Saturday, 19 April 2014

Silver Artifacts Conservation - April 17, 2014

April 17, 2014

Small trophy - full view - before
Tarnish is formed on the surface of silver when it is subject to high humidity, moisture or air pollution.  The silver combines with sulphur in the air or moisture to form silver sulfide (AgS) which is black in colour.  Removing this black material is usually done by polishing with a soft cloth.  Silver is easily scratched when polished.  In fact, no matter what you use to polish it there will be minute bits of silver worn away each time you polish often only visible by using a microscope.  Over time this will become a critical issue with silver laminates since the silver itself can be worn down to the base metal.  Not only do the scratches become unsightly but then you may have the brass colour showing through in some other parts of the artifact.  Often the base metal is a copper alloy such as brass but can be many other types of metal.  This photograph shows a trophy with a black sheen of tarnish, silver laminate on a copper alloy base, dust on the wooden base, and a strip of scotch tape.  The tape was loosely attached at the top back edge (reason unknown).

Small trophy - close-up - before
Conservation of silver involves several steps but the intent is to avoid  or at least minimize scratching the surface.  This is accomplished by using the softest materials readily available and a few simple techniques.  There are many commercial silver polishes available that are widely used and certainly perform the polishing quite readily.  However, what many conservators use is precipitated calcium carbonate which is a more pure form of chalk that is very gentle in the polishing process.  This material can be purchased relatively cheaply at most pharmacies.  You need very little every time perhaps only a tablespoon which you mix with distilled water to make a paste.  So the combination of the chalk with no impurities and a pure form of water will combine to make a polishing substance with very limited chance of scratching.

Small trophy - close-up - after
Small trophy - full view - after
Always use nitrile gloves to handle the silver to reduce contact with the oils, salts, and ammonia found on skin.  With every artifact the first step is to gently wipe the surface with a soft cloth to remove dust or any other grainy substances that may be present.  Moistening the cloth with distilled water may be done as well.  The polishing paste is then applied to the silver artifact with a soft cloth.  I apply it gently in a circular motion over the area to be polished.  I usually do this twice and find that in most cases there is still a bit more tarnish removed a second time. The polishing process is finished by a gentle wiping with a clean soft cloth. Again, the cloth can be moistened with distilled water.  

Lastly, the best way to store silver is wrapped in acid free paper within a tightly sealed polyethylene bag and at a relative humidity of 40% or less.  There are some conservation grade treatments that can be applied as a layer to the surface.  Also, there are anti-tarnish strips that can be stored in the bag with the artifact wrapped in the acid-free paper so it does not come in contact with the strips.

These photographs are of a trophy that was conserved in this manner.  This trophy dates from 1950 and was awarded during the Apple Blossom Festival in Kentville Nova Scotia Canada.