In addition to all of the casting problems normally encountered with karat gold jewelry alloys, there are some problems associated with sterling silver. Before tackling these silver casting problems, let’s examine why silver is more difficult to cast than gold. Most karat gold casting alloys contain gold, silver and copper along with a small percentage of zinc. White golds have nickel instead of silver, but still contain zinc. Also, most gold casting alloys contain one or more additives at trace level amounts to enhance castability. These special additives, plus zinc, act as deoxidizers, and are insurance against misuse during melting. Although melting karat gold alloys in air without protecting the metal from oxidation is not recommended, many casters do so with ad- equate results. Due to a lack of de- oxidizers, melting sterling silver in air will result in gas porosity, copper oxide inclusions and other defects.
Types of Problems
Casting problems with sterling can be classified into two general categories. The first includes problems encountered with all jewelry alloys. The causes of these problems are associated with all operations prior to melting: model-making, in- vesting, burnout, etc. The second category includes problems unique to sterling silver that occur during melting and casting. Prior to Melting: A great deal has already been written about the correct procedures for operations that are performed prior to melting. They include model-making, waxing, treeing, investing and burnout. A particularly good reference is Larry Diamond’s article that was presented at the 1987 Santa Fe Symposium on Jewelry Manufacturing Technology and published in the book for that event. The casting process consists of many smaller processes. When each process is correctly completed, a high-quality casting results. Some steps are more critical than others, so acceptable castings can still be produced when a few less critical procedures are compromised. The final quality of a casting is proportional to the number of steps done correctly. Since sterling silver is far less forgiving than karat gold alloys during melting and casting, it becomes important to perform those steps well. Here is a brief review of the steps that are most important and most of- ten overlooked.
Model-Making and Spruing
Shrinkage porosity is the most common defect in jewelry castings, and no other single factor can affect porosity as much as sprue size and placement. Shrinkage porosity occurs when the supply of molten met- al is choke off from a section of the casting that has not completely solidified. As that still-molten section solidifies, it shrinks and cannot be supplied with more molten metal. Therefore, a void is formed, as shown in Figure I. The upper sequence of diagrams in Figure 1 shows a properly sprued ring solidifying before, or at the same time as, the sprue. The lower sequence of photos shows an improperly sprued ring. The sprue solidifies before the ring, cutting off the supply of molten metal. The result? Shrinkage porosity! To minimize the chance of shrink- age porosity, pay careful attention to the sprue size and location. Choose a sprue that’s at least as thick as the thickest portion of the casting. If it is thinner, it will solidify before the casting and cut off the flow of molten metal into the casting, resulting in porosity in the areas the molten metal didn’t reach. Sprue location is equally important. Always sprue to the heaviest portion of the casting. If the metal has to run through a thin section be- fore reaching a heavy section, the thin section will solidify first, prematurely cutting off the supply of molten metal. If the casting has more than one thick section, consider multiple sprues. (See Figure 2. ) Avoid sharp corners at the points of attachment. A generous radius where the sprue is attached to the main tree will prevent pieces of in- vestment from breaking off and be- coming inclusions in the casting. (See Figure 3.) Figure 4 shows two common spruing mistakes. A necked sprue is shown in Figure 4A. Here, solidification will take place rather quickly in the neck region, thereby cutting off the supply of molten metal to the rest of the casting. Also, the necked sprue acts like a nozzle. So rather than flowing, the metal is sprayed into the casting. The sprue in Figure 4B is attached with square shoulders that inhibit the metal from flowing smoothly into the mold. The correct spruing technique is shown in Figure 4C. Here, notice the radius at the point where the sprue is attached to the casting. This allows the metal smooth entry into the casting with- out turbulence. The use of wide but thin, flat sprues is also a major cause of shrinkage porosity. These sprues solidify rather quickly due to their thinness. A sprue must be larger in all dimensions than the largest part of the casting to prevent it from solidifying before the casting. This is usually best accomplished with round sprues.
