I am Anna Funke, a recent graduate from the Master program in Conservation for Archaeology and Museums at UCL in London, UK. I am now doing a ten-week internship at the National Center for Preservation Technology and Training during which I am continuing Leah Poole’s 2014 research on the use of medical chelating agents for the removal of iron stains from marble. The blog post on Leah Poole’s original study can be found here.
Conservators have long used a variety of chelating agents such as EDTA and similar products to remove iron stains from stone objects and monuments. Marble, however, poses some particular challenges. Its primary component is calcium carbonate, which is highly sensitive to acidity. The surface pH of Colorado Yule, which was used in this study, is approximately 9.45. Any treatment solution with a lower pH than this may therefore cause damage to the stone’s surface. The problem is that chelating agents generally have a pH of around 5 or less. A successful treatment therefore has to buffer the chelating agent to a pH that allows it remove the iron without affecting the marble substrate.
Poole used UV-visual spectroscopy to test the chelating properties of five different agents both on iron and calcium carbonate. She tested ammonium citrate, cysteine, maltol, picolinic acid and acetohydroxamic acid. During these tests, she established two pH values that would cause minimal damage to the marble while still effectively chelating the iron. Actohydroxamic acid did not perform very well in these tests and was therefore replaced with thioglycolic acid for the research that I am undertaking this fall. Thioglycoloic acid has shown some promising results and has been reviewed throughout the conservation literature.
There are two parts to this phase of the study. One set of Colorado Yule samples will be exposed to the different chelating agents at both of the pH values established through Poole’s research. The samples will then be tested both for physical and chemical changes before treatment, after one treatment and then again after a second treatment with the chelators. The physical changes will be investigated using photography, mass, colorimetry, glossimetry, and laser profilometry. The chemical changes will be investigated using surface pH and FTIR (Fourier transform infrared spectroscopy).
The second set of samples will be artificially stained using different grades of steel wool and a time-set sprinkler system before it is treated with the different chelating agents. Although Leah did perform a similar test during her time at the NCPTT, the staining method used for that research was not very satisfactory and it was therefore decided to repeat this test. The same analytical techniques are used in this part of the study, to look at the effects that the chelating agents have on iron stains rather than the marble substrate.
This study aims to provide the data necessary to develop a treatment procedure for the removal of iron stains from historic marble that takes into consideration both the chelating agent’s effect on the iron stain as well as on the marble substrate. A future blog post will go into further detail about the findings of this study.