Electroplating was invented in the early 1800's as a way to cover preformed objects with a coating of more desirable metal. For industrial purposes, this may be to increase conductivity or resistance to oxidation. With jewelry, plating is used mostly for decoration.

The process starts with a watch case or bracelet made of base metal - typically cast white metal, brass, zinc, stainless steel, or even silver. The object is already formed into its final shape and its surface is polished. Any imperfection will be magnified since the metal being fused onto the item is more reflective than the base metal. The piece must also have any dirt or oil removed which can obstruct the plate from attaching to the base metal. Usually ultrasonic or electro-cleansing is used. Next, the object is immersed in an acid solution for about 1 minute. Formulas for this are proprietary and change according to the metals being used but a basic mixture is 10% sulfuric acid / 90% distilled water. Besides further ensuring a clean surface, the acid wash also activates the surface so it readily accepts the plating material. After the acid dip, the piece is rinsed in distilled or deionized water. Now that the object is free of any foreign agents, it can be plated. Sometimes multiple layers are used before the finishing metal. Copper & nickel plating are used to fill in porous areas and add brilliance to the finished product.

The plating metal is dissolved into a solution called a "bath". For example, one gallon of a typical rhodium bath mainly consists of these elements: 8 grams of rhodium, 50 ml of sulfuric acid, 100 ml of phosphoric acid, and distilled water. The bath is heated to between 32˚ and 46˚C and given a positive electric charge. The item is attached to a conductive metal rack and submerged into the solution. A small negative current is applied to the item, usually .15 to .40 amps at 1 to 5 volts. This causes the rhodium suspended in the bath solution to bond with the surface of the object. In general, the higher the electric current the thicker the plate becomes. In turn, the thicker the plate the longer it takes to wear off. You can see the base metal showing through on this vintage Elgin ladies watch:




Here are some terms for plating and the thickness they correspond to:

Gold Wash @.175 microns
Gold Flash 2 - 4 microns
Gold (Electro)Plate 7 - 40 microns
Heavy Gold (Electro)Plate 100+ microns

1 millimeter = 1000 microns

For reference, a hair follicle is about 125 microns in diameter.

"They don't make'm like they used to" definitely applies to gold plating. Nowadays, 7- 10 microns is acceptable and 15 - 20 is the exception. In the mid 20th century it was common to see plating 20 - 40 microns thick and up to 80 microns. Companies like Omega & Rolex even used thicker layers of gold back then. Nicknamed "golden eggs", these watches had a 200 - 240 micron thick shell of 14K or 18K over stainless steel. These were commonly called gold capped.

1969 gold capped Omega Constellation:




Gold filled cases were popular until the late 60's. A layer of gold alloy is bonded by heat & pressure to a base of brass. Normally it had 50 - 140 microns of 10 - 14K gold. A single layer on the top was called single clad, and if both sides were layered it was double clad. Rolled gold plate (or gold plaque) is basically the same as gold filled but uses less gold. US standards require that the minimum amount of gold be 5% of the total weight to be classified as gold filled.

 

I always struggle to keep this straight. This post really helps thought! Thanks!

 

I always understood that gold cap was a sheet/layer of gold pressed and bonded onto the case and not deposited via electrolysis as it seems to infer following on directly from the discussion on electroplating. Just for clarification?