What is firescale & why is Argentium Silver firescale free?
What is firescale?
Processes for removing or disguising firescale
How does Argentium Silver eliminate firescale?
Firescale tests
Comparing firescale resistance of commercially available sterling silver alloys
Related documents
Firescale has plagued the production of sterling silver articles for thousands of years.
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.
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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.
Processes for removing or disguising firescale | Disadvantages of processes |
| Abrasive processes: e.g.: filing, emerying, polishing | • Labour intensive • Added costs for tools, equipment and labour |
| Chemical processes: | • Workplace and environmental issues caused by the use and disposal of hazardous chemicals |
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How does Argentium Silver eliminate firescale?
Argentium Silver alloys contain a small quantity of germanium. When Argentium Silver is heated in air, the germanium oxidises preferentially to the silver and copper in the alloy. Germanium oxide is transparent and protects against the formation of dark, penetrating copper oxide firescale.
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Firescale tests
The micrographs below show cross-sections of annealed standard sterling silver (left) and Argentium 930 Silver (right). Firescale can be seen penetrating into the standard sterling sample, while Argentium 930 Silver shows no firescale. These samples were tested by CATRA (Cutlery & Allied Trades Research Association), alongside other commercially available sterling silver alloys. Of those alloys tested, Argentium Silver was the only sterling silver to be firescale free. (The CATRA firescale reports can be dowloaded from the list of files at the end of this section.)
Standard sterling silver Argentium 930 Silver
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Comparing firescale resistance of commercially available sterling silver alloys
The series of images below show a range of commercially available sterling silver alloys that have been torch annealed in air (x2), pickled and polished, to test whether these alloys are resistant to firescale: 
A) 7 commercially available sterling silver alloys, photographed as-received from the suppliers.
B) Alloys from image A), photographed after being annealed twice in air (natural gas/compressed air torch). Samples have not been pickled.
C) Annealed silver alloys from image B), photographed after being pickled and polished. The bottom edge of each sample has been polished hard to cut through any firescale.
Image C) shows the oxides that each alloy has formed during annealing. Oxides will vary due to different alloy compositions and a variety of colours are clearly visible in this photograph.
Interestingly, this test has proved that if a light colour is seen after annealling, it cannot be assumed that no firescale is present, for example, sterling alloy No. 5 shows little discolouration after two annealings (image B), however, dark penetrating firescale is revealed with polishing (image C).
Related documents
'Firescale-free Argentium Silver' data sheet (pdf download)'Environmental & economic benefits of eliminating firescale' data sheet (pdf download)CATRA* report 963443B: 'Resistance to Firestain Evaluation of Silver Alloys' (pdf download)(Testing the firestain (firescale) resistance of Argentium Silver and standard sterling silver in comparison with a range of US sterling silver alloys)
CATRA* report 963854: 'Resistance to Firestain Evaluation of Silver Alloys' (pdf download) (Testing the firestain (firescale) resistance of Argentium Silver and standard sterling silver in comparison with a range of European sterling silver alloys)
* CATRA (Cutlery and Allied Trades Research Association) website: www.catra.org
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