Silver alloy compositions, and master alloy compositions therefor

A silver alloy composition exhibiting the desirable properties of reduced fire scale, reduced porosity, reduced grain size and reduced oxide formation when heated, consists essentially of the following parts by weight: about 89-93.5% silver, about 0.02-2% silicon, about 0.001-2% boron, about 0.5-5% zinc, about 0.5-6% copper, about 0.25-6% tin, and about 0.01-1.25% indium. A master alloy composition adapted to be alloyed with silver, consisting essentially of the following parts by weight: about 5-35% zinc, about 5-80% tin, about 5-35% copper, about 0.05-14% silicon, about 0.01-1.25% indium, and about 0.05-17% boron.

TECHNICAL FIELD 
This invention relates generally to improved silver alloy compositions, 
and, more particularly, to improved sterling silver alloy compositions 
which exhibit the desirable properties of substantially-reduced formation 
of fire scale when the alloy is heated, reduced oxide formations in the 
melting and remelting of the composition, reduced porosity and reduced 
grain size. 
BACKGROUND ART 
Many silver alloy compositions are known. Some of these are used for 
jewelry and flatware, while others are used in brazing compositions and as 
electrical conductors. 
Among these various silver-containing alloys, sterling silver must contain 
at least 92.5% pure fine silver. The balance of this alloy may be some 
other metal, but typically includes a substantial percentage of copper. 
The presence of copper tends to increase the hardness of the resulting 
alloy. 
Sterling silver is typically sold in the form of pellet-like shot. Hence, 
the user may readily pour out a desired quantity of such shot, melt it, 
and then cast it into the specific form desired. Investment casting is in 
widespread use for this purpose. This technique contemplates the formation 
of a mold into which the molten sterling silver alloy is poured. The 
material of which the mold is formed typically contains calcium sulfate. 
Silver has a known affinity for oxygen, which affinity increases with 
temperature. When exposed to air, molten silver will absorb about 
twenty-two times its volume of oxygen. Like silver, copper also has a 
great affinity for oxygen, typically forming copper oxide. This may be of 
the cupric or cuprous variety, or both. Hence, unless air is excluded 
during the casting process, the cast article may be porous and 
characterized by the presence of internal voids. Thus, in melting sterling 
silver and other silver-copper alloys, care must be taken to prevent 
oxidation. 
Copper oxide, also known as fire scale, is typically a darkened portion 
which blemishes the cast article. Such fire scale is not limited to the 
surface of the cast article, as in the case of conventional tarnishes, but 
may penetrate the article to some depth. In some cases, such fire scale 
may not be removed by buffing and polishing. Moreover, the opportunity for 
the creation of fire scale exists when the alloy is initially formed as 
shot, when such shot is melted and recast to form the desired article, and 
subsequently if the cast article is thereafter annealed. In each of these 
cases, the alloy is heated, and, given the opportunity, may form fire 
scale. 
As previously noted, fire scale is more than a surface tarnish. Rather, it 
is a blemish which may permeate the cast article for some depth, and, in 
some cases, may not be removed by polishing. To the extent that it exists, 
the blemish caused by fire scale may lead to the rejection of as-cast 
parts. Moreover, such rejected parts may have to be re-refined into the 
elemental metals, and realloyed. 
Accordingly, it would be generally desirable to provide an improved silver 
alloy which substantially reduces the formation of fire scale, which 
exhibits reduced porosity and reduced oxide formation when recast, and 
which has a reduced grain size. 
DISCLOSURE OF THE INVENTION 
The present invention provides improved silver alloy compositions which 
exhibit the desirable properties of reduced fire scale, reduced porosity 
and oxide formation and reduced grain size. In another aspect, the 
invention provides an improved master alloy composition which may be 
melted with fine silver in the desired relative proportions. For example, 
to produce coin silver, 10% of the master alloy composition is melted with 
90% fine silver; to produce sterling silver, 7.5% of the master alloy 
composition is melted with 92.5% silver; and so on. Actually, coin and 
sterling silvers must contain at least 90% and 92.5% fine silver, 
respectively, with the balance of the silver alloy being the master alloy 
composition. 
In one aspect, the improved silver alloy composition broadly consists 
essentially of the following parts by weight: about 89-93.5% silver, about 
0.1-2% silicon, about 0.001-2% boron, about 0.5-5% zinc, about 0.5-6% 
copper, about 0.25-2% tin, and about 0.01-1.25% indium. The percentage of 
silver may be varied depending upon the quality of the alloy to be 
produced. The above ranges encompass both coin silver (i.e., containing at 
least 90% silver) and sterling silver (i.e., containing at least 92.5% 
silver). The master alloy for these silver alloy compositions consists 
essentially of: about 0.9-30.7% silicon, about 0.001-30.7% boron, about 
4.5-76.9% zinc, about 4.5-92.3% copper, about 2.2-30.7% tin, and about 
0.09-19.2% indium. 
