43rd Annual Meeting – OSG Tips Session, May 16, "Plaster Cleaning Tests" by Kathryn Brugioni

In this tip presented during the OSG Tips session luncheon, Kathryn Brugioni discussed the use of Reflectance Transformation Imaging (RTI) for evaluating whether certain dry cleaning methods for plaster abraded or damaged the surface of the object.
Dry methods are preferred over wet methods for cleaning plaster because of the risk of solubilizing the substrate during treatment. When Kathryn was presented with a heavily soiled plaster bust that required cleaning, she turned to the use of vinyl erasers as a cleaning method. Using previously published information that evaluated various types of erasers (Williams and Lauffenburger 1995; Pearlstein, et al. 1982) she decided to test two different PVC-based erasers made by Staedtler: the 526 50 Mars plastic eraser and the 527 05 Mars eraser strip refills, to evaluate not only how well they cleaned soiled plaster, but whether they abraded the surface.
Once she chose what dry cleaning method/materials to test, Kathryn was left with the question of how to evaluate surfaces after cleaning to determine the level of abrasion or scratches resulting from the treatment.  Examination using SEM imaging has been used (Wharton, et al. 1990) , but it is a technique that may not be available to all conservators.  So she looked to a method that could be more accessible: RTI.
RTI, or polynomial texture mapping, is an imaging technique that allows for an interactive display of an image under different lighting conditions.  Multiple images are taken of an object where the object is kept in a fixed position, but the light source moves.  The images are processed using using freely available software which combines all the images taken into a single image presented in an interface that allows for the direction of light to be moved across the image at different angles highlighting surface features. (The non-profit organization Cultural Heritage Imaging (CHI) is one of the leaders in this type of imaging for cultural heritage and has lots of information on its website about this technique, steps on how to do it and the software needed to process the images).  The software  also allows for different types of light or shadow effects to be rendered which may improve or further highlight surface examination.  All you need for RTI is a camera, moveable light source and some metal spheres (ball bearings) as markers that help the software determine the direction/angle of the light.  These are all things that conservators have on hand or can readily purchase (like the ball bearings) making this type of surface examination/imaging more accessible and much cheaper than an SEM.
Kathryn cleaned the surfaces of plaster test coupons using the erasers and imaged them with RTI before and after cleaning.  She soon saw that it was possible to see scratches on the surface using this technique.  However, she wanted a way to quantify the scratches and determine what the limit was in terms of scratch size observable using RTI.  She abraded plaster coupons with a range of grades of micromesh, from 400-1800, and then examined the surfaces using RTI.  She noted that you could detect scratches made with up to 800 grit micromesh, but higher grits, like 1800, created more subtle scratches that were not as easily discernable.
Comparing the scratches made by the two erasers on the plaster coupons to those of different micromesh grades, the scratches made by the Mars plastic eraser were similar to those made by 1200 grit micromesh (measured to be about 34μm size scratches) and the eraser strips made scratches similar to 466 grit micromesh (measured to be about 60μm sized scratches).  So the eraser strips are much more abrasive to plaster surfaces than the plastic eraser.
Based on Kathryn’s findings, it looks like RTI can be used to evaluate any surface scratches or changes caused through the abrasive action of erasers used for dry cleaning plaster. Though there are limitations to the use of this technique, and fine scratches may not be readily visible, RTI is a useful, and accessible, examination tool and can provide important information on surface changes caused by certain cleaning methods.

Pearlstein, E., D. Cabelli, A. King, and N. Indictor. 1982.The Effect of Eraser Treatment on Paper. JAIC 22(1): 1–12.
Wharton, G., S. Lansing Maish, W.S. Ginell. 1990. A Comparative Study of Silver Cleaning Abrasives. JAIC. 29(1): 13-31.
Williams, J. and J. Lauffenburger. 1995. Testing Erasers used to Clean Marble Surfaces. Objects Specialty Group Postprints, Vol. 3: 118-124.