One important aspect of my study on chelating agents was to produce samples that adequately represent the properties of naturally iron stained marble.
Previously, marble samples were wetted with water and then
covered in iron powder. However, this led to a crust rather than a stain forming on the samples.
I therefore decided to put the marble into a timed sprinkling system. This wetted the samples for a few seconds and then allowed them to dry for a brief period of time allowing the stain to be absorbed into the marble.
I used steel wool because it is a readily available source of low quality metal that would both degrade quickly as well as allow the water to run through it, carrying the staining corrosion product onto the marble.
Three different grades of steel wool were tested. A bit of each was simply placed into a cup of water and then observed to see how they would degrade. Grade 3 (coarse) degraded the fastest, turning the water murky after just hours, whereas Grade 0 (fine) took several days to show signs of rust. Grade 2 (medium) turned the water murky overnight.
I began the staining process using Grade 3 with the sprinkler on for 20 seconds and off for 30 seconds. However, the steel wool ended up deteriorating extremely rapidly, requiring replacing every few hours. It also quickly went beyond red iron (III) oxide (Fe2O3) and turned into black iron (II, III) oxide (Fe3O4). This caused a dark grey deposit on the marble, which only caused very limited staining.
After three days, I started to use Grade 0 steel wool instead, in the hope that it would not degrade as rapidly and cause more red staining. The sprinklers were also adjusted to be on for only 10 seconds. Fluffing out the steel wool also seemed to help as this allowed for more oxygen to enter the system, which caused more red iron (III) oxide to form.
After another three days the samples were moved onto a wire mesh support on a wooden frame to facilitate them drying during the times when the sprinkler was off. The sprinkling cycle was also adjusted
again to be on for only 2 seconds and off for 30 seconds. This optimized the corrosion process of the Grade 0 steel wool as well as the wetting and drying cycle, allowing the stain to soak into the marble. At this point the steel wool only had to be replaced once every 24 hours.
The staining process took a total of 10 days.
Once the staining was completed, the samples were placed in the oven over night at 60°C. They were taken out the next morning and all loose iron oxide powder that had formed was brushed off, leaving behind only the ingrained stain.
Here are a few more references that may be interesting on the topic of staining marble samples with iron:
Macchia et al, 2016. The treatment of iron-stained marble: towards a ‘green’ solution. International Journal of Conservation Science 7 (1), 323-332.
Spile et al, 2016. Effective cleaning of rust stained marble. Heritage Science 4 (12), 1-10.