Understanding Casting Grain

Today, High-qulaity, pre-alloyed casting grain is available in various karats and colors to satisfy jewelers’ investment casting needs. Color variation is quite simply a function of chemical composition where the relative contents of silver and copper play a major role in determining not only the color of the alloy, but also other characteristics such as strength and hardness. Alloy chemistries are formulated primarily for the purpose of attaining a desired color.

However, this approach can sometimes lead to a number of difficulties that are not insurmountable, but which are nuisances that require compromises. For example, high zinc content can produce larger- than-usual shrink voids. Red karat golds can be difficult to cast due to copper oxide slag formation, and also may be susceptible to gas porosity. Silver and copper content within certain ranges can produce extremely hard castings that are barely workable. To avoid these problems, it is usually wise to purchase “tried and true” compositions that have become standards in the industry.

Formulation of Gold Alloys

After having selected the karatage (typically 10k, 14k or 18k in the United States), the caster must choose the color suitable for a particular job. As previously noted, color in karat golds is established by the chemistry of the alloy. That is, by the relative amounts of gold (Au ), silver (Ag), copper (Cu), zinc (Zn), nickel (Ni) and other elements contained in the material.

Basically, colored karat golds (yellow, red, green) consist of alloys made with Au, Ag, Cu and Zn. White karat golds are alloys of Au, Cu, Ni and Zn, although “palladium white golds” (Au, Pd, Ag) are sometimes used. For a complete treatment of the metallurgical aspects of karat golds, readers are encouraged to consult the detailed work of McDonald and Sistare.

In addition to color concerns, a caster may have requirements regarding the mechanical characteristics of a chosen alloy. For example, formability is essential for ring prongs that must be bent to secure stone settings. Also, gold alloys should be reasonably strong to prevent being deformed while in use, and should be sufficiently hard to minimize abrasive wear. Some alloys exhibit very good strength and hardness in the as-cast condition, where- as other alloys are of the so-called “heat-treatable” or “age-hardenable” variety, meaning that strength and hardness can be increased by means of a suitable heat-treating cycle after casting.

Availability of Gold Grain

Recently, a standardized numbering system has been jointly developed by the Society of Automotive Engineers (SAE) and ASTM (formerly American Society for Testing and Materials ), and sponsored by the Gold Institute. Many of the compositions have been tabulated by the book Gold Usage by W.S. Rapson. These were also included in the SAE/ASTM Unified Numbering System as of December 1990. Table 1, which is shown in this article, provides a list of commercially available alloy compositions, several of which are now widely used, and therefore are considered to be “standards” of the industry.

Having recognized the existence of a variety of alloy compositions, colors, mechanical properties, etc., let’s get down to the business of choosing those alloys that are most popular and designed for use specifically as casting grain by the addition of elements to improve casting performance and appearance. The following list outlines some of the features necessary in a workable casting grain: -Color match. -Good flow and fill characteristics for fine detail. -A clean as-cast surface (invest- ment can be easily removed). -Free of undesirable impurities ( e.g. oxide inclusions or hard, stable compounds). .Minimal shrink characteristics. .Desirable mechanical proper- ties -ductile enough to facilitate settings. .Reasonably fine as-cast grain size. .Age-hardenable, if necessary. For example, 14k yellow golds may have silver contents ranging from 4% to 25%, and copper between 16% and 32%. Obviously, many different shades of yellow can be produced within these wide compositional brackets. Usually, zinc is also an ingredient of yellow golds because it lowers the melt/flow temperatures and is a good melt deoxidizer. Perhaps more importantly, zinc increases alloy fluidity, which enhances fine detail fill characteristics. In terms of color, it has some- what of a bleaching or whitening ef- fect, although this is not appreciably significant at the levels used in yel- low gold.

Red golds, at any karat, require very high copper levels to impart reddish color. A small amount of silver enhances the metal’s mechanical characteristics, and very small amounts of zinc are sometimes beneficial to deoxidize the metal.

Green golds, on the other hand, contain silver as the major alloying element, a small amount of copper as a property enhancer, and traces of zinc to improve de-oxidation and cast ability.

Effects of Additives

Minor additions of “performance elements,” within precise limits, are intentionally incorporated into most high-volume casting grains for the purpose of improving casting performance and appearance. These additives usually are boron and silicon and sometimes iridium, nickel, cobaIt, ruthenium and rhenium -each for a specific purpose.

Briefly, boron is very reactive and serves as an effective protector against oxidation during melting of the grain just prior to investment casting. Silicon, at closely controlled low levels, also aids as a de-oxidizer , improves flow characteristics, and yields a clean as-cast product for which minimal pickling is required. As a precautionary note, too much silicon is known to enhance grain growth during the solidification pro- cess, resulting in very large as-cast grain and promoting grain boundary concentration of silicon, which may induce cracking in these areas. This problem is often mistaken for alloy brittleness when, in fact, it is simply a silicon level above the soluble limit of the gold alloy. Iridium, cobalt, ruthenium and rhenium are used as grain-refining agents on a limited basis.

