When items of jewellery made of metals such as copper or nickel are placed in a solution having a salt of gold, a thin film of gold is deposited by

The deposition of a thin film of gold onto jewellery made of copper or nickel is primarily achieved through electroplating, which requires passing an electric current. In this process, the jewellery acts as a cathode and is submerged in an electrolyte solution containing gold salts, such as potassium gold cyanide [2]. When an electrical potential is applied, gold ions are attracted to the surface and reduced to metallic gold, forming a uniform layer. While 'immersion plating' (a displacement reaction) can occur by simply dipping the metal into a solution, it is self-limiting and typically used as a precursor for thicker layers [1]. For commercial jewellery and durable finishes, electroplating (passing a current) is the standard method to control thickness and ensure adhesion. Heating or cooling are not the primary mechanisms for this electrochemical deposition.

Sources

1. [1] https://www.sciencedirect.com/topics/chemical-engineering/gold-plating
2. [2] https://tau.ac.il/~chemlaba/Files/1.pdf