Melting

All metals used in any finished products have gone through at least one melting and freezing cycle during their production. It may seem trivial, but it is worth keeping in mind. Even cold finished wire, sheet, or die struck product was melted and allowed to freeze at some point in its processing history. Every metal has a processing history. It all starts with melting and this process step is worth a great deal of attention. Melting is an integral part of investment casting, so investment casting has been included in this section.

Raw Materials

Pure Metals

Only high-purity raw metals should be used to formulate and melt karat gold alloys for jewelry applications. Fortunately, gold, silver, copper, nickel and zinc and other elements used in precious metal alloys are readily available at purity levels of 99.9+ per cent. Copper and nickel have their own special situations, however.

Electrolytic tough pitch copper is commonly available, and, on a metallic basis, it is a high-purity metal. However, it is actually melted and processed to contain a residual amount of oxygen. This oxygen is typically present in the range of 0.020 to 0.040 percent. The presence of this amount of oxygen must be recognized and dealt with in melting operations.

Nickel is typically available as electrolytic or carbonyl nickel. Carbonyl nickel is purified at high temperatures by a vapor deposition process that uses carbon monoxide. Carbonyl nickel contains residual trace levels of carbon from the carbon monoxide. Melting pure nickel into a karat gold alloy is a tedious process because the melting point of nickel is so much greater than the melting point of typical karat gold alloys. The use of a master alloy which may be combinations of nickel and copper or nickel, copper and zinc make the alloying process much faster, easier and more reliable.

Master Alloys

Master alloys are a convenient tool for formulating karat gold alloys. These alloys contain all the elements at the required concentrations so that when an addition of fine gold is made to the master alloy, an alloy of desired karat, and color is produced. Only high-purity materials should be used to formulate master alloys. They must be melted with as much care as is used when melting karat gold alloys.

Sometimes individual master alloys are formulated so that they can be used to produce gold alloys of different karat qualities. For example, reducing the amount of fine gold added to a master alloy can create a 10K alloy instead of a 14k alloy. This works some of the time, but it might not work all of the time, especially when high karat, 18K alloys are being formulated. The lesson is, don’t expect a master alloy to be useful for formulating all karat qualities for all applications.

Karat Gold Scrap

Karat gold scrap can be used to prepare or formulate alloys if done with adequate care and information. Accurate, reliable information about the karat quality of the scrap must be available before any attempt is made to reuse it. Gold articles that are purchased from pawn shops for remelting and recycling require special consideration. If these articles were made before October 1, 1981, then the karat quality may be a full half karat less than the indicated quality. For example, an article stamped 14 karat may be only 131/2 karat. This may not sound like much, but 14K is equivalent to 583.3 as a decimal fineness. On the other hand, 131/2 karat has a decimal fineness of 562.5. If 131/2 karat scrap is used to prepare fresh, 14K metal, the composition will not comply with current hallmarking regulations unless extra fine gold is added to make up for the difference as indicated by the difference in decimal fineness. Current regulations which went into effect on October 1, 1981 allow unsoldered articles to be only 3 parts per thousand below the indicated decimal fineness; soldered articles are allowed to be 7 parts per thousand less than indicated decimal fineness.

Old high school and college class rings are used by some jewelry craftspersons as a source of gold. These types of emblematic jewelry typically have a year date that indicates when a student graduated. Do not make the mistake of thinking that this date is an indication of when such a ring was manufactured. Students sometimes purchase class rings years before their final graduation day. This means that a student who graduated in 1982 or 1983 could very well have purchased their ring before October 1 of 1981 and the ring could be a half karat less than the indicated quality.

Air Melting

Many different types of melting processes are used in metals processing. They range from the simple to the exotic. For example, parts that are used in a jet turbine engine have to survive for many operating hours in red hot, corrosive environments while people travel in safety and comfort. It would not be surprising if some exotic vacuum melting techniques were used to process these types of materials. However, it makes no sense to consider any exotic processes for precious metal alloys used in jewelry applications except in rare and unusual circumstances. These rare and unusual circumstances will be neglected in this discussion. Basically, all typical methods used to melt and formulate jewelry alloys are equivalent to “air melting”.

Melting metals typically requires large amounts of heat. This heat is first used to heat a metal charge to its melting point. Additional heat is required at the melting point of the metal to convert a solid into a liquid. Additional energy is then needed to superheat the metal to some casting temperature that is above the liquidus temperature of the alloy. Electricity and fuel gases are the two most common sources for the energy that is needed to melt a metal charge.

Induction melting has become the most common electrical method for melting. It is extremely clean, rapid, and highly efficient. Simple techniques can be used to monitor and control temperatures during the different phases of a melting practice. Different methods to prevent and reduce oxidation can be used at the same time without interfering with one another. It’s a preferred method.

Melting to formulate precious metal alloys is sometimes done by gas melting. However, gas melting has its own set of issues because it uses a combustion process as a source of heat. Natural gas is mostly methane, CH4, and is the most common fuel used for gas melting. When burned with combustion air, the products of combustion include carbon dioxide, carbon monoxide, water vapor, and nitrogen.

CH4 + 2O2 + 8N2 -> CO2 + 2H2O + 8N2

Combustion products from flames that are adjusted for maximum flame temperature and heat output will contain very little or no carbon monoxide. This type of flame will contain about 18 percent water vapor and 9 percent carbon dioxide.

The presence of carbon dioxide and water vapor can not be ignored in a metal melting operation that is expected to produce a high quality, gas free product. In gas fired furnaces, the metal charge is in intimate contact with the gas flame and the combustion products. At high temperatures, carbon dioxide can oxidize some of the elements in a karat gold alloy such as zinc, silicon, and boron. In other words, the flame products can consume the alloying elements that might have been added to deoxidize the metal. Water vapor can behave the same way. However, when water vapor oxidizes liquid metal, hydrogen is released. This hydrogen can be dissolved in the liquid metal and cause gas porosity when the metal is cast. Extra care must be used to protect liquid metal when gas-fired melting equipment is used because the products of flame combustion can react with the metal and cause serious quality problems.

