At what thickness is white bronze non-porous? If this was to be used after acid copper plating as a barrier before gold plating, would the non-porous thickness be enough to stop migration? L.J.
White bronze, often called Speculum, is used as a replacement for nickel in the manufacturing of jewelry and items that require a barrier layer between the base metal and the gold layer. The white bronze alloys contain between 40-60% tin are, as the name implies, white in color, tarnish resistant and are moderately hard. Because of the concerns about nickel in the environment, this material will probably see greater use in the future.
When white bronze is deposited as a barrier layer, the thickness of the deposit is usually between three and five microns. Whether a thinner layer would be adequate, I honestly don’t know.
There are a number of papers that have been published on this topic. A quick search of Surface Finishing Abstracts web site comes up with 31 references on white bronzes.