Fax forwarding DEP briefing materials for Mayor's meeting, March 2002

NYC 9/11 Public Portal Document ' f.',—

Friday 15 of Mar 2002, Mayor Office ->718 595 4544 Page 24 of 29

wash in order to remove all of the remaining alkaline substance from the substrate in order to at'oid burning or staining the historic falAic, The utifizatiOiY^f strong alkalis, with a |)H 13- 14. including sodium hydro.xide-based paint removers, should be avoided as their application can result in efflorescence or staining of the surface of masonry if not fiilly neutralized after application and removal. Hydroxide paint removers, including potassium, can ad\ersel\' react with fetrous elements found in certain stones (e.g. Indiana limestone) and cause subsequent staining. .Masonry surfaces should be tested with non-staining ph strip.s after remo\ als are completed in order to ensure that the substrate has been adequately neutralized.

Removal utilizing Bleaches- Certain alkali-based bleaches, calcium hypochlorite for example, are sometimes used in poultices. When stains are not easily removable, bleaching may reduce the appearance of a stain.

Organic Solvents and Paint Removers- Chemical cleaning may be utilized alone or in conjunction with a poultice. Organic solvents are sometimes used in the fonn of a paste or a gel that '.'.ill hold the solvent in place. The use of acidic or alkaline chemicals for cleaning can cause damage to building materials as well as pose health, safety, and environmental risks. .-Vny cleaning application should be applied by a qualified professional and tested in an inconspicuous area or on mock-ups prior to widespread application. ’'Off the shelf graffiti remo\ers should be a\oided as their application in conjunction with scrubbing often results in the running or redistributing of a paint or stain on the surface of the historic fabric. .Masonry surfaces should be tested with non-staining ph strips after removals are completed in order to ensure that the substrate has been adequately neutralized. .Ai’ea.s that are not being treated for the removal of applied paint (signage) should be appropriately masked and protected.

Abrasive Removal- .Abrasive cleaning inA olves mechanically abrading a surface to remove staining. Traditional abrasive cleaning techniques, such a.s ■•sand-blasting" and high pressure water applications, are not suitable for cleaning historic buildings as older building materials can easily be damaged. Low or medium water pressure washing (100-400 psi) can damage historic masottry. .-Vn acceptable form of micro-abrasion (inx ohcing fine abrasive materials applied at pressures of 35-40 psi) used to mechanically remove markings from historic masonry should be careftilly considered and implemented only by skilled workers under the direction of a building conservator or preservation professional. Abrasive cleaning should not be used on brick or glazed or polished surfaces.

Other Removal Methods- Removal method.'; such as laser cleaning have been recently developed. Experienced building conserxators and'or preservation professionals should implement a removal program including a method such as laser cleaning.

Cleaning Considerations: Buildings are frequently composed of several different masonry materials. Each material must be determined, evaluated, and tested by an experienced conservation or preservation specialist prior to implementing a cleaning system.

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