Removing Graffiti from Historic Masonry (Part A)
Successful graffiti removal from historic masonry depends on achieving a balance between breaking the bond between the graffiti and the masonrysurface without damaging the masonry. This generally requires knowledge both of the materials used to make the graffiti and the masonry on whichthe graffiti has been executed, as well as knowledge of cleaning methods and materials. Without this, masonry surfaces can be badly disfiguredor damaged during graffiti removal.
*The word graffito (graffiti, plural) -- is derived from the old Italian diminutive of graffio-to scratch, and the Latingraphire-to write. Graffiti in contemporary usage has cometo mean an inscription, drawings, or markings. Except in very formal or technical applications, graffiti is generally considered a "mass"noun and paired with a singular verb.
Graffiti. Most graffiti is made with spray paints. Although a number of solvents and paint strippers are capable of dissolving or breaking down these paints, some may permanently discolor or stain the masonry surfaceif not used correctly. As a result, the remaining paint may become moredifficult, or even impossible, to remove. Poorly thought-out and generallyhasty attempts to remove graffiti using harsh chemicals or abrasives canalso cause permanent damage to the masonry that may be worse than the graffiti.
The ability to identify the graffiti material is an important step insuccessful removal. Numerous kinds of spray paint (polyurethanes, lacquers,and enamels), and brush-applied paints (oils and synthetic resins suchas vinyls, acrylics, acetates, methacrylates, or alkyds), as well as permanent felt markers are the materials most often used to make graffiti. But other materials are also used for graffiti, including water-soluble felt markers, ballpoint pens, chalk, graphite and colored pencils, pastels, wax and oilcrayons, liquid shoe polish, and lipstick. The range of materials adopted by graffitists continues to expand.
Paints are composed of pigments that provide color and hiding power; binder that holds the pigments together and to the substrate; and a solventthat allows the pigment/binder mixture to flow. Some spray paints and markers may contain dyes instead of pigments. Paints are applied wet. Generally,as the solvent evaporates, the binder solidifies. The greater the solvent content of the paint, the greater the flow rate, and thus, the greater the ability of the paint to penetrate into masonry pores.
The two primary components contained in most graffiti materials-pigmentor dye, and binder-may simply remain on the masonry surface, or penetrateinto the masonry to varying depths depending on a number of factors, including the surface tension of the substrate and viscosity of the solvent or vehicle.Thus, even the total removal of the pigment or the binder may leave residuesof the other component actually in, or below, the surface of the stone. Residual stains, or graffiti "ghosts," such as those from anykind of red paint or the fine black pigments used in spray paints, maybe particularly difficult to remove (Fig. 4). With painted graffiti, itis helpful to establish how long it has been on the surface. For most paints that have been on the surface for several weeks or months, hardening processesare likely to be complete or well-advanced; the solubility of the paintis proportionately reduced and it will be more difficult to remove.
Masonry. The historic masonry substrate must also be identified.As used here, the term masonry encompasses all types of natural stones; manufactured clay materials, including brick and terra cotta; and cementitious materials, such as cast stone, concrete and mortar. The commonfactor among masonry materials is that they are porous, to a greater or lesser extent, and sensitive to abrasion. After identifying the masonry,its condition, including fragility, porosity and permeability, must also be assessed prior to beginning graffiti removal. For example, a smooth, newly-polished granite surface is comparatively easy to clean because itis relatively impermeable and paint vehicles tend to stay on the surface rather than penetrate into microscopic pores. A very smooth, polished surface also has no pits or crevices that will retain
particles of pigment or binder. In contrast, weathered marble or limestonemay be extremely porous and permeable, with a rough surface on which particlesof pigment can easily lodge. The fragility of such a surface can make itimpossible to clean the surface even with a bristle brush without riskingsevere surface loss. A difference in surface texture or finish may alsobe the reason that a particular cleaning agent will work in one situationbut not another.
Some types of masonry may react adversely to contact with the variouscleaning agents required to break or dissolve the bond between the graffitiand the masonry surface. Thus, for purposes of cleaning, masonry typesare often categorized according to whether they are acid-sensitive, non-acidsensitive, or alkali-sensitive. Acid-sensitive stones consistingof carbonate materials may be damaged or even destroyed by contact withacids. Although, in many instances, acidic cleaning compounds are not effectivefor graffiti removal and generally should not be used for this purpose,it is useful to know that some acid-sensitive materials include: stonessuch as limestone, marble, travertine, calcareous sandstones and shales;most polished stones; and glazed architectural terra cotta and glazed brick.Non-acid sensitive masonry materials include slate, granite, unglazedarchitectural terra cotta and unglazed brick. Alkali-sensitive stonesmay contain silicates, or ferrous, soluble iron compounds that can reactwith alkalis or water to form severe staining. Alkali-sensitive stonesinclude some granites, Indiana limestone, and many types of sandstone,especially those that are green or grey in color. Glazed and polished surfacestend to be damaged by both strong acids and strong alkalis.