Mayor's Office memo re: DEP meeting, Mar 2002

NYC 9/11 Public Portal Document a a auw~ .S va aqua A. A. a14vi GaaaGG —/ / lV Ja7J 1J22 Yager .V VZ G.7

exterior surfaces. Dust that has re-calcified, or hardened on surfaces should be carefully removed under the direction of a conservator or preservation professional so that historic fabric is not damaged. Debris and dust should be properly disposed of.

o The owner should insure that potential disaster related damages to the to the building (which may include, but are not limited to, openings in building facades and roofs, cracks, open mortar joints, and weakened or damaged materials) are identified by a qualified professional prior to cleaning the facade. Precautions should be taken prior to cleaning applications, as dust or cleaning agents may enter the building through openings in the facade or fabric. -

Cleaning Methods: (See \WTTC Site Specific Considerations First)

Based on building materials and overall building condition one of the following methods of cleaning will most likely be used:

Water Cleaning- Water cleaning utilizes the application of a gentle, low pressure, clean water spray. High-pressure water spray should not be used and can cause significant damage to building materials. Applied water pressures should not exceed 250 psi, but it must be noted that even low or moderate pressure water applications (100-400 psi) can damage some masonry. Prolonged application of water to a building can result in staining caused by the leaching out of minerals within the building material, the corrosion of embedded ferrous metals, and damage finishes on the interior of the building. Water cleaning should not be conducted during cold weather, as freeze/thaw cycles may cause damage to facades.

Chemical Cleaning- Chemical cleaning may be utilized alone or in conjunction with a water cleaning. Water cleaning may be accompanied by the use of a nonionic detergent applied with fiber brushes and followed with subsequent water applications as specified in the determined cleaning program. The use of acidic or alkaline chemicals for cleaning can cause damage to building materials as well as pose health, safety, and environmental risks. Cleaning poultices are composed of absorbent powders mixed with a liquid to create a wet paste that can be applied to a building. Poultices allow controlled application and removal and minimize rinsing requirements.

Abrasive Cleaning- Abrasive cleaning involves mechanically abrading a surface to remove soiling. Traditional abrasive cleaning techniques are not suitable for cleaning historic buildings as building materials c~•i easily be damaged.

Acceptable and gentle abrasive cleaning methods are highly sophisticated. Consult Historic Building Facades: The Manual for Maintenance and Rehabilitation published by New York Landmarks Conservancy for information regarding acceptable methods of abrasive cleaning.

18

NYC-WTC 000093807