Chelating agents have long been used by conservators to remove iron stains from historic stone. The chemical composition of marble, however, presents a particular challenge because its main component—calcium carbonate—is highly sensitive to acidity and chelating agents are acidic by nature. Chelators chemically bond with metal ions, making them removable with water. This allows conservators to simply wash away metallic stains. While chelating agents are generally extremely effective at removing iron stains, their acidity can be damaging to the marble substrate. While these effects can be minor and limited to dulling a marble surface, they can be much more severe and even result in the permanent etching of the stone. This study looks specifically at the use of chelating agents, which are chemically analogous to chelators used in the medical profession to treat heavy metal poisoning and similar conditions. These products are therefore highly stable and well understood, as they have been through rigorous analysis and testing to gain approval for medical use.

 

This study investigates the use of five different chelating agents for their efficacy in the removal of iron stains as well as their physical and chemical effects on marble surfaces: ammonium citrate, cysteine, maltol, picolinic acid, and thioglycolic acid. One group of samples cut from Colorado Yule marble is artificially stained with iron oxide while another group is left unstained. Each chelating agent is tested at two different pH values that were chosen through UV-visible spectroscopy. The samples are analyzed before and after cleaning. Colorimentry, glossimetry, and laser profilometry readings are taken of all samples at each state of this study in order to establish a thorough understanding of how these chelating agents affect the physical properties of the marble surface and to quantify the effectiveness of the chelators in removing the iron stains. Surface readings of the pH values of the samples as well as FTIR spectra are also taken at each stage in order to gain a better understanding of the chemical effects that the chelating agents have on the marble surface.