Cynthia  Eid
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SNAG News Technical Article by Cynthia Eid: Argentium Sterling Silver

The Society of North American Goldsmiths

TECH NEWS

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alloy relatively unimpeded. When Argentium® Sterling Silver is heated, the germanium precipitates out of the alloy, and forms its own crystal structure. Because the germanium crystal structure has a different geometry than the silver/copper crystal structure, the two structures interlock, thus making the metal harder.

Why does germanium stop firescale and tarnish?

Germanium, being a “mobile” metalloid, readily migrates to the surface. In this context, the word mobility describes the movement of the electrons within a solid. Germanium constantly diffuses to the surface, where it combines with oxygen, and forms a thin layer of germanium oxide, GeO2. This thin, nearly transparent layer of germanium oxide is impervious to oxygen, and thus prevents cuprous oxide (Cu2O/firescale/firestain) from forming in the interior layers of metal. (Firescale forms when oxygen penetrates the sterling, and combines with copper.)

Germanium oxide is preferential—the germanium oxide forms so readily and easily that it forms BEFORE the copper and silver are able to oxidize. Germanium is highly reactive. (Chemistry & Physics definition of reactive: Tending to participate readily in reactions.) This makes sense when we remember that germanium is not found as a separate element in nature. Even the surface of pure germanium has a very thin layer of germanium dioxide, GeO2.

The only time Argentium® Sterling Silver discolors when heated is if oxygen is missing so that the germanium cannot oxidize. (For instance, I’ll notice a discoloration on the side of a sheet of Argentium® laid against a soldering pad when heated.) This discoloration is only on the surface, and fully removable with pickle.

Germanium stops the oxygen from penetrating---with no oxygen inside, there is no firescale inside the metal. (Silver is one of the few metals to allow oxygen to penetrate.) Germanium on the surface of the metal combines with oxygen to form a protective barrier layer of GeO2, germanium dioxide.

Some people like to say that Argentium® Sterling Silver is "self- healing" in its tarnish-resistance, because this germanium oxide layer will naturally grow over time. Heating speeds this process, so, if finishing a piece has abraded the surface, heating the Argentium® Sterling Silver will re-build the tarnish-resistant layer of germanium oxide more quickly. The other day, as I was trying to reticulate some Argentium® Sterling that was discolored from being in a kiln with the door closed for 45 minutes at 1050ºF, I was intrigued to see that the longer I heated it with the torch, the WHITER the silver became(!).

Some Background: The Correct Meanings of the Words Oxide and Oxidize, which are commonly used by jewelers and metalsmiths to mean patina, tarnish, antiquing, or coloration from heat.
  •  Tarnish is not an oxide; it is silver and copper sulfide. Tarnish occurs when silver is exposed to sulfur containing compounds. The sulfur may be in the air, or it may be in a chemical that is applied to the silver. There can be many contributing factors that can cause an alloy to tarnish: environmental conditions, perfume, deodorant, chemicals used in manufacture, solder temperature, packaging, skin conditions, water, or conditions where sulfur or chlorine is present.
  •  When jewelers and metalsmiths darken recesses of silver for contrast (antiquing) we often say that the silver has been "oxidized." In fact, though, that gray/black stuff (produced by liver of sulfur, for instance) is actually silver sulfide (NOT an oxide!)
  •  Metalsmiths typically think of an oxide as being black, but this is not always the color of an oxide. Glass, for instance, is silicon oxide - it ’s transparent.
  
  •  We rarely actually see silver oxide. It is a light to medium gray color—it is that pale gray color we see if we use nitric acid to etch or strip sterling silver.
  •  Germanium oxide is somewhat transparent and whitish.
  •  Sterling silver forms several oxides, including cuprous oxide and cupric oxide. Cuprous Oxide (Cu2O) is what we commonly call firescale, and cupric oxide (CuO) is the gray or black surface film that is easily removed by mild acid, such as sparex, sulfuric, or citric acid.
  • Silver (the element) is unusual, in that it lets oxygen in. That is why other metals have a surface oxide only, not the underlying firescale problem that we have with regular sterling silver.
Who else has been using Argentium® Sterling Silver?

Some large jewelry manufacturers, including Tiffany, Jacmel, and New England Sterling, as well as Magic Novelty (headpins, earwires, jump rings, and eye pins) and Hallmark Sweet (beads) are selling products made in Argentium® Sterling Silver. The World Poker Tour Invitational tournament trophy was made from Argentium® Sterling Silver. 5

Is there a downside to using Argentium® Sterling Silver?

  •  Argentium® Sterling costs more than regular sterling silver.
  •  There are not many findings available in Argentium® yet.
  •  Because it does not conduct heat as well as regular sterling silver, a different soldering technique is needed.
  •  Sagging can be a problem when soldering flat sheet.
  •  It has a paler glow when red hot that can be difficult to see.

WORKING WITH ARGENTIUM® STERLING SILVER


Tips and Procedures:

ANNEALING

Argentium® Sterling has a melting point around 60 degrees lower than regular sterling silver. Similarly, it has a lower annealing temperature range, of 1050ºF to 1150ºF.

Argentium® Sterling Silver displays a paler color when heated. In practice, I find the color hard to see, and it is easy to overheat if I anneal in a lighted room. Annealing and soldering in the dark make it easier to avoid overheating. When that is not practical, I use dabs of paste flux as a temperature indicator; when the flux is fluid, but still a bit bubbly (not yet as runny as it looks when silver solder is ready to flow) I figure the Argentium® Sterling is annealed.

Recently, Peter Johns told me that it is a good practice to use a clean, new soldering board for Argentium, and to keep it separate from the surfaces that are used for other metals, especially copper alloys. This avoids the possibility of the Argentium® Sterling having its surface contaminated by oxides in the bricks, which could create stains on the metal or other problems. Prior to this, I had not been using fresh soldering surfaces, and had not noticed any problems. It is sensible though, so I have now begun using a separate, labeled soldering board for Argentium® Sterling.

QUENCHING

Argentium® Sterling Silver retains heat longer than standard sterling silver. It’s important to wait for any visible red heat to disappear from the alloy before quenching (this is best judged in

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This technical article was first published by the Society of North American Goldsmiths in its newsletter, under Tech News. All SNAG members automatically receive the newsletter five times a year. To sign up for membership, go to www.snagmetalsmith.org, or (630) 778-6385.

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