There are mixed messages and opinions in the jewelry industry about the role of silicon as an alloying element in karat gold casting alloys. Dieter Ott (1) summarized the situation very well when he stated that “the use of silicon is almost a philosophical question (in karat gold alloys).”
Silicon is known to perform useful deoxidation and ‘brightening’ functions. It is also known to increase the apparent fluidity of molten alloys and help in the production of investment castings which faithfully replicate intricate details in patterns. However, silicon promotes the development of large grains in cast structures and in high concentrations can cause the formation of low melting phases or precipitates which reduce mechanical properties. Figure 1 shows the brittle fractures associated with coarse grains in 14 karat (14K) yellow gold alloys containing silicon.
Two recent studies of silicon-deoxidized 9K and 14K yellow golds indicate that the behaviour of silicon in these alloys is linked to the total precious metal content in a karat gold alloy. Normandeau (2) studied the effects of silicon in 10, 14, and 18K yellow gold jewelry alloys. In his study, the effect of silicon was correlated with the total concentrations of gold and silver in a casting alloy. The results indicate that as the
total amount of gold and silver in a jewelry casting alloy increases, the alloy demonstrates loss of ductility and embrittlement when silicon exceeds a critical level. In other words, the alloys become less tolerant of silicon as caratage increases. Silicon-rich particles were observed on the grain boundaries of 14K fracture specimens when nominal silicon levels exceeded 0.175 wt% (3). Silicon concentration greater than 0.050 wt% in 18K alloys resulted in severely embrittled specimens for tensile testing.
Grice (4) studied silicon-deoxidized low karat golds and reported the appearance of an alloy phase in the grain boundaries of alloys with silicon contents in the range of 0.10 – 0.75 wt%. He concluded that 9K alloys containing high concentrations of silicon can be made to fail during devesting operations because of cracking and hot tearing effects caused by these grain boundary segregates.
The results of these two investigations indicate that silicon is demonstrating a unique behaviour when present as an alloying element in yellow karat golds: silicon is more severely segregated in karat golds than might be expected. An investigation was undertaken at Stuller Settings Inc to evaluate the microsegregation of silicon during freezing events in jewelry alloys.