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Second Bank Pennsylvania Blue

First Bank of the United States
Pennsylvania Blue Marble, also known as Montgomery County Marble, King of Prussia Marble or Henderson Marble was quarried from veins within a limestone belt located in Upper Merion and Whitemarsh Townships, Montgomery County and in West Whiteland Township, Chester County, Pennsylvania. The light blue stripe running from east to west  on the above geological map of Montgomery and Chester Counties indicates the limestone vein in which the Pensylvania Blue Marble would have been found.

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Application of the Material on the Second Bank
 In 1996 the Architectural Conservation Laboratory (ACL) of the University of Pennsylvania, in conjunction with Independence National Historical Park (INHP), began a program of stone characterization and physio-mechanical testing of potential consolidation methods for Pennsylvania Blue marble.

According to a study conducted by Jocelyn Kimmel at the University of Pennsylvania in 1996, at least three varieties of Pennsylvania 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 grey in color and nearly homogenous in texture, with 1% or fewer accessory materials. Its interlocked crystalline structure is responsible for its lack of porosity and its resistance to disaggregation.

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The Stone Described
The stone is a metamorphosed limestone formed 450 million years ago during the Middle Cambrian or Lower Ordovician period; folded and metamorphosed 50 million years ago during the Canadian period.
The characteristics are semi-crystalline, coarsely crystalline, weakly metamorphosed calcite marble with fine to coarse grains containing 1% to 10% accessory minerals consisting mostly of muscovite, quartz, and graphite. The stone appears in uneven to even, thin to thick beds colored white, yellow-white, white with blue veins, mottled, clouded, light blue, blue, gray, and black. Pennsylvania 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 marble, 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 marble as a building stone.

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 marble in conjunction with variations of exposure to weathering and the elements.
front of building
The First Bank of the United States had the principle facade built with Pennsylvania blue marble, although the rest of the structure was built with brick.
secb column tops
The Merchants' Exchange building located on northeastern corner of 3rd and Walnut was also designed by William Strickland. Built between 1832 and 1834, this structure is also constructed from Pennsylvania marble.
Pennsylvania Marble and Other Buildings of Note
Once a building stone acquires a record of poor weatherability and is no longer commercially viable, there usually is little motivation for continued use or testing of the material. In most cases, it is only through conservation studies of deterioration and treatment of historic buildings that information about the properties and performance of specific building materials and technologies is updated or reevaluated. In the case of Pennsylvania marble, its short-lived fame and restricted use have resulted in limited modern scientific study of its deterioration and responses to conservation treatments. Buildings of note that were constructed of Pennsylvania marble include:
  • 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