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Pennsylvania
Blue Marble:
Pennsylvania
Blue Marble, quarried less than 30 miles from the city of Philadelphia,
was an important regional building stone in the first half of the
nineteenth century. Builders in the mid-Atlantic States used it
extensively. The appearance of Pennsylvania Blue, combined with
its reputedly fine quality and accessibility, made it a natural
choice of Philadelphia architects. It was used primarily for monumental
public building projects and Greek Revival architecture from 1790-1860.
By the middle of the 19th century, however, the poor performance
of the marble, the inability of the large grained stone to take
detailed carving, changing design tastes, and improved transportation
systems that increased the availability of white marbles from New
England and Georgia, all contributed to the demise of Pennsylvania
Blue Marble as a building stone.
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Pennsylvania
Blue Marble
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Also
known as:
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Pennsylvania
White Marble, Montgomery County Marble, King of Prussia Marble,
Henderson Marble |
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Location:
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Quarried
from veins within a limestone belt located in Upper Merion and
Whitemarsh Townships, Montgomery County and in West Whiteland
Township, Chester County, Pennsylvania |
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Age:
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Limestone
formed 450 million years ago during Middle Cambrian or Lower
Ordovician period; folded and metamorphosed 50 million years
ago during Canadian period |
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Characteristics:
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Semi-crystalline,
coarsely crystalline, weakly metamorphosed calcite marble
with fine to coarse grains
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contains
1% to 10% accessory minerals consisting mostly of muscovite,
quartz, and graphite
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appearing in uneven to even, thin to thick beds
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colored
white, yellow-white, white with blue veins, mottled,
clouded, light blue, blue, gray, and black
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Buildings
of note:
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Samuel
Blodget's First
Bank of the United States, 1795-97
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John
Haviland's Franklin Institute (Atwater Kent Museum), 1825
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William
Strickland's Merchants' Exchange, 1832-33
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Thomas
U. Walter's Girard College, 1833-47
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According
to a study conducted by Jocelyn Kimmel at the University of Pennsylvania
in 1996, at least three varieties of Pennsylvania Blue Marble appear
to have been used in the construction of the Second Bank. All consist
of at least 90% calcite (with replacement magnesium to some extent),
but differ in the amount of accessory minerals present. The presence
of various shapes and sizes of accessory minerals weakens the interlocking
calcite matrix of Pennsylvania Blue Marble contributing to varying
degrees of deterioration and failure.
Marble
used for the secondary facades still remains relatively intact.
It is comparatively dark in color and nearly homogenous in texture,
with 1% or fewer accessory materials. Its interlocked structure
is responsible for its lack of porosity and its resistance to disaggregation.
Medium-grained
white marble was used to build the front and rear facades. This
stone has a slightly higher proportion of foreign material and a
higher porosity and permeability; hence it is often more deteriorated
than the darker marble used for the side facades.
Fine-grained,
porous, white marble was used at the cornice level, and presumably
wherever sculptural details required a workable stone. The high
proportion of accessory minerals - namely mica, quartz, and orthoclase
- combined with the stone's relatively high porosity and location,
all contribute to the high levels of salts, disaggregation, and
spalling at the cornice level.
The
deterioration of the Second Bank's exterior stone is clearly related
to the inherent geo-chemical and micro-fabric characteristics of
Pennsylvania Blue Marble in conjunction with varying degrees of
exposure to weathering and the elements.
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