Fine silver (99.9% pure) is usually too soft to produce large objects that are to have a function (such as silverware); so, the silver is usually alloyed with another metal to give it strength.
This alloy is called sterling silver. It consists of 92.5% by weight of silver and 7.5% by weight of other metals, usually copper.
Manufacturers are always experimenting with other metals toreplace the copper, in order to improve various properties such as reducing casting porosity, eliminating firescale, and increasing resistance to tarnish.
The metals they experiment with include germanium, zinc and platinum, as well as a variety of other additives, including silicon and boron.
However, no one alloy has emerged to replace copper as the industry standard, and alloy development is a very active area.
What is firescale?
When sterling silver is heated in air for processes such as soldering and annealing, oxygen reacts with copper in the alloy to form a dark coloured copper oxide known as ‘firescale’. At elevated temperatures, silver absorbs oxygen: this enables copper to oxidise below the surface of sterling silver, thus causing firescale to penetrate deep into the alloy.
This sterling silver watch had originally been silver plated to hide the firescale. The dark stain is, however, clearly revealed where the plating has worn away.
Firescale is removed or disguised by various processes. These processes add extra labour costs and often incorportate the use of harsh chemicals, e.g. cyanide.
