Botanic Gardens Building—Site 4

[Illustration: The Botanic Gardens building is made of limestone.]

This limestone building was built in 1931. Like many of the limestone
buildings in Washington, this building has been cleaned, so it does not
have an accumulation of surface dirt. However, the cleaning and regular
washing by rainfall have accentuated the fossils in the stone, which
dissolve less readily than the calcite matrix. Some of the sculpted
heads above the arches of the building show small black crusts. On the
east side of the building, microorganisms naturally present in the stone
contribute to the deterioration (blackening) of the stone, where water
drips from a joint in the roof.

[Illustration: Fossil details stand out on the limestone at the
Botanic Gardens building; the fossils are more resistant to
dissolution than the calcite matrix that holds the fragments
together.]

The Botanic Gardens Building is the last stop in the Capitol area of
the tour; you may wish, however, to see some bronze alteration on the
Garfield Memorial (First St. and Maryland Ave.) and the accentuated
fossils with surrounding algae or fungi on the limestone posts near
the Capitol Reflecting Pool. The next stop, the Jefferson Memorial, is
about 3.5 kilometers (2.2 miles) from the Capitol.

Jefferson Memorial—Site 5

[Illustration: The Jefferson Memorial is made of marble and was
dedicated in 1943.]

The Jefferson Memorial is a marble building, dedicated in 1943. One of
the most striking deterioration features to observe here is the loss of
silicate mineral inclusions in the marble columns because of dissolution
of the calcite matrix. Close examination of the grooves shows flakes of
mica and sometimes grains of pyrite. Blackened crusts are visible on the
column capitals that are sheltered from rain and from regular washing of
the monument.

[Illustration: Several of the column shafts at the Jefferson
Memorial have grooves that follow the inclusion traces in the
marble, where the mineral inclusions have weathered out and been
lost.]

[Illustration: A close look at some of the weathered grooves in the
columns shows that small bits of mica and pyrite remain.]

The National Park Service began a survey of the condition of this
memorial and the Lincoln Memorial in 1992. The results will be used to
help make decisions on treatment, cleaning, and preservation. The
information gathered from the survey will serve as a known baseline for
the condition of the stone, so that future changes in the condition of
the buildings can be assessed. In May 1990, a part of one of the column
capitals (called a volute) broke off and fell onto the northwest
portico. This failure raised concern about all the volutes at the
Memorial, so several other cracked volutes were removed, and studies are
being conducted to determine why they cracked. Because of where and how
the volute broke, it is unlikely that acid rain or air pollution
contributed to the failure. The broken pieces will probably be replaced,
but only when the reason for their failure is understood, so that an
appropriate replacement technique can be chosen.

[Illustration: Part of one of the column capitals at the Jefferson
Memorial broke off and fell onto the portico in 1990.]

The next stop is the Lincoln Memorial, 1.8 kilometers (a little more
than a mile) northeast of the Jefferson Memorial.

Lincoln Memorial—Site 6

The Lincoln Memorial, dedicated in 1922, is made of marble from
Colorado. This building has few alteration crusts, in part because it
receives regular cleaning and in part because of the design of the
building. Except for the features around the entablature (the edge of
the roof), there are few sheltered areas where alteration crusts can
accumulate. With the aid of binoculars, you can see some alteration
crusts along the underside of the roof overhang; in these places the
marble is very badly crumbled under the alteration crusts. Some columns
show preferential weathering or loss of inclusions, but some of this
damage might be from graffiti removal. The most visible dissolution
feature is sugaring, where the stone has lost its polish and the surface
now feels rough. Visitors have affected this popular memorial too;
several of the columns, especially the limestone columns inside the
chamber, show darkening and rounding of edges where visitors have
touched them over the years.

One interesting feature at the Lincoln Memorial is differences in stone
condition that must come from variations in the stone. At several places
around the outside of the memorial, adjacent blocks of marble show very
different surface roughness. Since the blocks of stone have the same
orientation with respect to wind, rain, and pollution, the difference in
condition cannot be due to exposure and must be related to basic
characteristics in the stone that was used.

