Insiders ’ guide to precious metal coin production
In this article we explore some of the techniques and equipment The Perth Mint ’ s Coining department uses to make exquisitely finished precious metal coins .
BURNISHING
One of the first operations the Coining team undertakes is burnishing the blanks .
Burnishing is carried out inside spinning drums in which a mix of stainless-steel shot and tumbling ‘ soap ’ is gently rolled against the blank surfaces .
This process is designed to remove any blemishes or scratches , smoothing the blanks in preparation for pressing . Burnishing can take up to two hours to
complete , depending on the type of metal and size of the blanks .
The team also employs ‘ acid wash ’ and ‘ ultrasonic wash ’ techniques . Very similar from an operational point of view , they remove any contaminants from the blank surfaces with acid or by delivering vibration waves through a liquid . Air drying the burnished blanks completes the process .
PICKLING
The Perth Mint is renowned for the accuracy of the weight of each coin it makes . If for any reason a precious metal blank is underweight , it is immediately rejected .
Blanks may , however , end up being slightly overweight . One of the ways we ensure they meet specification is through pickling .
Another acid process , pickling is used to dissolve away excess precious metal from any overweight blanks . Large numbers of blanks can be treated with incredible accuracy in our computer controlled pickling plant .