For this last blog post on my lab research on the use of chelating agents to remove iron stains from marble, I will look at some of the challenges associated with preparing the cleaning solutions and discuss my treatment set-up as well as go over some general observations.
As has been discussed in a previous blog post, five different chelating agents were tested: cysteine, thioglycolic acid, maltol, picolinic acid and ammonium citrate. Each of these chelators was then tested at pH values 9.2 and 10, which were established by Leah Poole in an earlier stage of this study.
One of the first things that should be considered is the careful monitoring of the pH of the solutions during preparation. Some of the chelators have a buffer zone just above pH 7 at which point they become highly sensitive to any adjustments. It would therefore be of some advantage to have a digital pH reader available during this process. Having different concentrations of NaOH (to increase pH) and HCl (to decrease pH) available can also be of great help as some back titration is likely to be needed.
The primary focus of this study was to determine the damage that the ten different treatment solutions cause to the marble substrate. This was then combined with data on how effectively they reduce iron staining to produce a ranking of different solutions and when they can be used most effectively. Given this focus on damage caused to marble, it was decided to overexpose the samples to the chelators to replicate a worst case scenario if the agent was left on the object or structure for too long. The samples were therefore suspended in the treatment solutions for 30 minutes, before they were removed and thoroughly rinsed with deionized water.
Three samples were treated once with each solution for replication purposes. Then one of each of these sets of samples were treated a second time with the same chelating agents and pH to determine the effect of repeat treatments. The treatment solutions used on the unstained samples generally became cloudy after a few minutes indicating that they were leaching calcium carbonate from the marble.
The extent of this clouding could give an indication of the amount of damage that was caused to the surface of the samples. Some of the solutions discolored quite heavily when used on the stained
samples as the iron was reacting with the chelating agent. This, however, did not seem to directly correspond to the chelator’s effectiveness at removing the stain.
A number of different tests were conducted on the samples after each round of treatment. After they were photographed, colorimetry, glossimetry, laser profilometry, surface pH and FTIR data was collected for all of them. The results of this data indicate that physical rather than chemical damage seems to be the primary cause for concern with these treatments. It also shows that there are chelators that are much better than others at specifically targeting iron and are therefore effective cleaning agents without causing too much damage to the marble substrate.
This blog post will be updated with more specific results on the individual chelators once this study has been published.