A MAJOR PORTION OF THE JEWELRY MANUFACTURING process is affected by the quality of the master model. The way to successfully lower casting rejection rates lies in the execution of a precise master model. In lostwax casting, a design moves from model to mold to wax to investment to the final cast. Since it is inevitable that a fraction of detail will be lost at each stage, it is vital that the operation begin with the sharpest possible master model. In my opinion, a master model should be assembled from individual components, as opposed to carved out of a single piece. This helps ensure maximum accuracy. A well-constructedmaster model combines precision and quality without sacrificing a design that is flowing and fashionable.
A well-executed master model is a perfectly engineered piece that eliminates the need for excessive, laborintensive finishing costs. Ideally, a master model should look as beautiful in all respects as the finished jewelry piece.
Modelmakers often receive assignments from either pictures or actual physical pieces. The first thing a modelmaker must do is sit down and study the objects or pictures.
Physical pieces provide modelmakers with better blueprints to work with than pictures. Modelmakers can handle an object to learn how previous craftsmen made it, and perhaps determine how to improve on it.
But whether working from a physical piece or a design rendering, good modelmakers never regard the original as a perfect creation. Rather, they study it with the intention of improving its overall design and sharpness of detail. They consider how to build the master model to make it workable for efficient, uninterrupted mass production.
When building the master model, craftsmen must keep in mind that a cenain thickness is desired in critical areas to allow for an easy and fast flow of the wax as it is being injected into the mold, and later, as the metal flows into the investment.
Identifying these critical areas and knowing how thick to make them are decisions of considerable consequence for master modelmakers. To prevent porosity in casting, they must have a well thought-out design that incorporates correct dimensions for a smooth, graceful transition between narrow and wide areas and promotes a good flow of metal throughout the design.
Silver: The Material of Choice
Having considered all this, the modelmaker must now choose the right materials with which to work. Even the best craftsman can’t function well without the best materials; any problems that develop are only magnified and compounded if the wrong materials are used. I do not recommend the use of wax for making master models for rings or any other intricately detailed or filigreed jewelry. Wax does not lend itself effectively to sharp and accurate detail. The only time to make models out of wax is when making prototypes of cumbersome or massive pieces that are hard to manipulate in sterling.
Many current-day modelmakers elect to use brass when fabricating the components they will use to make their model; I feel this is a mistake. Brass should be avoided because of the alloy’s high tin and copper content. The low melting point of tin, about 500°F, means it cannot stand up to the lowest flow of silver solder. And when heated, copper forms a fIlm of contamination that impedes silver soldering.
Sterling silver is the material of choice for fabricating the components used to build master models for rings and other intricate jewelry. There is no worry about silver oxidizing or collapsing. You need to use an alloy composed of 7.5 percent copper, with silver making up the balance. This formula is very important. The added copper lends its characteristic property of hardening, but the content is low enough not to cause any trouble during fabrication.
Silver is a forgiving material for producing models. It is elastic and malleable. Silver can be hammered and it won’t crack or split, and can be rolled into thin sheets. And silver is well suited to the numerous soldering operations that go into the building of a model. A master modelmaker has control over sterling silver and trust in it for measurements, dimensions and transitions.
And finally, the melting points of silver solders are considerably lower than the melting points of sterling components. This gives modelmakers more leeway as they perform the many soldering operations involved in crafting the model.
The kind of solder used in constructing a model will have a great affect on the accurate assembling of precision components. Modelmakers must select appropriate solders that have the right melting points for each of the various stages of construction.
In the initial stages of building a model, it’s important to use a solder that has less tendency to flash to other areas. At this stage, modelmakers want to use the highest melting point silvet solder. Later, after all the components in the initial stage are in place and secured, modelmakers will work with easier flow silver solders.
Some tradesmen use tin solder on sterling. There are several problems with tin. One is that tin will artack sterling, and actually burn holes in it. Also, tin solder bonds can deteriorate later during the vulcanizing process, causing the components to come apart in the mold.
Art and Science
From here, the work of the modelmaker is a combination of talents both innate and acquired. There is a natural order of working which almost seems more spiritual than physical.
In many regards, modelmaking is both an art and a science. For example, in styling the master model, experienced modelmakers measure with their eyes. When it comes to acrually fabricating the piece, however, they rely on precise instruments.
To be a good technician-one who can establish the correct measurements for all the components-is one thing. But for design, modelmakers must also have an eye and feel for balance. Each must be in harmony with the other.
James Tuma has more than 40 years of experience in modelmaking; and is certified by the US. National Bureau of Standards and Measurements. He specializes in modelmaking and consulting for the lost-wax process. For more information, contact: S&A Tuma Co., 22 Community Drive, Cramton, RI, 02905; (401) 181-3285.
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