A request from his daughter Kirsteen for Ian MacLeod to clean all of grandma’s silver tea service and bring it to Melbourne on his next trip from Perth saw him discover a numbered silver dish from “Wm Rogers of Hamilton Ontario, EP on Copper WMMTS 33/103” which was covered with a very dense silver sulphide patina.
Surface testing with water confirmed that most of the surface was extremely hydrophobic and the surface felt plastic with all the characteristics of polyurethane. Only a few patches where the lacquer had broken down were responsive to the use of proprietary silver cleaners (Silvo at home office). A bath in 20 gram per litre caustic overnight had minimal effect as 90% of the coated areas retained the coating.
So his thoughts turned to hot chlorinated solvents as acetone had no effect. Slowly the brain kicked in and it was decided to wrap the object in commercial cooking grade aluminium foil, sprinkle the bright metal foil with salt and then add some hot water and run in a gas oven for two hours at 80-90C. After this interval the oven door was opened and the characteristic smell of hydrogen sulphide was detected and the bright foil had turned a dull grey. The delightful thing was that the treatment had not only reduced the thick sulphide to metallic silver it had also removed the organic coating. Since my office does not have an FTIR I cannot confirm it was a polyurethane but the contact angles with water, the smoothness and the touch were all consistent with the polymer being of that ilk.
Following the aluminium treatment the upper side of the plate was more lustrous that the more heavily corroded lower part of the plate which had become a dull silver colour. On application of polish both surfaces were restored to a fully lustrous state.
The only reason I am writing this news item is that in small museum and even in state museum collections there are heaps of black lacquer coated silver items and they do not have access to expensive conservators or conservators are not allowed to use toxic organic solvents etc. So here is a simple solution to a complex problem. Give it a go—I will guarantee its efficacy.
Ian D MacLeod
Heritage Conservation Solutions