DOCUMENTATION OF SURFACE RELIEF CHANGES CAUSED BY ABRASIVE CLEANING SYSTEMS AT THE NEBRASKA STATE CAPITAL

Evaluation of abrasive cleaning methods used on monumental structures is routinely conducted by conservators. Soiled stone surfaces are chosen for trials. Visual inspection of cleaned surfaces is then made and the appropriate cleaning method chosen. Photos may be used to document results.

To more fully characterize changes in surface relief caused by abrasive cleaning, evaluation of a system to prepare molds of soiled limestone surfaces was conducted. The trial included preparation of molds for small areas of soiled stone surfaces both before and after trial cleaning was done. The resulting molds were used to cast replicas of the surface that could be examined at high magnification in an effort to more fully characterize any surface changes than is possible by ordinary visual examination in the field. The casts could also serve as reference standards for comparison when full cleaning was conducted.

Evaluation of a two-component commercially available polymeric material normally used to detect microscopic defects of metal components was conducted in the field. Molds of stone surfaces were prepared in the field both before and after evaluation of several abrasive stone cleaning systems. Casts of the soiled and cleaned areas were then made in the laboratory and examined microscopically. It was determined that the method could capture and adequately characterize surface relief changes caused by abrasive cleaning.

Unfortunately field evaluation of multiple cleaning methods using this surface-replication system resulted in “sample overload” making it difficult to make meaningful interpretations in a timely manner. And while a mold could capture actual relief changes of a surface exposed to abrasive cleaning, no surface color could be detected. These difficulties led to development of a simple and entirely different system that allowed rapid collection of highly magnified uniformly lit stereo pair images of a low relief dimension stone surfaces. This system can be used to rapidly collect and evaluate small scale changes that occur during abrasive cleaning. It has been used to detect low relief microscopic features of building facades related to other construction related difficulties.