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Material Characterization of Pennsylvania Blue Marble
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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.

Pennsylvania Blue Marble
Also known as:
Pennsylvania White Marble, Montgomery County Marble, King of Prussia Marble, Henderson Marble
Quarried from veins within a limestone belt located in Upper Merion and Whitemarsh Townships, Montgomery County and in West Whiteland Township, Chester County, Pennsylvania
Limestone formed 450 million years ago during Middle Cambrian or Lower Ordovician period; folded and metamorphosed 50 million years ago during Canadian period
  • Semi-crystalline, coarsely crystalline, weakly metamorphosed calcite marble with fine to coarse grains
  • contains 1% to 10% accessory minerals consisting mostly of muscovite, quartz, and graphite
  • appearing in uneven to even, thin to thick beds
  • colored white, yellow-white, white with blue veins, mottled, clouded, light blue, blue, gray, and black
Buildings of note:
  • Samuel Blodget's First Bank of the United States, 1795-97
  • John Haviland's Franklin Institute (Atwater Kent Museum), 1825
  • William Strickland's Merchants' Exchange, 1832-33
  • Thomas U. Walter's Girard College, 1833-47

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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