[Illustration: The Lincoln Memorial is made of marble and was
dedicated in 1922.]

[Illustration: Under the roof overhang is one of the few places at
the Lincoln Memorial where alteration crusts have developed on the
marble.]

[Illustration: Some columns at the Lincoln Memorial have flattened
chalky areas where inclusions have weathered differently from the
surrounding calcite.]

[Illustration: The marble guttae on the roof overhang are crumbling
and falling apart underneath blackened alteration crusts.]

Buildings along 17th Street, NW

Some notable examples of stone deterioration are included in this part
of our tour. We will begin this segment at the northeast corner of
17th Street and Constitution Avenue, with the Capitol Gatehouse. We
will then cross 17th Street and examine several buildings along 17th
Street as we walk north towards Pennsylvania Avenue. The total
distance is about three-fourths of a kilometer (half a mile).

Capital Gatehouse—Site 7

[Illustration: The Capitol Gatehouse, now located at 17th Street and
Constitution Avenue, is made of the same sandstone used in the White
House and the center part of the Capitol, but it was left unpainted.
Deterioration of this stone is due to the clay it contains, not to
the effects of acid rain.]

This small sandstone building was built around 1828 at the west entrance
to the Capitol. In 1880 it was moved (along with a twin and four
gateposts) to its present site. This building is made of the same
sandstone that was used in the central part of the Capitol and in the
White House. Three types of deterioration are readily visible at the
gatehouse: spalling, pock marks, and preferential weathering of clay
layers in the stone. This stone may be more degraded than stone in the
Capitol or the White House, because of variations in stone quality and
maintenance to the buildings and because it has never been painted.

[Illustration: This kind of sandstone was soon found to be a poor
building stone because of its tendency to spall. (detail on Capitol
Gatehouse)]

To continue, we will cross 17th Street and examine parts of several
buildings as we walk north.

Organization of American States Building—Site 8

[Illustration: The Organization of American States Building is made
of marble and was dedicated in 1910.]

This marble building was dedicated in 1910. Two sculptures in the front
of the building show some alteration crusts in sheltered areas and
dissolution in exposed areas. In back of the building the marble
balusters on the patio are covered with blackened crusts, especially on
the sides facing the garden. In many places the crusts have blistered or
spalled off, exposing new surfaces to alteration. In general, the patio
sides of the balusters are in much better condition than the sides that
face the garden, perhaps because washing of the patio has washed off the
gypsum crusts on that side of the balusters.

[Illustration: Blackened gypsum crusts may blister and spall off,
exposing a crumbling stone surface to further pollution.]

Continue north through the garden and parking lot and cross C Street
to the Daughters of the American Revolution (DAR) Buildings.

DAR—Constitution Hall—Site 9

[Illustration: DAR—Constitution Hall is made of limestone and was
built in the 1930’s.]

The main damage on this limestone building, built in the 1930’s, is the
blackening of the side balustrade from algae or fungi. The stone is
porous and therefore retains moisture, thus encouraging growth of
organisms. The limestone in this building is quite uniform and shows
little preferential dissolution, except in a few places. On the top of
the balustrade along C Street, for example, some of the calcite matrix
has dissolved from around the fossil fragments, and some holes are
filled with calcite crystals.

[Illustration: Some of the blackening on limestone surfaces may be
from algae or fungi that readily grow in the rough surface in
Washington’s warm, humid climate.]

Continue east along C Street to Memorial Continental Hall.

DAR—Memorial Continental Hall—Site 10

[Illustration: Memorial Continental Hall, built in 1909, is part of
the Daughters of the American Revolution building complex.]

[Illustration: Carvings at the base of the columns on the south side
of Memorial Continental Hall show that carved details and sharp
edges remain on sheltered areas.]

