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 24 of 29

wash in order to remove all of the remaining alkaline substance from the substrate in order to avoid burning or staining the historic fabric. The utilization of strong alkalis, with a pH 13- 14, including sodium hydroxide-based paint removers, should be avoided as their application can result in efflorescence or staining of the surface of masonry if not fully neutralized after application and removal. Hydroxide paint removers, including potassium, can adversely react with ferrous elements found in certain stones (e.g. Indiana limestone) and cause subsequent staining. 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.

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 form of a paste or a gel that will 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. Any 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 removers should be avoided 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. Areas that are not being treated for the removal of applied paint (signage) should be appropriately masked and protected.

Abrasive Removal- Abrasive cleaning involves mechanically abrading a surface to remove staining. Traditional abrasive cleaning techniques, such as "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 masonry. An acceptable form of micro-abrasion (involving fine abrasive materials applied at pressures of 35-40 psi) used to mechanically remove markings from historic masonry should be carefully 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 methods such as laser cleaning have been recently developed. Experienced building conservators 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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