This fall, I was fortunate enough to attend the triennial meetings of the International Council of Museums — Committee for Conservation Metal Working Group, in Edinburgh, Scotland.
One of the papers presented there, “Effects of the Cleaning of Silver with Acidified Thiourea Solutions,” should be of great interest to collectors with silver items in their collections.
The researchers described using “mint” condition Mexican silver coins (98 percent silver and 2 percent copper) for their experiments.
Acidified thiourea solutions
“Acidified thiourea solutions” is the technical term for silver dips. Silver dips have been used over the years by collectors and conservators to remove silver sulfide (i.e., toning).
Silver sulfide is the mineral acanthite. It is one of the most insoluble metal salts known.
As such, a strong combination of chemicals is needed to dissolve it.
Although coin dip recipes may vary from product to product, the two principal components are a strong acid and a sequestering agent. Sulfuric, formic, hydrochloric and phosphoric acids have all been used in silver dips.
The concentration of the acid is quite low. Thiourea is the sequestering agent.
When a silver coin is placed in the dip the tarnish is dissolved by the acid and removed from the surface of the coin.
The thiourea sequesters or seizes the silver ions and holds them in suspension, allowing the acid to remove more tarnish.
A by-product of this reaction is hydrogen sulfide (rotten egg gas) resulting from the sulfur ions combining with hydrogen.
Many dips are perfumed to hide the hydrogen sulfide smell.
Dips also often contain a detergent to remove grease and a colorant so as to distinguish them from water.
‘Safe’ tarnish remover
Many consider silver dips a “safe” means of removing tarnish because the dips are reputed to remove only the tarnish.
The tarnish dissolves more readily than either copper or silver, but studies have shown that the acids in the dips can dissolve copper from a silver alloy and the silver as well.
As a result, applying a dip to the surface of a coin with a swab was recommended rather than dipping the coin in the solution.
In the past we advised collectors to rinse pieces cleaned with silver dip for a minimum of one hour under running water to remove any residual chemical from the dip.
The research presented at Edinburgh shows that no amount of rinsing will remove the thiourea.
You may ask why this a problem? Thiourea’s chemical formula is NH2CSNH2. The “S” in thiourea is sulfur.
Research has shown that a silver dip leaves an adsorbed stable film of aqueous thiourea on the object and that the bond between the sulfur atom of the thiourea and the silver is so strong that it is impossible to break.
In other words, cleaning with a silver dip leaves behind sulfur, which, in turn, will cause the silver to tarnish or recorrode.
The researchers also found that the acid in the coin dips leached copper out of the silver alloy, resulting in the surface of the metal being slightly etched. This makes it even more reactive to the sulfur, which causes it to recorrode faster.
Their study concluded that silver dips are not recommended for cleaning silver objects. I concur.
So, if you cannot use a silver dip, what can you use? Two alternative cleaning techniques are preferred: galvanic cleaning or the use of abrasives.
I will review these techniques in a future column.
Susan L. Maltby, Toronto, is a private conservation consultant, with an interest in numismatic preservation.