FEMA correspondence regarding historic building cleaning treatments, February 2002

NYC 9/11 Public Portal Document mMn. i ). [Liu[ i i :[4Mni 1VU. I YL r.

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

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 Starmdards for Cleaning Masonry Exteriors).

The following information is explained in greater detail in Preservation Brief . Nunrber' 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. Masonry 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, pressure, clean water spray. Water alone eau sometimes remove paint. If the low

application of low-pressure water is not successful removing the applied paint, a neutral non-ionic in'

• 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 fQrm a paste that calm 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 staining 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 mock-ups prior to widespread on

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

2)

NYC-WTC 000093772