Suggestions for Removing Graffiti from Historic Masonry (Part I)
Graffiti Removal Method Health and Safety Cautions
- Paint Remover based on N-methyl-2-pyrrolidone (NMP).
** Organic Solvent such as acetone, lacquer-thinner, orpetroleum-based compound such as dimethyl adipate.
*** Bleach such as calcium hypochlorite.
- N-methyl-2-pyrrolidone (NMP) is mildly toxic and may haveadverse reproductive effects.
Solvents and petroleum-based compounds have toxic vapors, areflammable, and require well-ventilated conditions.
These are suggestions to assist in graffiti removal. Methodsshould always be tested first under the supervision and guidanceof an architectural conservator.
Permanent magic marker
Water soluble marker
Crayon, Lipstick, Shoe Polish
- It is important to pre-wet the masonry surface when using an alkalinepaint remover; it is also advisable to pre-wet the masonry surroundinga graffitied area to dilute the effect of any cleaning agents that mightbe inadvertently splashed or spilled on the unsoiled surface. Do notwet the area to be cleaned if the cleaning agent is solvent-based or incompatiblewith water.
- Always rinse the cleaning agent off the masonry surface starting atthe bottom and moving up. This prevents the cleaning agent from runningdown and staining a lower surface.
- Air temperature can be a factor in graffiti removal. Most paint removersdo not work when the air temperature is either very cold or very hot. Thismay sometimes explain why a method that worked in one instance may notbe effective again in another, similar situation.
- Variations within the same type of stone, such as bedding planes, density,finish, or degree of weathering, may explain why some areas of the samestone sometimes clean better that others.
- Even if advance testing has been done and a treatment plan exists,at least some on-the-spot testing will probably be necessary.
- Mortar joints react differently from masonry units, and may requirea different cleaning material and/or method to be cleaned effectively.
- Graffiti removal may result in an obviously "clean" spot.Always clean the entire masonry unit that is bounded by mortar joints (butnot the joints themselves, unless necessary). The prominence of the cleanspot may be minimized by fanning the cleaning out from the spot, and "feathering"it by gradually reducing the strength or thoroughness of the cleaning.
- If it is not possible to completely remove all traces of graffiti withoutremoving some of the masonry surface, it may be preferable to leave themasonry alone. Some graffiti ghosts become less noticeable with time dueto fading of the dyes used in paints and markers. Sometimes it may be possibleto conceal more obvious graffiti ghosts with carefully-matched paint.
- After graffiti removal, the masonry surface should always be testedwith Ph strips to make sure all the cleaning materials have been completelyremoved. Non-staining Ph strips, available from chemical supply companies,will indicate whether acids or alkalis remain on the masonry surface.
- Although alkaline paint removers are sometimes ineffective on modernformulations of aerosol paints, they can work well in removing multi-layeredgraffiti because they last longer.
- What removes graffiti in one instance may not always work again evenin what appears to be an identical situation.
- More than one cleaning material and technique may be required to cleana heavily graffitied area if different materials were used to make thegraffiti. For example, shapes are often outlined with broad-tipped feltmarkers and then filled in with spray paint.
- Effective graffiti removal often depends on trial-and-error testing,as well as a knowledge of masonry materials, graffiti materials and cleaningtechniques.
This Preservation Brief was developed under a cooperative agreementbetween the New York Landmarks Conservancy and the National Park Service.Mark A. Weber, Director, Technical Services Center, served as projectcoordinator for the Conservancy. The author, Martin E. Weaver, isthe Director of the Center for Preservation Research at Columbia University.He is an internationally recognized expert in the conservation of architecturaland cultural resources, a noted lecturer, and author of Conserving Buildings:A Guide to Techniques and Materials, as well as numerous articles onthe subject.
Anne E. Grimmer, Senior Architectural Historian, Technical PreservationServices, Preservation Assistance Division, National Park Service, coordinatedthe development of this Preservation Brief and served as Technical Editor.Technical review of this publication by the following is gratefully acknowledged:Frances Gale, Training Coordinator, National Center for Preservation Technologyand Training, National Park Service, Natchitoches, LA; Judith M. Jacob,Architectural Conservator, Building Conservation Branch, Northeast CulturalResources Center, National Park Service, NY, NY; Andrea Mones-O'Hara, RegionalHistoric Preservation Officer, National Capital Region, General ServicesAdministration, Washington, DC; Nicolas F. Veloz, Conservator of OutdoorSculpture and Monuments, National Capital Area Office, National Park Service,Washington, DC; and Michael J. Auer, Timothy Buehner, Charles E. Fisher,and especially Kay D. Weeks, Preservation Assistance Division, NationalPark Service.
Washington, DC. October, 1995