Authored by Eddie Bell
Rio Grande was founded in 1944 by my father, Saul Bell. A jeweler for more than 80 years, Saul was involved in every imaginable aspect of the jewelry industry, including manufacturing, diamond-setting, watch-making, wholesaling, and retailing. He loved to share his vast knowledge of the jewelry-making process. My brothers, sisters and I still benefit daily from his knowledge and wisdom, and I hope to pass on some of what I have learned from him and from other master jewelers through this series.
We get a lot of calls regarding 18-karat rose gold casting problems because it doesn’t follow the rules that yellow, green and white gold follow, especially when casting stones in place. The problem? Stones and rose gold require opposite cooling techniques. Rose gold needs to cool quickly to keep it from cracking. Gemstones cannot tolerate uneven cooling; yet, because most are thick in the center and thin around the edge, it’s difficult to cool them evenly—unless they’re in a controlled environment (e.g., left in the investment to cool).
Rose gold is made by alloying gold with copper and copper is not completely soluble in gold. When a gold/copper alloy cools slowly past around 700°F (370°C), the atoms rearrange themselves into what metallurgists call an “ordered array structure.” An ordered array structure is a strata of gold atoms with bands of copper atoms between them—and this stuff is really brittle. If 18-karat rose gold is quenched quickly from a dull red heat to below 700°F, it will have a cubic crystalline form like yellow gold, and it’ll be nice and malleable. The same quench, however, will break the stones. If you’re new to rose gold, the frustration of trying to cast with stones in place can be maddening.
There are three options when dealing with rose (or pink) gold:
Warning: Protect yourself from breathing in silica particles when quenching investment molds. It has been demonstrated that the steam rising from a flask-quenching operation contains respirable silica particles that can cause silicosis, a deadly desease. Wear OSHA approved protective gear for silica dust protection or have engineered ventilation to prevent inhalation of silica particles. Your casting area must be kept clean by wet mopping or using a vacuum cleaner equipped with a HEPA filter. Never use compressed air to blow investment powder. The dangerous particles are sub-micron (too small to see with the naked eye) and once airborne can stay in the air of a room for a long time. A vacuum cleaner with a normal filter only fills the air with silica particles.
Read Eddie’s article Considering the Difference Between Gold Alloys (Casting Grain) and Master Alloys, and find more articles by Eddie Bell in the Resource Center.