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Barral Plating

   Years and years ago, a friend of mine from Trinidad told me
that he had worked in a factory where gold plating was done
simultaneously with tumbling. 

It’s called barral plating. Used often for small parts which
are inconvenient to rack up for conventional plating. The
costume industry uses this widely for gold plated findings and
similar small parts. Your plated tie tac backs, for example,
were almost undoubtably barrel plated… The concurrent
burnishing action increases the integrity and uniformity of the
deposit, and the process means there is no attachement point on
the piece to create an anomoly in the plating, plus the constant
movement in the process reduces uneveness caused by differences
in current density in recesses. The anode is in the solution
but seperated from the rolling, perforated tumbling barral,
either inside it but up at the top, or external to it. The
tumbling barral is the cathode, and is coated with insulating
material on the outside, but conductive on the inside where
peices contact it, and parts get plated any time they are in
contact, electrically, with the barrel, either directly or
through contact with the rest of the parts. The disadvantage, of
course, is that you’re also plating the barrel and any additional
metallic burnishing media, and you have to periodically strip
excess plated metal off the machine. The needed machines are
considerably more complex than the small tumblers or
electroplating setups we jewelers use, but the process itself is
a widely used one that does indeed give very good results.

Peter Rowe