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.