Melting gold is a common practice used by jewelers and refiners for purification, testing, and casting new pieces. The process involves heating the metal to its melting point of \(1,064^\circ\)C, which inevitably results in some reduction of the initial mass. This weight loss is typically small, but the exact amount is highly dependent on the starting material’s purity and the specific melting method employed. Understanding the factors that contribute to this mass reduction is necessary for maximizing the final yield of pure metal.
Mechanisms of Gold Mass Reduction
The apparent “loss” of gold mass during the melting process is primarily due to the removal or separation of non-gold materials from the alloy. Gold is a noble metal that resists oxidation and destruction under normal melting conditions. The most significant cause of mass reduction in lower-karat scrap is the oxidation of alloying metals mixed with the gold.
Gold jewelry is typically alloyed with base metals like copper, silver, and zinc to increase hardness and durability. When these alloys are heated, the non-gold metals readily react with oxygen, forming oxides. These oxides, referred to as dross or slag, float to the surface of the molten gold. The formation of this slag is the main mechanism by which the initial weight of the scrap material is reduced.
A second factor is the mechanical trapping of fine gold particles. As the sticky slag forms, tiny droplets of pure gold can become embedded within the non-metallic waste material. This gold is not chemically lost but is unrecoverable in the primary melt, adhering instead to the slag or the internal walls of the crucible.
Vaporization of gold is generally negligible under controlled conditions. Pure gold has an extremely high boiling point of approximately \(2,860^\circ\)C, far above standard melting temperatures. However, some base metals in the alloy, such as zinc, have boiling points lower than the gold melting temperature and may vaporize, causing a small mass loss. If the gold is severely overheated, a minor amount of gold vaporization can occur, contributing slightly to the overall loss.
Practical Expected Loss Percentages
The amount of mass reduced after melting is directly proportional to the impurity level of the starting material. When melting clean, fine gold, such as 24-karat grain, the mass reduction is minimal. Under highly controlled conditions, the loss is often less than \(0.1\%\) of the total weight. This minimal reduction accounts for slight mechanical losses or trace impurities.
Scrap gold and common jewelry alloys have a noticeably higher expected loss percentage. Materials like 10-karat or 14-karat jewelry contain a greater proportion of base metals that will oxidize and be removed as slag. In these cases, the mass reduction typically ranges from \(1\%\) to \(5\%\) of the starting weight. A typical melt of 14-karat scrap gold might see a loss of around \(2.3\%\) due to the removal of non-gold alloys and impurities.
Heavily contaminated or “dirty” material yields the highest percentage of weight reduction. This category includes floor sweeps, polishing dust, or scrap containing non-metal attachments like stones, enamel, or organic dirt. Since the weight of all these non-gold materials is included in the initial measurement, the final weight of the refined gold bar will show a significant reduction as the contaminants are burned off or separated. This higher reduction rate is simply the removal of non-valuable mass, not an actual loss of gold content.
Techniques for Maximizing Yield
Maximizing the final yield requires proactive steps to minimize both the formation of slag and the mechanical trapping of gold. Thorough pre-cleaning of the scrap material before it enters the crucible is essential. Removing non-metallic items like gemstones, enamel, and any dirt or organic matter ensures that the initial melt is not burdened with contaminants.
The controlled use of a chemical agent known as flux is a primary method for reducing gold loss during melting. Fluxes, commonly composed of substances like borax or soda ash, are added to the molten metal to react with the base metal oxides. The flux effectively gathers these impurities, creating a liquid, glassy slag that is less viscous and floats cleanly on top of the heavier molten gold. This separation prevents fine gold particles from becoming trapped within the waste material, which can account for a significant portion of the loss.
Temperature control throughout the melting process helps maximize the recovered metal. Reaching the gold’s melting point quickly is necessary to achieve a homogeneous liquid state without prolonged exposure to heat. Overheating the gold encourages faster oxidation of the base metals, which increases the amount of dross formed. Precise temperature management ensures that the base metals are not vaporized and that the gold does not adhere excessively to the crucible walls.
Finally, proper handling of the tools and materials helps ensure complete recovery. Using clean, properly seasoned crucibles made of high-density, non-wetting materials can reduce the amount of gold that sticks to the surface. After pouring the main ingot, the remaining slag and any material adhering to the crucible should be processed separately to recover any mechanically trapped gold particles, ensuring the highest possible yield.