One specific, and preferred, sterling silver alloy composition consists 
essentially of: about 92.5% silver, about 0.5% copper, about 4.25% zinc, 
about 0.02% indium, about 0.48% tin, about 1.25% of a boron-copper alloy 
containing about 2% boron and about 98% copper, and about 1% of a 
silicon-copper alloy containing about 10% silicon and about 90% copper. 
This particular composition would translate to a sterling silver alloy 
consisting essentially of: about 92.5% silver, about 2.625% copper, about 
4.25% zinc, about 0.02% indium, about 0.48% tin, about 0.025% boron, and 
about 0.1% silicon. This sterling silver composition exhibits the 
desirable properties of reduced fire scale, both when originally alloyed 
and when subsequently recast, of reduced porosity, and of reduced grain 
size. The master alloy composition for this particular sterling silver 
alloy would therefore consist essentially of: about 35% copper, about 
56.67% zinc, about 0.27% indium, about 6.4% tin, about 0.33% boron, and 
about 1.33% silicon. 
Another sterling silver composition consists essentially of the following 
parts by weight: about 89-93.5% silver, about 0.02-2% silicon, about 
0.001-2% boron, about 0.5-5% zinc, about 0.5-6% copper, about 0.25-6% tin, 
and about 0.01-1.25% indium. The master alloy for this composition 
therefore consists essentially of: about 5-35% zinc, about 5-80% tin, 
about 5-35% copper, about 0.05-14% silicon, about 0.01-1.25% indium, and 
about 0.05-17% boron. 
Accordingly, the general object of this invention is to provide an improved 
silver alloy composition. 
Another object is to provide an improved silver alloy composition which 
shows a substantially-reduced tendency to form fire scale when heated. 
Another object is to provide an improved sterling silver alloy composition 
which, by virtue of its reduced propensity to form fire scale, reduces the 
number of rejected parts when such alloy is subsequently recast. 
Another object is to provide various improved silver alloy compositions 
which exhibit the desirable properties of reduced oxide formations when 
originally melted and when subsequently remelted. 
Another object is to provide various improved master alloy compositions 
which may be alloyed with fine silver to produce silver alloy compositions 
exhibiting the foregoing desirable properties. 
These and other objects and advantages will become apparent from the 
foregoing and ongoing written specification, and the appended claims.

MODE(S) OF CARRYING OUT THE INVENTION 
As noted above, the present invention provides improved silver alloy 
compositions and master alloy compositions which when alloyed with fine 
silver will produce the improved silver alloy compositions. The improved 
composition is deemed to have particular use as both coin silver and 
sterling silver. However, these particular compositions are only 
illustrative. Hence, the invention should not be limited to compositions 
containing the minimal amounts of silver necessary to qualify for such 
grades. The invention broadly provides improved alloys, which are 
particularly suited for use in investment casting procedures, which 
exhibit the desirable properties of substantially reducing the formation 
of fire scale on the recast product, of greatly reduced porosity, a 
reduction in grain size, and reduced oxide formation during melting. 
Coin silver, by definition, must contain at least 90% fine silver, with the 
balance being other metals, typically copper. Sterling silver, on the 
other hand, must contain at least 92.5% fine silver. Here again, silver is 
typically alloyed with copper because the alloyed material is harder than 
either of its constituents. Metals other than, or in addition to, copper 
may also be used in such alloy. Silver-copper alloys may oxidize to form a 
black- or red-colored blemish commonly known as "fire scale". Upon 
information and belief, fire scale is copper oxide. The cupric variety 
will produce a blackened blemish, whereas the cuprous variety will form a 
reddish blemish. In either case, this blemish is not limited to the 
surface of the article, as in the case of superficial tarnish (typically 
silver sulfide), but may penetrate the article more deeply. In some cases, 
the penetration is such that the defect or blemish cannot be removed by 
buffing and polishing. 
In one aspect, the invention broadly provides an improved silver alloy 
composition, which consists essentially of the following parts by weight: 
about 89-93.5% pure fine silver, about 0.1-2% silicon, about 0.001-2% 
boron, about 0.5-5% zinc, about 0.5-6% copper, about 0.25-2% tin, and 
about 0.01-1.25% indium. In the foregoing composition, silicon acts as a 
deoxidant, which reduces the porosity of the recast alloy, and has a 
slight hardening effect. Boron is added to reduce the surface tension of 
the molten alloy, and to allow it to blend homogeneously. Zinc is added to 
reduce the melting point of the alloy, to add whiteness, to act as a 
copper substitute, as a deoxidant, and to improve fluidity of the alloy. 
Copper is added as a conventional hardening agent for silver, as well as 
the main carrying agent for the other materials. Tin is added to provide 
tarnish resistance, and for its hardening effect. Indium is added as a 
grain refining agent, and to improve the wetability of the alloy. Silver 
must, of course, be present in the necessary minimal percentages to 
qualify as either coin silver or sterling silver, as appropriate. These 
metals, when alloyed in the ranges indicated, have been found to produce 
castings free of normal fire scale, with the additional advantages of 
greatly-reduced porosity rates and a reduced grain size. 