Iridium is probably the subject of most research today, since very low levels of it have a pronounced effect on as-cast grain size. This characteristic is especially desirable for thin cross-sections, such as settings, where improved strength properties and toughness are sought, not to mention the elimination of the “orange peel effect” commonly seen on slightly worked parts. Proper tech- nique and equipment selection is important with these alloys, since iridium reacts with certain elements contained in commercial crucibles. Stable, hard, intermetallic compounds are sometimes formed and cannot be removed during finishing operations, so caution is recommended.

Grain Manufacture

Today’s major manufacturers possess the metallurgical and analytical means to produce and maintain consistently high-quality standards, especially in the area of the so-called performance elements. Casting grain production lot sizes normally range from 30,000 to 180,000 pennyweights per charge, with state-of- the-art induction melting equipment designed to promote “electronic stirring,” which is necessary for multi-component alloy system homogeneity.

The melting process is carried out under reducing conditions to eliminate (or reduce to a very low, harmless level) the metal oxygen content. Performance elements are usually produced in master alloy form by major manufacturers, since high purity levels (or low impurity levels) and metallurgical acceptability are not easily found through master alloy suppliers. These extremely low levels of impurities remain part of the analytical quality control system.

In addition, the actual granulation process of the casting grain is performed through a protective atmosphere. This prevents oxygen pickup at a time when the alloy is most vulnerable as molten metal. During this operation, a sample is taken and routed to the analytical laboratory for complete chemical analysis. A portion of this sample is also used for the investment casting test, where parts are actually produced, polished, evaluated and metallographically examined before the grain is made available to customers. Clearly, quality control is an integral phase of the casting grain production cycle.

Purchasing vs. Alloying

The obvious advantages of purchasing karated grain from a reliable supplier are the availability of various karats and colors, the gold content guarantee, and the assurance that the grain will produce good castings. While in-house alloying, whether “starting from scratch” or using a base metal master alloy, may appear to be cheaper, the investment caster must be cognizant of the risks and hidden costs.

“Karat insurance” is one of the most important aspects for a grain manufacturer. Major producers of quality casting grain employ complete analytical facilities. Grain charges, as previously stated, are very large, and homogeneity is maintained by equipment design. Therefore, assay control costs can be kept to a minimum, gold content can be guaranteed, and content of all other elements can be maintained within a very high degree of confidence and repeatability. Obviously, it would be very difficult and costly to achieve this type of control in small melts of 1,000, 3,000 or even 5,000 penny-weights. But, when lot sizes are routinely in excess of about 100,00 penny-weights, this expense is proportionately decreased.

If you “make your own,” the following items must be included in any cost comparison against the purchase of a quality, karat-guaranteed product.

-Equipment: Additional crucibles, pour cups, fluxes or protective gases, maintenance and equipment setups, safety equipment for the operator, and cleaning and drying equipment.

-Cost of recoveries: Melt losses and recovery expenses via spent crucibles, pour cups, stirring rods, etc. Somewhere in this area, the costs of spent cleaners or acid disposal (hazardous waste), and compliance with environmental regulations have to be considered.

-Energy costs: Additional melts for the purpose of granulating. Fuel gases or electrical power. Some percentage of fixed overhead.

-Labor: Additional time, and in some cases an additional employee, may be required.

-Assaying costs: If you do not analyze each lot, you will, no doubt, have deviant assays on occasion. Fire assaying alone won’t do it, since color is sensitive to the silver/copper ratio. Zinc will gradually burn off, resulting in gold content higher than the designated plumb karatage. Impurities may find their way into the system (from unclean master, recycling, solders, etc.). These factors, if left unchecked, can cause a multitude of quality problems, especially during finishing. Thus, assay of melt is recommended to maintain high quality standards. Assay costs are proportionally higher as melt size decreases since more individual melts must be analyzed.

-Inventory management: How much gold do you need, or can yOu afford to carry, to get your product out the door? Making your own translates to constant weighing, karat corrections due to off-assays, karat drifts due to zinc burnout, scrap cleaning, and recycling-all while keeping tally of the net fine ounces, finance charges, etc.

Many jewelry manufacturers alloy their own karat gold with each investment by mixing fine gold with a purchased master alloy. Still, there are risks involved in this practice. Segregation, due to insufficient mixing, can easily occur, creating high- and low-karat areas within the same investment. This condition can be exaggerated in torch or gas-fired melting without adequate stirring. Impurity levels in master alloys can be excessive, causing numerous problems, as previously stated. Therefore, it is advisable to request a complete certification of analysis, with impurities listed, from the master alloy vendor. In addition, there is the possibility of weight errors when creating many small melts. These may go undetected, since obviously it would not be economical to analyze each small melt.

In summary, the investment caster must be highly aware of the potential and costly risks that may be brought about by alloying karat gold. And he should carefully weigh those risks against purchasing pre-alloyed grain that has been thoroughly tested and inspected, and has a guaranteed karat age.