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    How To Remove Graffiti
    By Enrico Uva | January 1st 2013 11:30 PM | 6 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    He stood by the door of a subway car, looking from side to side to see if anyone was looking. Maybe my clothing camouflaged me against the background of my seat. More likely, I did not pose a threat, and certainly he didn't realize I was a teacher at his school. He opened his long coat, and for a moment I thought he was going to flash the ghosts of the subway tunnels when suddenly he pulled out a marker from his inner pocket and tagged an advertisement on the wall.

    Unfortunately I did not have my 95% ethanol with me. After I transferred to another school, I still encountered the occasional tagging. Interestingly "graffiti" is rooted in the Italian word for "scratch". Tagging, technically, is less artsy than true graffiti. While the former is used by street gangs to mark territory, most individuals' scratches and scribbles are merely an attempt to publicly declare individuality while they disregard others' property.

    When that property is a metallic surface such as a locker, the ethanol wipes the ink away beautifully, without a trace. It's also effective on wood surfaces when the markings are fresh, but unfortunately absolute ethanol can also dissolve the varnish.

    That's one of the challenges in removing graffiti. The most common medium, spray can paint, can host a variety of compounds: polyurethanes, lacquers and enamels. For each of these, there are solvents capable of forming intermolecular bonds with the compounds that are stronger than those between the latter and the background. Examples include butanone (MEK= methyl ethyl ketone) and xylene. But in attempting to remove graffiti, there's the risk of letting the paint penetrate deeper and of damaging the surface itself. It's advisable to begin by testing a solvent on small areas. 

    When cleaning masonry especially, the cleaning product is best applied to a poultice, a porous solid filled with solvent. This extends contact time between the cleaning fluid and the surface and prevents spreading of pigments to unaffected areas. Since water does not dissolve in organic solvents, the surface should be dry. In cases where the tagging is recent, an ammonia solution with a pH less than 13 also works and is safe on most surfaces. And since ammonia is water-soluble, the surface could be wet. Some granites and most sandstones, especially those of a green or grey color, are sensitive to alkaline solutions, so highly concentrated solutions of NaOH or KOH are not recommended. At the other pH-extreme, most acids are not only useless in attacking paints, but they will also dissolve and thus damage anything with carbonates: marble, limestone and terracotta.

    If there are still residual pigments after treatment with a solvent, they can be bleached with swimming pool disinfectant: calcium hypochlorite, Ca(OCl)2. Since this compound is only slightly alkaline (it's the product of a weak acid and strong base), it's innocuous towards both acid-sensitive and alkali-sensitive surfaces. Most commercial products use a shotgun approach by blending several agents. For example, an old recipe uses a blend of Ca(OCl)2, pine oil and ammonia. Another employs base, ether, ethanol and a ketone. Concoctions, however, can  sometimes be less than the sum of their parts, and the more meticulous approach is recommended for prized masonry. More recent patent-pending formulas are interesting from an environmental point of view because they use a combination of esters and surfactants.

    Since solubility and evaporation rates are both temperature-dependent, an attempt to remove graffiti in extreme temperatures will render the cleaning operation less efficient. Sometimes the desire to avoid harsh chemicals tempts one to use high-pressure washing or abrasives, but these damage masonry, at times even etching a permanent outline of the graffiti. 

    A better alternative to grinding, albeit expensive and tedious, is the use of lasers, which have come to the rescue of defaced historical artifacts. When an Nd:YVO4 (neodymium-doped yttrium orthovanadate) laser was used on various granites, the effectiveness was not affected by the type of rock, but it did vary with the composition of paint used, especially if it was reflective.
     
    Sources:

    Sasha Chapman. Laser technology for graffiti removal Journal of Cultural Heritage Volume 1, Supplement 1, 1 August 2000, Pages S75–S78 


    Martin E. Weaver. Removing Graffiti from Historic Masonry
    http://www.usheritage.com/form/brief38.pdf

    T. Rivas. Nd:YVO4 laser removal of graffiti from granite. Influence of paint and rock properties on cleaning efficacy  Applied Surface Science Volume 263, 15 December 2012, Pages 563–572

    Comments

    rholley
    The most common medium, spray can paint, can host a variety of compounds: polyurethanes, lacquers and enamels.
    That takes me back!  My first job after leaving university was working in paint research.

    One project involved assessing solvents as paint strippers.  Solubility parameter (matching the electronic properties of the solvent to the paint) was, as known a priori, important.  But one factor stood out, and that was molar volume.  Small molecules such as dichloromethane (aka methylene chloride) were much more aggressive than larger ones.

    However, pre-soaking a paint film in ethanol seemed to “de-stress” it, and subsequent treatment with dichloromethane was slower, and the film did not expand and crumple up to the same extent.
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    UvaE
    Hopefully Robert you did not spend too much time with methylene chloride. In those days wearing a mask/respirator was not always the norm.
    In Singapore they cane the kid leaving an indelible ugly scar covering the entire back. This method cures the problem at the source, and is less labor intensive, green as well for lack of caustic chemicals. One application serves a lifetime.

    UvaE
    One application serves a lifetime.
    In theory. And if they cane a kid for mere tagging or graffiti, I hate to ask what they do for more severe infractions!

    rholley
    About the same time as this blog appeared, there appeared (01 Jan 2013) a letter to the Telegraph  about anniversaries, including this year’s centenary of the composer Benjamin Britten.  Among the on-line comments, there were some about two sculptures in England that often get vandalized. 



    One of these concerned a memorial to Britten, namely the Scallop, by Maggi Hambling, in Aldeburgh.  The commenter said

    The Maggi Hambling Scallop is beautiful; sadly, every time we’ve seen it, some numpty has chucked paint over it.
    While another replied
    I agree it is a nice work but disliked by someone in Aldeburgh. You have to walk a mile to get to it so whoever is dishing out the paint is a determined individual.

    A sculpture perhaps more asking for such treatment is Another Place, by Antony Gormley, of which Wikipedia says:

    «In common with most of Gormley’s work, the figures are cast replicas of the artist’s own body. As the tides ebb and flow, the figures are revealed and submerged by the sea.

    Another Place is a subject of local controversy in Merseyside. Some consider the statues to be “pornographic” due to the inclusion of a simplified penis on the statues,[citation needed] whilst others see them as beautiful pieces of art which have brought increased tourism revenue to the local area.»

    About this another commenter said:
    The same thing happens to various members of Antony Gormley’s Another Place. The dressing up of them is fine – the clothes and accoutrements quickly fall off – but some of the paint has remained for years.

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    UvaE
    Stonehenge is apparently another popular graffiti target and has been for centuries. Some of the older graffiti were true "scratches" because the markings were carved onto the surface.