MSc Student, Geology
I am a Geology major for those of you who do not know. My thesis is on copper corrosion so I tend to research more about it. I wanted to get some information on copper stabilization and in particular, alkaline dithionite.
This method was first used in 1979 by MacLeod and North, but was investigated further by MacLeod in 1987 for silver. A solution of 40 g/l of NaOH and 50 g/l of Na2S2O4, sodium dithionite. Artifacts are sealed quickly in a container with the solution to help keep atmospheric oxygen levels to a minimum. Bronze objects tend to change color from blue green (copper trihydroxychlorides) to a yellow orange (copper I hydroxide) to a chocolate brown (metallic copper). The change occurs in minutes of being immersed in the alkaline dithionite solution.
Here’s a look at the geochemistry occuring:
3Cu2(OH)3Cl + S2O4 2- + OH- = 6Cu(OH) + 3Cl- + 2SO4 2- +4H+
6Cu(OH) + S2O4 2- = 6Cu + 2SO4 2- + 2H2O + 2H+
3Cu2O + S2O4 2- + OH- = 6Cu + 2SO4 2- + H+
The alkaline solution is a reducing agent but can alter the patina, so use will need to be taken into account before using this as a stabalization technique. MacLeod used it on several Greco-Roman coins, but they were so badly corroded that no inscription would be possible to see. After treatment, the corrosion on the coins was reduced to a loosely adherent powder that when brushed off revealed the original inscription. Artifacts needs to be rinsed after treatment, MacLeod used deionized water. He found that this is a viable treatment for consolidation of heavily corroded bronzes and may even reconsolidate the original surface. This treatment is not meant for every bronze artifact; some fragile artifacts may disintegrate in alkaline dithionite.
This treatment is costly. High-grade chemicals can be expensive and commercial-grade reagants can have impurities. After the treatment is over, the left-over chemicals must be disposed of properly because it will imbalance the microbiological balance of sewage systems.
I got my information from David A. Scott “Copper and Bronze in Art: Corrosion, colorants, conservation” The Getty Conservation Institute. 2002.