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

NYC 9/11 Public Portal Document Friday 15 of Mar 2002, Mayor Office ->718 595 4544 Page 23 of 29

to applications as dust or cleaning agents may enter building through openings in facades or fabric.

Cleaning Methods: The intent of these standards is to provide general guidelines for the removal of applied paint (or signage) from historic building exteriors that comply with the Secretary of the Interior's Standards for the Treatment of Historic Properties. As mentioned above, masonry surfaces should be free of dust prior to implementing a removal program (see Performance Standards for Cleaning Masonry Exteriors).

The following information is explained in greater detail in Preservation Brief Number 38 "Cleaning Graffiti from Historic Masonry" by Martin E. Weaver:

Substrate Identification- Identification of the type of stone substrate will help determine the type of removal methods. Masonr; materials can be categorized as being acid-sensitive, non-acid sensitive, or alkali-sensitive. The application of inappropriate paint removers or cleaning agents can damage stone. Acid-sensitive materials include, but are not limited to: limestone, marble, travertine, calcareous sandstones and shales; most polished stones; and glazed architectural terra cotta and brick. Non-acid sensitive materials include, but are not limited to: slate, granite, unglazed architectural terra cotta and unglazed brick. Alkali- sensitive materials include, but are not limited to: glazed brick, glazed terra cotta, some granites, Indiana limestone, and many types of sandstone. Strong acids and strong alkalis can damage glazed and polished surfaces.

Removal with Water and Detergent- Water cleaning utilizes the application of a gentle, low pressure, clean water spray. Water alone can sometimes remove paint. If the application of low-pressure water is not successful in removing the applied paint, a neutral non-ionic detergent may be used in conjunction with a water application. Any detergent being used should first be tested in a small inconspicuous area prior to widespread application. The pressure of water applied to a masonry surface should be carefully monitored (see "Abrasive Removal"). Inappropriate detergents can cause permanent staining and damage to substrates.

Removal using Poultices- Poultices are powdered materials (e.g. clay, paper, cellulose products, fuller's earth) that are mixed with water or a cleaning agent to form a paste that can be applied to a building surface. Poultices are often the most effective means of removing applied paint (signage) from masonry as they allow the cleaning solution to remain in place as long as possible, and draw out the e'..i.i.img material from the masonry. An experienced and qualified professional should apply poultices as improper application can sometimes draw out and redeposit stains on the opposite face of masonry surfaces. Any cleaning application should be tested in an inconspicuous area or on mock-ups prior to widespread application.

Removal utilizing Alkaline Compounds- Alkaline compounds are typically used in poultices in the form of liquids (reagents), applied to markings/stains on non-alkali sensitive masonry. The application of alkaline compounds should be followed by a weak acid wash and a water

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