The porch area on the south side of this marble building built in 1909
is a good place to look at some contrasts in marble deterioration. Parts
of the balustrade have been replaced, as shown by differences in color
and surface roughness of the stone. The exposed stone surface along the
top of the balustrade is rougher than the surfaces in more sheltered
areas. The columns on this porch are carved around the base, so you can
examine the effects of exposure to rain on the carving details. The more
exposed carvings have lost their sharp edges and definition compared to
the sheltered carvings. The bases of the columns contain small amounts
of pyrite, which is more resistant to weathering than is the calcite in
the marble surrounding the pyrite. The sheltered part of the window-sill
support on the west side of the porch shows an alteration crust, a dull
gray accumulation on the stone surface.

[Illustration: On an exposed portion of the carving on the columns
at Memorial Continental Hall, the edges of the marble have rounded
and the surface has roughened.]

[Illustration: Pyrite grains stand in relief where calcite and micas
have weathered out of the marble at Memorial Continental Hall.]

[Illustration: A dull gray surface on the marble on the window-sill
support shows where an alteration crust is just beginning to
develop.]

At the corner of 17th and C Streets, turn left and walk north along
17th Street. On our way to the Corcoran Gallery, we will pass the Red
Cross building (marble, 1917). Some of the same types of marble
deterioration observed at other locations are also present here.

Corcoran Gallery—Site 11

[Illustration: The Corcoran Gallery is built mostly of marble.]

The Corcoran Gallery is marble with a granite base. It was built in 1879
and enlarged in 1927. Ornate carvings around the roof, doors, and
windows have blackened crusts of gypsum, as do parts of the marble
pedestals supporting the bronze lions at the front door. The marble
bases also have inclusions that stand out above the surrounding calcite,
which has been dissolved away.

[Illustration: Marble bases for bronze lions outside the entrance to
the Corcoran Gallery have feldspar inclusions that stand in relief
compared to the roughened surrounding calcite.]

Continuing north along 17th Street towards Pennsylvania Avenue, you
will see several modern granite office buildings and the Executive
Office building (formerly the State-War and Navy building), which was
built from granite and completed in 1888. These granite buildings show
little deterioration. Turn right onto Pennsylvania Avenue and proceed
to the Renwick Galley on the northeast corner of the intersection of
17th Streets and Pennsylvania Avenue.

Renwick Gallery—Site 12

[Illustration: The Renwick Gallery, made of brick and sandstone, was
completed in 1859.]

This building of brick and sandstone, completed in 1859, is interesting
from a stone preservation point of view. The decorative sandstone panels
were badly deteriorated, so in 1968 the panels were saturated with epoxy
to strengthen them. This treatment actually accelerated the
deterioration because when water penetrated behind the epoxy-filled
area, large portions of the treated panels spalled off. A second
renovation attempt was therefore necessary two years after the first,
and the present panels are cast sandstone. A post of the original
sandstone stands at the southeast corner of the building.

[Illustration: Casts of ground sandstone and epoxy replaced the
original carved sandstone decorative trim at the Renwick Gallery
when a first attempt to preserve the carved stone failed.]

The next part of the tour begins at 15th Street and Pennsylvania
Avenue S. To get there, walk east along Pennsylvania Avenue, past
Blair House and between Lafayette Park and the White House. Lafayette
Park has a number of bronze statues that have been cleaned fairly
recently. The White House is built of sandstone that was painted
white; the paint was used in part to improve the durability of the
stone. After you pass the White House, you will come to the Treasury
Building. Turn right onto 15th Street and walk south, towards the
Washington Monument and the Mall. The total distance from the Renwick
to the corner of 15th and Pennsylvania is about three-fourths of a
kilometer (half a mile).

Federal Triangle Buildings—Site 13

On the east side of 15th Street, beginning at E Street, is the Commerce
Department building, which was constructed of limestone in the 1930’s.
This building is part of the Federal Triangle, a cluster of Federal
office buildings in the area bounded by Pennsylvania Avenue,
Constitution Avenue, and 15th Street, built primarily during the New
Deal administration of President Franklin D. Roosevelt. Some sculptures
on the buildings were done by participants in the WPA program. These
buildings were cleaned in the 1960’s, probably by sandblasting. Look for
fossils in relief and alteration crusts in some sheltered places on the
carved work. Some of the blackening on this building is from dirt and
organic material trapped or growing in the rough surface of the stone.