In one specific composition, which qualifies as sterling silver, an alloy 
was produced to have: about 92.5% silver, about 0.5% copper, about 4.25% 
zinc, about 0.02% indium, about 0.48% tin, about 1.25% of a boron-copper 
alloy containing about 2% boron and about 98% copper, and about 1% of a 
silicon-copper alloy containing about 10% silicon and about 90% copper. 
Fine silver was weighed and placed in a crucible for melting. Zinc was 
weighed, and placed in the crucible. The crucible was then heated to melt 
the silver and zinc. The boron-copper alloy, tin, indium and 
silicon-copper alloy were then added to the melt as a pre-prepared blend, 
under a protective cover of natural gas to prevent unnecessary oxidation. 
When the mixture was molten, it was stirred and poured through a tundish 
into water, which solidified and shaped the granules in the form of shot. 
Quantities of such shot were then provided to recasters for testing. The 
shot was measured, remelted and poured into investment castings to produce 
desired articles. In all known cases, the recast article was shown to be 
substantially free of fire scale, to have considerably reduced porosity, 
and to have a finer grain structure than conventional sterling silver 
alloys. Labor time in finishing the cast article was reduced due to the 
elimination of the step previously needed to remove fire scale. Moreover, 
the rejection rate of the recast articles was substantially reduced over 
conventional silver-copper alloy compositions. The master alloy 
composition for this particular sterling silver composition consists 
essentially of: about 35% copper, about 56.67% zinc, about 0.27% indium, 
about 6.4% tin, about 0.33% boron, and about 1.33% silicon. When 7.5% of 
this master alloy composition is alloyed with 92.5% fine silver, the 
resulting sterling silver alloy will have the desirable properties 
mentioned below. 
Another group of compositions have been found to exhibit 
substantially-reduced oxide formation when melted and subsequently 
remelted, in addition to all of the various properties mentioned above. 
These compositions consist essentially of the following parts by weight: 
about 89-93.5% fine silver, about 0.02-2% silicon, about 0.001-2% boron, 
about 0.5-5% zinc, about 0.5-6% copper, about 0.25-6% tin, and about 
0.01-1.25% indium. The percentage of silver may be varied depending upon 
the quality of the ally to be produced. The above ranges encompass both 
coin silver (i.e., containing at least 90% fine silver) and sterling 
silver (i.e., containing at least 92.5% silver). 
One specific and preferred sterling silver alloy composition consists of: 
about 92.6% silver, about 1.85% zinc, about 0.05% indium, about 4% tin, 
about 0.5% copper, about 0.5% of a boron-copper alloy containing about 2% 
boron and about 98% copper, and about 0.5% of a silicon-copper alloy 
containing about 10% silicon and about 90% copper. This particular 
composition translates to a sterling silver alloy consisting essentially 
of: about 92.6% silver, about 1.44% copper, about 1.85% zinc, about 0.05% 
indium, about 4% tin, about 0.01% boron, and about 0.05% silicon. This 
alloy exhibits all of the desirable properties mentioned above (i.e., 
substantially reduced fire scale, reduced porosity, reduced grain size, 
and reduced oxide formation during melting and remelting). 
The invention provides various silver alloy compositions when silver is 
used as the main metal. However, the non-silver metals can constitute a 
master alloy composition, which may be used by manufacturers who prefer to 
purchase pure silver and add a master alloy to the pure silver to form 
sterling silver. A master alloy for such purpose consists essentially of 
the following elements by weight: about 5-35% zinc, about 5-80% tin, about 
5-35% copper, about 0.05-14% silicon, about 0.05-17% boron, and about 
0.01-1.25% indium. 
One specific and preferred master alloy composition consists essentially 
of: about 25% zinc, about 54% tin, about 0.75% indium, about 6.75% copper, 
about 6.75% of a boron-copper alloy containing about 2% boron and about 
98% copper, and about 6.75% of a silicon-copper alloy containing about 10% 
silicon and about 90% copper. This translates to a master alloy consisting 
essentially of: about 25% zinc, about 54% tin, about 0.75% indium, about 
19.44% copper, about 0.135% boron, and about 0.675% silicon. The above 
master alloy composition, when alloyed with pure silver in the range of 
about 7.5-10% (i.e., containing about 7.5-10% of the master alloy 
composition, with the balance being pure silver), produces castings with 
all of the desirable properties mentioned above. 
Therefore, while several preferred forms of the improved silver-alloy 
compositions have been described, and certain modifications thereof 
discussed and suggested, persons skilled in this art will readily 
appreciate that various additional changes and modifications may be made 
without departing from the spirit of this invention, as defined and 
differentiated by the following claims: