Gold-filled is a composite material: a base metal core with a layer of karat gold bonded to it. Its gold layer is roughly five to ten times thicker than ordinary gold plating. In watchmaking, it gave cases that were stiff enough for screw backs and screw bezels.
How is gold-filled made?
The gold is joined to the core with heat and pressure. No electroplating is involved. In double-clad sheet, both faces carry gold, and the gold is split equally between them. Under US FTC standards, the gold must make up at least one-twentieth of the item’s total weight. Material made the same way with less gold may legally be described as rolled gold plate or gold overlay instead.
Where it comes from
In 1859, James Boss was granted a patent for forming watch cases from gold-filled material. His material was a brass sheet sandwiched between two thin gold sheets. Boss shaped his cases by rolling the sheet. The traditional method used soldering and cutting. His rolled cases had a harder gold surface than electroplated, gold-washed cases, so they wore less easily. The earliest gold-filled watch cases were marketed as ‘gold stiffened’ cases.
How does a gold-filled case wear?
Over time, the outer gold can wear through to the base metal beneath. This is known as ‘brassing’. For this reason, makers guaranteed filled cases for a set number of years. Solid gold cases never show brassing. Solid gold is softer than gold-filled material, though, and bends more easily. So each has its own weak point. A filled case can lose its gold surface, while a solid gold case can lose its shape.
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