[Illustration: All of the Federal buildings that form the Federal
Triangle (between Pennsylvania Avenue, Constitution Avenue, and 15th
Street) are made of limestone.]

Continue south on 15th Street to Constitution Avenue. From the corner
of 15th Street and Constitution, follow some of the foot paths half a
kilometer (three tenths of a mile) to the Washington Monument.

Washington Monument—Site 14

This monument was begun in 1848, but it was not finished until 1885; the
change in color about 150 feet up marks a change in the type of marble
used to face the monument. Although it is made of marble, its smooth,
straight shape and the massive blocks used in this monument have
minimized the effect of acid precipitation. Dissolution does occur in a
few areas, but the amount of stone material lost from dissolution is
insignificant compared to the mass of the stone.

[Illustration: The straight shape and massive stones in the
Washington Monument minimize the impact of acid precipitation to
this important landmark.]

Our tour ends here, but there are many more stone buildings and
monuments in Washington and in other cities that may also show the
effects of urban pollution and acid precipitation. However, as seen on
this tour, not all of the deterioration of stone buildings is caused
by acid rain. To protect our historic stone buildings and monuments,
we need to limit air pollution that contributes to acid rain
deterioration, and we need to develop effective maintenance and
preservation procedures that will not further harm the stone.

Glossary of Geologic and Architectural Terms

Antefix—an ornament along the edge of a roof, often shaped like a shell
or shield

Baluster—a post or support for a handrail

Balustrade—a rail and row of supporting posts, especially along a stair
or porch

Bronze—a metal alloy of copper and tin; although brown when fresh, it
weathers to a more commonly seen green color

Calcite—a mineral composed of calcium carbonate: CaCO₃

Column Capital—The top portion of a column. There are three main types;
from least ornate to most ornate, they are Doric, Ionic, and Corinthian

Diabase—a dark-gray to black, fine-textured igneous rock, composed of
the minerals feldspar and pyroxene

Feldspar—a common mineral composed of silica (Si), aluminum (Al), and
calcium (Ca), potassium (K), or sodium (Na): CaAl₂Si₂O₈, KAlSi₃O₈,
NaAlSi₃O₈

Fossil—remains or trace of a plant or animal preserved in a rock from
some past, prehistoric time

Granite—a light or speckled, coarse-grained igneous rock composed of
quartz, feldspar, and mica

Guttae—small, drop-like rounded ornaments under a roof overhang

Gypsum—a mineral composed of hydrated calcium sulfate: CaSO₄·2H₂O

Igneous—formed from melted or partially molten rock

Inclusion—a mineral phase that is distinct (in composition or
appearance) from the main mineral components of a rock

Limestone—a sedimentary rock, chiefly composed of calcium carbonate;
formed from the shells of marine animals

Marble—a metamorphic rock, chiefly composed of calcium carbonate

Metamorphic—changed; usually by heat or pressure to a different form but
the same composition

Mica—a silicate mineral that forms sheets or layers and contains
aluminum, hydroxyl, and alkali ions: K₂Al₄Si₆Al₂O₂₀(OH,F)₄,
K₂(Mg,Fe)₆Si₆Al₂O₂₀(OH,F)₄

Mineral—a naturally occurring inorganic substance with an ordered
structure; rocks are made of one or more minerals

Portico—a porch, with a roof supported by columns

Pyrite—a mineral (“fool’s gold”) composed of iron sulfide: FeS₂

Pyroxene—a silicate mineral containing two metal oxides: CaMgSi₂O₆,
CaFeSi₂O₆, (Mg,Fe)SiO₃

Quartz—a mineral composed of silicon dioxide, or silica: SiO₂

Sandstone—a sedimentary rock composed chiefly of quartz grains

Sedimentary—formed from particles that were transported by water or wind
and deposited in layers

Volute—a scroll-like ornament on an Ionic column capital