Melt Protection

In any type of air melting operation whether it is done with induction or gas heating, serious steps must be taken to protect liquid metal from the effects of oxygen at all times. Air contains 20% oxygen. Oxygen is a very reactive element, especially at elevated temperatures. Air also contains significant amounts of water vapor. Molten metal will react with oxygen to produce oxides at the smallest provocation. As mentioned earlier, molten metal can also be oxidized by contact with water vapor. This is particularly serious, because, as metal is oxidized by water vapor, hydrogen is released. This hydrogen can and will dissolve into the liquid metal. When the metal subsequently solidifies during any casting operations, hydrogen gas is released, creating gas porosity.

The amount of water vapor in air is typically determined by weather conditions. Values for “dew point” are commonly used to describe the amount of moisture in the air when local weather conditions are reported. Dew point temperatures can be used to evaluate the concentration of water vapor in ambient air. Some values for dew point, and percent of moisture in air are summarized in Table I.

TABLE I
Volume percent moisture in air at various dew point temperatures

The data in the Table 1 indicates that air can contain up to 4 percent water vapor on a humid, summer day. It is a reported fact that commercial metal casters have more problems with gas contents in metals during the summertime because of increased humidity in the air. One induction melting operation known to this writer changes melting practices in the summer to control hydrogen content in metal that is caused by water vapor.

Fluxes

For the previously mentioned reasons, molten metal must be protected at all times from contact with the oxygen and water vapor in ambient air. Flux layers are typically used to cover the surface of liquid metal to provide this protection. These layers provide physical barriers to prevent contact with oxygen. Common fluxes include charcoal, boric acid, and boric oxide.

Charcoal is a chemically active flux, because it can reduce some metallic oxides back to the metallic state. Boric acid or boric oxide are “glassy” fluxes and are chemically passive to liquid metal. They can and do dissolve and collect metal oxides. Analyses of glassy fluxes used in melting operations have indicated that high concentrations of zinc oxide and smaller concentrations of copper oxides are present.

Inert or reducing gases and reducing gas flames can be used as “gas covers” in melting operations. Gases and flames are unstable and do not stay in the same place all the time. It is wise to use excess amounts of gas and flame to compensate for their buoyancy and instability with regard to their physical location

Deoxidation

If a metal has been melted in air, the issue of dissolved oxygen and oxides in the metal must be addressed. The degree of deoxidation that is required will depend on the future processing that is planned for the metal. If the metal is to be cast into some ingot form and processed through cold working and annealing operations, only a minimal amount of deoxidation is required. Enough deoxidizer should be added to the metal to remove dissolved oxygen without leaving residual amounts that will interfere with the plastic deformation which will happen during cold working.

Many jewelry people do not believe it is necessary to deoxidize a metal that is going to be cold worked. This is not true. All alloys that have been melted in air require some type of treatment to control the oxygen. The real problem is to consistently control the dosage rates of the deoxidizer. Only small amounts are generally required and if overdone, problems with cracking and reduced ductility can be created. It’s a situation where more is not necessarily better. Figure 1 describes “blisters” that have formed on a piece of cold rolled gold alloy that was treated with insufficient deoxidizer during melting. These blisters formed when the material was annealed in a reducing atmosphere. The reducing gas reacted with oxides in the metal to form steam. Expansion of the steam at elevated furnace temperatures resulted in the raised “blisters” on the surface of the gold alloy.

Metal that is going to be used in an investment casting process typically requires higher levels of deoxidizer than if it was going to be fabricated by cold working operations. When casting grain is first produced, enough deoxidizer has to remain in the casting grain to survive a second melting step when the final casting is poured. The amount of deoxidizer that survives and is available for this second melting step depends on the equipment, the melting methods, and the techniques for metal protection that are used. Some important aspects of these issues will be discussed.

Deoxidizer Additions

Melting techniques that protect metal as much as possible while it is in the liquid state are required for two reasons. The first is to prevent or minimize the formation of oxides. The second is to achieve control over the deoxidation process that is used to remove whatever oxygen does get into the metal. A deoxidation procedure that is in control results in an alloy with very low residual levels of oxygen in the metal and residual levels of deoxidizer that are within some desirable, pre-determined range.
Residual oxygen levels of only several parts per million should be consistently and routinely achieved by simple deoxidation practices.

Crucibles used in precious metal melting operations are commonly made of graphite or carbon. Carbon functions as a deoxidizer when used in this application. Charcoal can be used as a flux for the purpose of deoxidation as well as melt protection.

The zinc that is present in most precious metal jewelry alloys is the first line of defense against oxygen. Zinc oxide forms very readily. If oxygen dissolves into a melt, a significant portion of it will react with the zinc. The zinc oxide that is produced is removed from the metal when it dissolves into a flux, if present. As mentioned previously, high concentrations of zinc oxide have been measured in fluxes after melting operations. This makes sense because of the inherent deoxidizing power of zinc.

Other elements can be added to karat gold alloys to remove oxygen. It makes sense to consider elements that combine more readily with oxygen than zinc. Some of these elements are silicon, boron, phosphorous, calcium, aluminum, magnesium, and lithium. Silicon and boron are the most common deoxidizing additions that are used in karat gold casting alloys. Lithium is reported to be used in some circumstances. Silicon is not a suitable deoxidizer for use in products that are to be cold worked. It can cause reduced ductility and cracking in rolling and wire drawing operations.