Marble columns, iron grillwork, decorative cornices, brass details on doors; these are extra touches that make a building unique. When marble is polished and clean, ironwork and cornices free of rust, and brass buffed to an impeccable shine, the combined effect radiates charm and boosts your building's curb appeal. The city can be a hostile environment, however, with the combined forces of nature, pollution and everyday wear-and-tear wreaking havoc on these design elements. The origin and history of the building materials can provide insight into the best methods for maintenance and restoration.
Since the late 19th century, marble has been a popular choice for building faÃ§ades. Marble is a metamorphic stone formed, as the name suggests, through change - either from the tectonic movement of earth masses, or through the heating of rocks by molten material called magna. Marble - the most popular metamorphic rock for architecture - is basically re-crystallized limestone. The stone takes on various appearances depending on the absence or presence of other materials in the formation. Darker marble contains more sediment and tends to be softer than lighter shades, which are generally more durable and better able to withstand the ravages of time.
By understanding that marble is created through a process of change, it follows logically that it does not stop changing simply because it becomes part of a building. According to "Historic Building FaÃ§ades; The Manual for Maintenance and Rehabilitation" edited by William G. Foulks, there are five general causes of deterioration to stone. First is salt crystallization - the most common offenders being the salts in deicers and alkali cleaning products. Second is acid rain. Pollution in the air increases the acidity of normal rain, which then eats away at the mineral calcite in limestone and marble resulting in changes to the surface and loss of detail. Freezing water is the third factor. Since water expands when it freezes, it exerts pressure on the rock. The level of damage from freezing and thawing varies based on level of saturation. Related to freezing is simple wetting and drying which is more likely to affect the softest and most porous forms of stone. The last main category is biodeterioration or deposits and stains from algae, moss, birds and other animals.
When an expert is called in to assess damage to a building faÃ§ade, the first step is to try to determine what was done to it in the past both by nature and by those who were trying to care for the stone, according to James Hunt, vice president at Port Morris Metal and Marble in the Bronx.
"Thirty to 40 years ago, they used caustic chemicals to clean the marble," says Hunt, adding that these chemicals often did more harm than good. "They also used sealants, which did not allow the stone to breathe and trapped water inside."