Investing is a rather straightforward process that involves mixing water and investment powder ac- cording to the manufacturer’s directions-much like making a cake. Unfortunately, some people choose to take shortcuts and do not weigh or measure the ingredients, or properly time the process. Here is a brief summary of the important steps. .Follow the manufacturer’s directions closely. .You must weigh and measure the amounts of water and powder . .Use the recommended water temperature; too hot shortens the working time; too cold extends it. .Know what your total working time is; time each step carefully. .Leave one or two minutes after vacuuming the flasks for gloss-off to occur . .Work carefully. Have proper ventilation. Wear proper respiratory safety equipment. Practice good housekeeping; contamination can greatly affect quality.
Improper burnout can lead to several problems. Incomplete burnout that leaves wax residue in the mold can react with the molten metal to form gases that can be absorbed by the metal. Heating too quickly at the start of the burnout cycle can boil the wax in a turbulent manner that can damage the still-soft investment, resulting in rough cast surfaces and/ or a cracked mold. Here is a typical burnout cycle as recommended by most investment manufacturers. Step 1: 300°F for two hours. Low temperature allows wax and moisture to leave gently. Step 2: 600°F for two hours. Step 3: 900°F for two hours. Step 4: 1325°F for four to six hours. The molds must reach this temperature for complete burnout. Do not exceed this temperature or the investment will begin to break down. Step 5: Casting temperature for two hours. You must allow at least two hours for the molds to drop down to the casting temperature.
Melting & Casting Problems
Sterling silver is an alloy consisting of at least 92.5% silver. The remainder is usually copper, but occasionally small additions of nickel, tin or zinc are used. Wrought sterling (sheet, wire, tubing) usually is deoxidized with phosphorus. But, phosphorus has a tendency to cause hot-shortness (cracking at elevated temperatures) in some castings. Silver has the ability to absorb a tremendous amount of oxygen in the molten state. Figure 5 shows the silver-oxygen phase diagram. This diagram illustrates that at room temperature, silver is just about incapable of absorbing any oxygen. The solubility increases very slightly until the melting point is reached. At this point, the silver can absorb 10 times its volume of oxygen} the silver itself does not oxidize, but the copper does. Two different oxides of copper form. The first is cuprous oxide (CU20), which is a reddish color, and when viewed under polarized light, is a bright blood- red color. The second is cupric oxide (CuO), which forms when oxygen continues to be supplied to Cu20. Cupric oxide is stable. Cuprous oxide can be reduced back to copper with carbon or hydrogen, or it will form cupric oxide in an oxidizing atmosphere. Thin layers of cupric oxide are easily removed by pickling in a hot 7% solution of sulfuric acid.
There are two types of porosity: gas and shrinkage. Gas porosity looks like air bubbles in the metal, while shrinkage porosity looks like jagged or torn voids. Figure 6A shows gas porosity and Figure 6B shows shrinkage porosity. Shrinkage porosity is caused by uncontrolled solidification. It can be minimized or prevented by control- I ling the way in which a casting solidifies. Since the heaviest area will , freeze last, it must be able to receive a continuous supply of molten metal until it is completely solidified. By examining how molten metal solidifies, this concept can be better understood. Metal solidifies into crystals. In a casting, these crystals are called denrites. Figure 7 A shows crystals or dendrites forming in molten metal that is beginning to solidify. Figure 7B shows a three-dimensional view of a dendrite. Solidification begins at the mold wall and the dendrites grow inward as it progresses. To prevent shrink- age porosity, molten metal must flow through the sprue to the heaviest section of the casting until complete solidification has taken place. To do this, the molten metal has to flow through a thickening dendritic net- work that tends to impede its flow. This is why the sprue must be larger than the heaviest section of the casting. When solidification progresses from the outermost area of the casting toward the sprue, the casting will be free of shrinkage porosity. Gas porosity results from gas en-‘ trapped in the metal during solidification. As metal solidifies, it at- tempts to reject all impurities, including gas. When there is too much dissolved gas or when solidification progresses in a non-directional manner, the gas will be entrapped. Gas porosity is fairly rare in karat gold casting alloys containing special additives that act as deoxidizers, or in those containing more than 0.5% zinc. Sterling silver casting alloy usually does not contain sufficient amounts of deoxidizers, and it is highly susceptible to oxygen pickup. (See