Daniel Boone Trail Highway marker in North Wilkesboro, NC, showing rust staining.

Daniel Boone Trail Highway marker in Wilkesboro, NC, showing rust staining.

I am Leah Poole, a graduate student at the University of Texas at Austin, and I have been working in the Materials Conservation department with Jason Church. Prior to starting graduate school at the University of Texas, I received an undergraduate degree in chemistry.

The project we have been working on this summer is the continuation of NCPTT grant MT-2210-01-NC-05, started by Martha Singer and Dr. Charles Caldwell. The aim of the project is to find a chelating agent that will remove metallic stains from marble while not damaging the marble. These metallic stains are often caused by iron and copper, and the chelating agents will complex the water insoluble metals into a water soluble form. The danger of using chelators is their potential to remove the calcium of the marble’s calcite. Because of the possibility of calcium chelation, we are looking at medical chelating agents, which have a much greater affinity for iron than calcium.

During the experiment, five chelating agents were analyzed: acetohydroxamic acid, cysteine, maltol, picolinic acid, and ammonium citrate. The latter is a chelating agent commonly used by conservators to remove iron staining from marble.

Leah Poole preps marble samples for testing.

Leah Poole preps marble samples for testing.

According to the mole ratio method often employed to determine metal complexes, the amount of chelating agent was varied against the amount of ferric ion, and the resulting solution was analyzed using vis-spectrometry. This analysis occurred for each chelating agent at four different pH values—7.4, 8.5, 9.2, and 10.0. The resulting spectra showed that the complexes formed from each chelating agent varied with pH and their intensities were dependent on the mole ratios.

Solutions of the chelating agents were made at the four different pH values and added to known amounts of calcium carbonate and ferric oxide, which represented calcite and rust, respectively. From that, the amount of chelated calcium carbonate or ferric oxide was calculated. After analyzing how much substrate was complexed per mole of chelating agent, the results showed that pH was an important factor in the calcium chelation. In general, more calcium is chelated at a lower pH. In order of least calcium chelated to most is: picolinic acid, acetohydroxamic acid, cysteine, maltol, and ammonium citrate.

Marble samples with rust staining used in the cleaning tests.

Marble samples with rust staining used in the cleaning tests.

After applying the chelating agent to iron and rust stained marble samples, it was found that cysteine cleaned the marble the best, followed by ammonium citrate, maltol, picolinic acid, and acetohydroxamic acid. During this process, both colorimetric and photographic documentation were utilized to analyze the marble samples.

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3 Responses to Testing the Use of Medical Chelating Agents to Remove Iron Stains on Marble

  1. Andrew Thorn says:

    I published an article on iron stain removal you may like to consider the results of. The method is not listed in your tests but we have found it far superior to ammonium citrate.

    Thorn, A. 2005 Treatment of heavily iron-stained limestone and marble sculpture. ICOM-CC 14th Triennial Congress Preprints 2005 888-894.

    I can send a pdf.

    Would love to know of your final conclusions in this study.

    Regards,

    Andrew Thorn

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