Silver is a unique member of the metals family -the “whitest” of all metals. In its pure form this moon-coloured metal is highly lustrous, and can be polished to a mirror finish. Silver was known and used by primitive man. The ancient Hebrews called it by a name meaning pale. The Greeks knew it by a name meaning shining. American Indians called it “tears of the moon”. The chemical symbol for silver, Ag, comes from its Latin name, argent.
Like gold, silver is considered a precious metal, and is extremely malleable and ductile. It is harder than gold, but softer than copper. Silver can be hammered into sheets so thin that it would take 100,000 of them to make a stack an inch high. Silver has a specific gravity of 10.5, and a melting point of 1760F (960C.)- almost 200F below that of gold.
The uses of silver:
Silver is used for jewellery, coins and silverware. Other principal uses are electrical contacts, photographic processes, metal joining, silver paints and dental applications.
The highest purity of silver commercially attainable is 99.95% (nominally considered 100%), but because of its softness and susceptibility to damage, silver is rarely used in pure form.
The use of the term “sterling” has a historical derivation. In the 12th century, five towns in eastern Germany banded together to form the Hanseatic League – an entity which engaged in substantial commerce with England. In payment for English cattle and grain, the League used its own currency -silver coins called “Easterlings” The English soon learned that these coins were extremely dependable, and it is believed that, under the reign of Henry II, the Easterling was used as a basis for standardizing English coinage. As time went on, the name was shortened to “Sterling”- which is still in use today, referring to the English monetary system, and to a particular alloy of silver.
The word “sterling” represents the best known and most respected quality marking in use today. It signifies that the article so stamped is made from silver with a silver content of at least 92.5% by weight, with no stipulation about the remaining 7.5%. The reason for the relatively “low” silver content in sterling is a practical one: finer grades of silver are too soft for everyday use. Alloying elements are needed to increase strength and hardness. Since copper provides the best combination of wear qualities, it is the most common alloying element used by jewellers and silversmiths.
Although the legal minimum silver content for sterling is 92.5%, it is significant to note that all Handy & Harman sterling silvers are produced by starting with an alloy that has 92.7% silver. The extra percentage of silver compensates for any minute loss of silver during processing, and guarantees that all finished sterling will end up with at least 92.5% silver content.
Characteristics of Sterling Silver:
Some metallurgical facts about silver-copper alloys:
Molten silver and copper are completely soluble in each other in all proportions. However, alloys which have copper contents ranging from about 2% through 27%, when solidified and examined under a microscope, exhibit two discrete constituents: one is nearly 100% silver; the other is a silver-copper “eutectic” (71.9% silver; 28.1% copper), whose melting point is 1435F (780C.) (Note: In a two-metal alloy system, the “eutectic” is a specific ratio of the two metals that exhibits the lowest melting point.)
When standard sterling silver is cooled, microscopic analysis shows both of the above constituents to be present in the solidified sterling. The alloy is entirely liquid at 1640F (890C.) -and entirely solid at 1435F (780C.) However, the degree of copper solubility in the solid alloy depends on the heat treatment used, and the overall physical properties of the sterling can be materially affected, not only by heating the silver to different temperatures, but also by employing different cooling rates.
Controlling hardness and workability of silver alloys:
Silver alloys are normally supplied soft- for easy working. If desired, the alloys can be supplied in various tempers, by reducing (working) the alloy without annealing it.
Though virtually all sterling silvers consist of the same alloy of copper and silver, their properties are greatly affected by working and by heat treatment, such as annealing and quenching.
In sterling and coin alloys, the copper tends to dissolve into the silver, resulting in a homogeneous, large- structure, which is naturally soft and ductile. Cold working these alloys-by rolling, pressing, hammering or wire drawing-causes some of the crystals to become deformed and smaller, which reduces the alloy’s ductility.
Heat treatment can also be used to increase hardness (and decrease ductility). The process, known as precipitation hardening, is described below, and involves heating and cooling the silver in such a way as to cause copper to precipitate out of solid solution, thereby producing a fine- binary structure. This type of structure is hard, but it is also difficult to work, and has a tendency to crack.
