The casting of bronze – or in this case spandrels at times are the result of a missing piece or possibly casting an entire set where spandrels may have been changed due to fashion changes. This process of casting and fire gilding ( Bronze Doré-mercury gilding) has been practiced throughout Europe as the way my step by step procedure will describe since the early 17th century.
The bond between the gold and copper alloy is achieved by creating an interdiffusion bond during the heating process. This formation of mercury-copper intermetallic compounds at the interface where the mercury is preferentially retained, rather than diffusing more extensively as it would into silver. These compounds will not allow the mercury to be completely driven off during heating. Some mercury remains bound up with the copper. Upon heating during the application , the gold immediately elevates to the surface amalgam, with mercury through lower levels of the coating and slightly into the copper. Copper has a low concentration at the surface. Mercury gilding reached its pinnacle in 18th century France. It was used to give a luxurious decorative effect.
Two Methods of Mercury Gilding
1. Coat the base metal with a coat of hot mercury and then lay the gold leaf into it.
2. Amalgam gilding