[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.