As mentioned, silver alloys can be supplied with custom tempers (hardness), defined by the degree of cold-working performed. “Half hard” describes a reduction of the cross- by two B & S gauges; “hard” has been reduced by four B & S gauges, and “spring hard” represents a reduction of 6-10 B & S gauges.
When a specific degree of hardness is desired in the finished article of jewellery or silverware, it is best obtained by controlling the amount of work done on the article after the final anneal, with all work being performed uniformly over the entire piece to prevent cracking at stress points.
Precipitation hardening involves the following procedure:
Heating the alloy to 1375-1400F (745-760C.)
Holding at temperature for 15 minutes.
Quenching rapidly in cold water.
The alloy is now in a softened condition, and can be re-hardened by heating to 600F (316C.) for 30-50 minutes and then air cooling. The resulting hardness is equivalent to the hardness obtained by cold working to a 50% reduction.
The importance of annealing:
Annealing is an effective method for re-softening silver alloys that have lost their ductility due to working or heat treatment. It permits sterling to be worked with reductions of 90% and even more. When the metal becomes too hard for further working, it is simply annealed and re-softened. Specific annealing procedures for sterling alloys should be available from the supplier, and these recommendations should be followed scrupulously, since even small differences in temperatures and/or cooling rates could significantly affect the physical characteristics of the alloy.
When sterling silver is annealed, care must be taken to avoid “overheating”-a condition that increases hardness by promoting undesirable grain growth and a significant loss of ductility.
In torch annealing, it is particularly important not only to see that no part of the work is overheated, but also that all parts of the object or article are brought to the full annealing temperature. Since sterling silver anneals so rapidly, it is not necessary to hold it at the annealing temperature for very long.
Obviously, the use of a closed furnace has certain advantages, since the temperature of the object can be more precisely controlled-and the heat can be absorbed more uniformly. However, the annealing time must be established by trial and error for articles of different size and shape -and for different size furnaces.
Controlling colour and finish of silver:
It is always preferable to anneal silver alloys in a neutral or reducing atmosphere, in order to prevent the formation of copper oxides. In addition to using a controlled atmosphere for annealing, the alloy can also be protected from the air by coating it with flux. Any residues can easily be removed by pickling and rinsing the article in hot water.
When silver alloys are annealed in open air, copper oxides will form. These oxides are of two types. The upper layer is cupric oxide, which has a black color; beneath the layer of cupric oxide there may also be another layer of oxide (cuprous oxide), which, because of its reddish color, is called “fire”
The black surface layer of cupric oxide can be removed by dipping the article in a “pickling solution”- a 5%-10% water solution of sulphuric acid. The pickling action can be accelerated by heating the solution.
After pickling, if a reddish layer is visible, it may be removed by polishing, but if it does not polish out, it can usually be removed by dipping the area in a cold, 50% solution of nitric acid. Since the nitric acid bath removes silver very rapidly, the operator must remove the article from the bath as soon as the fire is dissolved and rinse immediately with water. Any cloudy residue should be polished off immediately.
The best annealing temperature for normal softening of sterling silver is between 1100F and 1200F (593C.-649C). Temperatures above 1200F (649C.) tend to dissolve the copper-rich phase, and, unless the cooling rate is rigidly controlled, maximum softness will not be achieved. At temperatures above 1300F (704C.) the article, if worked, will develop an “orange peel” surface. At temperatures below 1100F (593C.), the time required to achieve the desired results increases to a point where it becomes uneconomical.
By applying Flux to a section of the article prior to heating, the operator has an excellent visual aid for determining when the article reaches the proper annealing temperature, since the Flux will become clear, like water, at 1100F (593C.).
After annealing, the article may be either air-cooled or quenched in water.
Casting sterling silver:
Sterling silver can be cast with excellent results, using the same techniques and precautions recommended for gold casting. The silver for casting is provided in a grain form, which helps facilitate the speed and uniformity of melting.
Because sterling contains pure silver as well as a silver-copper eutectic, it must be heated to 1650F (899C.) to be completely molten. Complete solidification does not occur until the sterling cools to 1435F (779C.).
For coin silver, the melting range is 1435F (779C.) (melting begins) to 1635F (891C.) (melting is complete).