Figure 5.) Unless the molten sterling is protected from oxygen pickup, gas porosity will result. Figure 6A shows gas porosity in sterling silver that was melted without such protection. Protecting the molten sterling from oxygen pickup can be accomplished in several ways. It is important not to let air come in contact with the molten sterling during melting and casting. Here are several methods of protecting the sterling during melting and casting. .Cover the melt with an inert gas such as argon or nitrogen. .Cover the melt with a reducing flame such as propane or municipal gas. The advantage of a flame versus an inert gas is that the flame is visible, allowing you to see if the melt is adequately covered. .Use a graphite disk slightly smaller than the crucible. The disk will float on the sterling and protect it in two ways. First, it acts as a barrier between the air and the sterling. Second, it will reduce any copper oxide that may form. Just leave the disk in place while pouring; and it will float and act as a skimmer. Do not use small pieces of graphite or they will fall into the flask when pouring. This method does not lend itself to most centrifugal casting operations, but works especially well for vacuum-assist casting, where the metal is poured by hand into a flask- .Melt in a vacuum. This should not be confused with vacuum-assist casting, where only the flask is subjected to a vacuum. During vacuum melting, the crucible is contained in a large chamber that is evacuated prior to melting and usually back-filled with an inert gas so melting occurs in the absence of oxygen. In those methods, it also is important to protect the metal during pouring. When centrifugal casting, the mold is filled almost instantly, so no protection is necessary. Only when pouring by hand into a flask is it necessary to protect the pouring stream. This is usually best accomplished with a reducing flame. The flame should be turned on as soon as the flask is in place, and the flame should cover the flask opening to eliminate the air that is in the flask.
Copper Oxide Formation
When sterling is melted without protection and oxygen pickup occurs, the copper will oxidize. Copper oxide in the casting can cause two problems. First, copper oxide inclusions will occur throughout the casting. They really only cause a problem when they are near the surface, where they act as hard spots that stick above the polished surface. Second, a layer of copper oxide can form in and around shrinkage porosity. This is seen on a polished surface as gray, cloudy spots. These spots are usually too deep to be polished out. Figure 8 shows a cross section of a sterling ring; shrinkage porosity can be seen throughout. Also evident are two darker areas: one at the up- per right and the other in the lower left of the casting. These dark areas indicate copper oxide surrounding surface porosity. The presence of cuprous oxide can be confirmed with an optical microscope using polarized light. Figures 9 through 11 show the various views of sterling silver with normal illumination and with polarized light. The first photo of each sequence (9A, 10A, lIA) shows various inclusions that appear black. The second photo in each sequence (9B, 10B, IIB) shows the inclusions as bright red in color, confirming that they are cuprous oxide. In some cases, when the molten sterling is left unprotected for an ex- tended amount of time and/or is overheated, the copper oxide formation is so severe that the surface of the molten sterling develops a rather tenacious skin. This copper oxide skin greatly reduces the fluidity of the metal. The usual result is that many of the fine details on the castings do not fill. In some instances, major portions of the casting do not fill. A telltale sign of this condition is a reddish color on the surface near the incomplete fill. For further assistance regarding any casting problems, it’s advisable to contact your metal supplier.
1 Diamond, L. “Casting Defects from Model to Finished Product,” The Santa Fe Symposium on Jewelry Manufacturing Technology 1987. Met-Chem Research, Boulder, CO, pp. 149-203. 2 Chaston, J .C. “Oxygen in Silver,” Silver. Krieger, Florida, pp. 304-307. The author thanks Jeff Stewart for his fine metallography and photography. The author and AIM thank The Santa Fe Symposium in Albuquerque, NM, for permission to reprint a revised version of this article, which first appeared in the book titled “The Santa Fe Symposium on Jewelry Manufacturing Technology 1990. ” Call (505) 344-3357 for details. Rich Carrano is director of technical services at Stem-Leach in Attleboro, MA. Rich has 22 years experience in the jewelry and precious metal industry. He holds a bachelor’s degree in metallurgy from the University of New Haven. Stem-Leach is a supplier of karat gold, gold filled and silver mill products for the jewelry industry and its allied fields. Rich Carrano can be contacted at; Stem-Leach, 49 Pearl St., Attleboro, MA, 02703,. (508) 222- 7400.
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